Geology

Geology

Page 1 of 14

Geology

by Roy Porter Introduction Throughout history humans have sought to control and understand their environment. Practical activities like agriculture and quarrying naturally lead to enhanced knowledge, and science suggests further ways of utilizing the Earth. Growing interaction with the Earth has been important in the development of numerous sciences - not just geology but cosmogony and geophysics ; alchemy and chemistry ; mineralogy and crystallography ; meteorology, physical geography, topography, and oceanography ; natural history, biology, and ecology. Distinct investigation of the Earth itself - geology - has been a recent development. Geology (literally ` Earth - knowledge ´ ) does not date back more than two hundred years. Antiquity:-

Scientific thinking about the Earth grew out of traditions of thought which took shape in the Middle East and the Eastern Mediterranean. Early civilization needed to adapt to the seasons, to deserts and mountains, volcanoes and earthquakes. Yet inhabitants of Mesopotamia, the Nile Valley, and the Mediterranean littoral had experience of only a fraction of the Earth. Beyond lay terra incognita . Hence legendary alternative worlds were conjured up in myths of burning tropics, lost continents, and unknown realms where the gods lived. The Greeks:-

The first Greek philosopher about whom much is known was Thales of Miletus ( c. 640 - 546 BC ). He postulated water as the primary ingredient of material nature. Thales' follower, Anaximander, believed the universe began as a seed which grew ; and living things were generated by the interaction of moisture and the Sun. Xenophanes ( c. 570 - 475 BC ) is credited with a cyclic worldview : eventually the Earth would disintegrate, returning to a watery state. Like many other Greek philosophers, Empedocles ( c. 500 - c. 430 BC ) was concerned with change and stability, order and disorder, unity and plurality. The terrestrial order was dominated by strife. In the beginning, the Earth had brought forth living structures more or less at random. Some had died out. The survivors became the progenitors of modern species. The greatest Greek thinker was Aristotle. He considered the world was eternal. Aristotle drew attention to natural processes continually changing its surface features. Earthquakes and volcanoes were due to the wind coursing about in underground caves. Rivers took their origin from rain. Fossils indicated that parts of the Earth had once been covered by water. Ptolemy and Pliny:-

In the 2nd century AD , Ptolemy composed a geography that summed up the Ancients' learning. Ptolemy accepted that the equatorial zone was too torrid to support life, but he postulated an unknown land mass to the south, the terra australis incognita . Antiquity advanced a ` geocentric ´ and ` anthropocentric ´ view. The planet had been designed as a habitat for humans. A parallel may be seen in the Judaeo - Christian cosmogony. The centuries from Antiquity to the Renaissance accumulated knowledge on minerals, gems, fossils, metals, crystals, useful chemicals and medicaments, expounded in encyclopedic natural histories by Pliny ( AD 23 - 79) and Isidore of Seville ( AD 560 - 636). The great Renaissance naturalists were still working within this ` encyclopedic ´ tradition. The most eminent was Konrad Gesner, whose On Fossil Objects was published in 1565, with superb illustrations. Gesner saw resemblances between ` fossil objects ´ and living sea creatures. The Christian view:-

file://C:\Documents and Settings\s h a f i q\Desktop\Geology.htm

3/22/2007

Geology

Page 2 of 14

At the same time, comprehensive philosophies of the Earth were being elaborated, influenced by the Christian revelation of Creation as set out in ` Genesis ´ . This saw the Earth as recently created. Bishop Ussher (1581 - 1656) in his Sacred Chronology (1660), arrived at a creation date for the Earth of 4004 BC . In Christian eyes, time was directional, not cyclical. God had made the Earth perfect but, in response to Original Sin, he had been forced to send Noah's Flood to punish people by depositing them in a harsh environment, characterized by the niggardliness of Nature. This physical decline would continue until God had completed his purposes with humans. The scientific revolution:-

The 16th and 17th centuries brought the discovery of the New World, massive European expansion and technological development. Scientific study of the Earth underwent significant change. Copernican astronomy sabotaged the old notion that the Earth was the centre of the system. The new mechanical philosophy (Descartes, Gassendi, Hobbes, Boyle and Hooke) rejected traditional macrocosm microcosm analogies and the idea that the Earth was alive. Christian scholars adopted a more rationalist stance on the relations between Scripture and scientific truth. The possibility that the Earth was extremely old arose in the work of ` savants ´ like Robert Hooke. For Enlightenment naturalists, the Earth came to be viewed as a machine, operating according to fundamental laws. The old quarrel as to the nature of fossils was settled. Renaissance philosophies had stressed the living aspects of Nature. Similarities between fossils and living beings seemed to prove that the Earth was capable of growth. Exponents of the mechanical philosophy denied these generative powers. Fossils were petrified remains, rather like Roman coins, relics of the past, argued Hooke. Such views chimed with Hooke's concept of major terrestrial transformations and of a succession of faunas and floras now perished. Some species had been made extinct in great catastrophes. The significance of fossils:-

This integrating of evidence from fossils and strata is evident in the work of Nicolaus Steno (1638 1686). He was struck by the similarity between shark's teeth and fossil glossopetrae . He concluded that the stones were petrified teeth. On this basis, he posited six successive periods of Earth's history. Steno's work is one of the earliest ` directional ´ accounts of the Earth's development that integrated the history of the globe and of life. Steno treated fossils as evidence for the origin of rocks. The Enlightenment:-

Mining schools developed in Germany. German mineralogists sought an understanding of the order of rock formations which would be serviceable for prospecting purposes. Johann Gottlob Lehmann (1719 1776) set out his view that there were fundamental distinctions between the various Ganggebergen (masses formed of stratified rock). These distinctions represented different modes of origin, strata being found in historical sequence. Older strata had been chemically precipitated out of water, whereas more recent strata had been mechanically deposited. Abraham Gottlob Werner (1749 - 1817) was appointed in 1775 to the Freiberg Akademie. He was the most influential teacher in the history of geology. Werner established a well - ordered, clear, practical, physically - based stratigraphy. He proposed a succession of the laying down of rocks, beginning with ` primary rocks ´ (precipitated from the water of a universal ocean), then passing through ` transition ´ , ` fl ú tz ´ (sedimentary), and finally ` recent ´ and ` volcanic ´ . The oldest rocks had been chemically deposited ; they were therefore crystalline and without fossils. Later rocks had been mechanically deposited. Werner's approach linked strata to Earth history. The development of stratigraphy:-

Thanks to the German school, but also to French observers like Guettard, Lavoisier, and Dolomieu, to Italians such as Arduino, and to Swedes like Bergman, stratigraphy was beginning to emerge in the 18th century. Of course, there were many rival classifications and all were controversial. In particular, battle raged over the nature of basalt : was it of aqueous or igneous origin? The Wernerian, or Neptunist, school saw

file://C:\Documents and Settings\s h a f i q\Desktop\Geology.htm

3/22/2007

Geology

Page 3 of 14

the Earth's crust precipitated out of aqueous solution. The other, culminating in Hutton, asserted the formation of rock types from the Earth's central heat. The ideas of Buffon and Hutton:-

A pioneer of this school was Buffon (1707 - 1778). He stressed ceaseless transfigurations of the Earth's crust produced by exclusively ` natural ´ causes. In his Epochs of Nature (1779) he emphasized that the Earth had begun as a fragment thrown off the Sun by a collision with a comet. Buffon believed the Earth had taken at least 70,000 years to reach its present state. Extinction was a fact, caused by gradual cooling. The seven stages of the Earth explained successive forms of life, beginning with gigantic forms, now extinct, and ending with humans. Though a critic of Buffon, James Hutton shared his ambitions. Hutton (1726 - 1797) was a scion of the Scottish Enlightenment, being friendly with Adam Smith and James Watt. In his ` Theory of the Earth ´ (1795), Hutton demonstrated a steady - state Earth, in which natural causes had always been of the same kind as a present, acting with precisely the same intensity ( ` uniformitarianism ´ ). There was ` no vestige of a beginning, no prospect of an end ´ . All continents were gradually eroded by rivers and weather. Debris accumulated on the sea bed, to be consolidated into strata and thrust upwards by the central heat to form new continents. Hutton thus postulated an eternal balance between uplift and erosion. All the Earth's processes were gradual. The Earth was incalculably old. His maxim was that ` the past is the key to the present ´. Hutton's theory was much attacked in its own day. Following the outbreak of the French Revolution in 1789, conservatives saw all challenges to the authority of the Bible as socially subversive. Their writings led to ferocious ` Genesis versus Geology ´ controversies in England. The 19th century:-

New ideas about the Earth brought momentous social, cultural, and economic reverberations. Geology clashed with traditional religious dogma about Creation. Modern state - funded scientific education and research organizations emerged. German universities pioneered scientific education. The Geological Survey of Great Britain was founded, after Henry De la Beche (1796 - 1855) obtained state finance for a geological map of south - west England. De la Beche's career culminated in the establishment of a Mines Record Office and the opening in 1851 of the Museum of Practical Geology and the School of Mines in London. Specialized societies were founded. The Geological Society of London dates from 1807. In the United States, the government promoted science. Various states established geological surveys, New York's being particularly productive. The US Geological Survey was founded in 1879, under Clarence King and later John Wesley Powell. In 1870 Congress appointed Powell to lead a survey of the natural resources of the Utah, Colorado and Arizona area. The stratigraphical column:Building on Werner, the great achievement of early 19th - century geology lay in the stratigraphical column. After 1800, it was perceived that mineralogy was not the master key. Fossils became regarded as the indices enabling rocks of comparable age to be identified. Correlation of information from different areas would permit tabulation of sequences of rock formations, thereby displaying a comprehensive picture of previous geological epochs. Britain:-

In Britain the pioneer was William ` Strata ´ Smith (1769 - 1839). Smith received little formal education and became a canal surveyor and mining prospector. By 1799 he set out a list of the secondary strata of England. This led him to the construction of geological maps. In 1815 he brought out A Delineation of the Strata of England and Wales , using a scale of five miles to the inch. Between 1816 and 1824 he published Strata Identified by Organized Fossils , which displayed the fossils characteristic of each formation.

file://C:\Documents and Settings\s h a f i q\Desktop\Geology.htm

3/22/2007

Geology

Page 4 of 14

France:Far more sophisticated were the French naturalists Georges Cuvier (1769 - 1832) and Alexandre Brongniart (1770 - 1837), who worked on the Paris basin. Cuvier's contribution lay in systematizing the laws of comparative anatomy and applying them to fossil vertebrates. He divided invertebrates into three phyla and conducted notable investigations into fish and molluscs. In Researches on the Fossil Bones of Quadrupeds (1812), he reconstructed such extinct fossil quadrupeds as the mastodon, applying the principles of comparative anatomy. Cuvier was the most influential paleontologist of the 19th century. Fossils, in Cuvier's and Brongniart's eyes, were the key to the identification of strata and Earth history. Cuvier argued for occasional wholesale extinctions caused by geological catastrophes, after which new flora and fauna appeared by migration or creation. Cuvier's Discours sur les r é volutions de la surface du globe (1812) became the foundation text for catastrophist views. Older rock types:-

Classification of older rock types was achieved by Adam Sedgwick (1785 - 1873) and Roderick Murchison (1792 - 1871). Sedgwick unravelled the stratigraphic sequence of fossil - bearing rocks in North Wales, naming the oldest of them the Cambrian period (now dated at 500 - 570 million years ago). Further south, Murchison delineated the Silurian system amongst the grauwacke . Above the Silurian, the Devonian was framed by Sedgwick, Murchison and De la Beche. Shortly afterwards, Charles Lapworth developed the Ordovician. Uniformitarianism:Werner's retreating - ocean theory was quickly abandoned, as evidence accumulated that mountains had arisen not by evaporation of the ocean, but through processes causing elevation and depression of the surface. This posed the question of the rise and fall of continents. Supporters of ` catastrophes ´ argued that terrestrial upheavals had been sudden and violent. Opposing these views, Charles Lyell advocated a revised version of Hutton's gradualism. Lyellian uniformitarianism argued that both uplift and erosion occurred by natural forces. Expansion of fieldwork undermined traditional theories based upon restricted local knowledge. The retreating - ocean theory collapsed as Werner's students travelled to terrains where proof of uplift was self - evident. Geologists had to determine the earth movements that had uplifted mountain chains. Chemical theories of uplift yielded to the notion that the Earth's core was intensely hot, by consequence of the planet commencing as a molten ball. Many hypotheses were advanced. In 1829, Elie de Beaumont published Researches on Some of the Revolutions of the Globe , which linked a cooling Earth to sudden uplift : each major mountain chain represented a unique episode in the systematic crumpling of the crust. The Earth was like an apple whose skin wrinkled as the interior shrank through moisture loss. The idea of horizontal (lateral) folding was applied in America by James Dwight Dana to explain the complicated structure of the Appalachians. Such views were challenged by Charles Lyell in his bid to prove a steady - state theory. His classic Principles of Geology (1830 - 33) revived Hutton's vision of a uniform Earth that precluded cumulative, directional change in overall environment ; Earth history proceeded like a cycle, not like an arrow. In Principles of Geology , Lyell thus attacked diluvialism and catastrophism by resuscitating Hutton's vision of an Earth subject only to changes currently discernible. Time replaced violence as the key to geomorphology. Lyell discounted Cuvier's apparent evidence for the catastrophic destruction of fauna and flora populations. For over 30 years he opposed the transmutation of species, reluctantly conceding the point at last only in deference to his friend, Charles Darwin, and the cogency of Darwin's Origin of Species (1859). Ice ages:-

file://C:\Documents and Settings\s h a f i q\Desktop\Geology.htm

3/22/2007

Geology

Page 5 of 14

Landforms presented a further critical difficulty. Geologists had long been baffled by beds of gravel and ` erratic boulders ´ strewn over much of Northern Europe and North America. Bold new theories in the 1830s attributed these phenomena to extended glaciation. Jean de Charpentier and Louis Agassiz contended that the ` diluvium ´ had been moved by vast ice sheets covering Europe during an ` ice age ´ . Agassiz's Studies on Glaciers (1840) postulated a catastrophic temperature drop, covering much of Europe with a thick covering of ice that had annihilated all terrestrial life. The ice - age hypothesis met opposition but eventually found acceptance through James Geikie, James Croll, and Albrecht Penck. Syntheses were required. The most impressive unifying attempt came from Eduard Suess. His The Face of the Earth (1885 - 1909) was a massive work devoted to analyzing the physical agencies contributing to the Earth's geographical evolution. Suess offered an encyclopedic view of crustal movement, the structure and grouping of mountain chains, of sunken continents, and the history of the oceans. He made significant contributions to structural geology. Suess disputed whether the division of the Earth's relief into continents and oceans was permanent, thus clearing the path for the theory of continental drift . Around 1900, the US geologist and cosmologist Thomas C Chamberlin (1843 - 1928) proposed a different synthesis : the Earth did not contract ; its continents were permanent. Continents, Chamberlin argued, were gradually filling the oceans and thereby permitting the sea to overrun the land. The 20th century:-

By 1900, study of the Earth had become fragmented into specialisms like stratigraphy, mineralogy, crystallography, sedimentology, petrography, and palaeobotany, and there was no universally - accepted unifying research programme. Geophysics increasingly provided intellectual coherence. Geophysics emerged as a distinct discipline in the late 19th century. Study of the Earth's magnetic field came to early prominence. In 1919 the American Geophysical Union was formed, and 1957 was designated the International Geophysical Year. The modern term ` earth sciences ´ , to some degree replacing geology, marks the triumph of geophysics. Fieldwork in the 19th century had set the agenda for an enduring tradition of stratigraphic surveying and investigation of landforms. These traditions continued to yield valuable harvests. Immensely influential was the US geomorphologist William Morris Davis (1850 1934). Davis developed the organizing concept of the cycle of erosion. He proposed a stage - by - stage life - cycle for a river valley, marked by youth (steep - sided V - shaped valleys), maturity (flood - plain floors), and old age, as the river valley was imperceptibly worn down into the rolling landscape he termed a ` peneplain ´ . Use of radioactivity for dating purposes:-

Geology during the 19th century built on the idea of the cooling Earth. Lord Kelvin's estimates of the Earth's age suggested a relatively low antiquity, but this was soon challenged from within physics itself, for in 1896 the discovery of radioactivity revealed a new energy source unknown to Kelvin. In The Age of the Earth (1913), Arthur Holmes (1890 - 1965) pioneered the use of radioactive decay methods for rock - dating. By showing the Earth had cooled far more slowly than Kelvin asserted, the new physicists undermined the ` wrinkled apple ´ analogy. Continental drift:-

Amidst such challenges Alfred Wegener(1880 - 1930) went further and declared that continental rafts might actually slither horizontally across the Earth's face. From 1910 Wegener developed a theory of continental drift. Empirical evidence for such displacement lay, he thought, in the close jigsaw - fit between coastlines on either side of the Atlantic, and notably in palaeontological similarities between Brazil and Africa. Wegener was also convinced that geophysical factors would corroborate wandering continents. Wegener supposed that a united supercontinent, Pangaea, had existed in the Mesozoic. This had developed numerous fractures and had drifted apart, some 200 million years ago. During the Cretaceous, South America and Africa had largely been split, but not until the end of the Quaternary had North America and Europe finally separated. Australia had been severed from Antarctica during the Eocene.

file://C:\Documents and Settings\s h a f i q\Desktop\Geology.htm

3/22/2007

Geology

Page 6 of 14

The causes of continental drift:What had caused continental drift? Wegener offered a choice of possibilities. One was a westwards tidal force caused by the Moon. The other involved a centrifugal effect propelling continents away from the poles towards the equator (the ` flight from the pole ´ ). In its early years, drift theory won few champions, and in the English - speaking world reactions were especially hostile. A few geologists were intrigued by drift, especially the South African, Alexander Du Toit(1878 - 1948), who adumbrated the similarities in the geologies of South America and South Africa, suggesting they had once been contiguous. In Our Wandering Continents (1937), Du Toit maintained that the southern continents had formed the supercontinent of Gondwanaland. The most ingenious support for drift came, however, from the British geophysicist Arthur Holmes. Assuming radioactivity produced vast quantities of heat, Holmes argued for convection currents within the crust. Radioactive heating caused molten magma to rise to the surface, which then spread out in a horizontal current before descending back into the depths when chilled. Such currents provided a new mechanism for drift. The real breakthrough required diverse kinds of evidence accumulating from the 1940s, especially through oceanography and palaeomagnetism. Advances in palaeomagnetism arose from controversies over origins of the Earth's magnetic field. The evidence for changing directions of the magnetic field recorded by the rocks was linked to a baffling anomaly : in many cases the direction of the field seemed to be reversed. This led geophysicists to suspect that the terrestrial magnetic field occasionally switched. Over millions of years, there would be intermittent reversal events in which the North and South magnetic poles would alternate. Remnant magnetization would record these events, and, if the rocks could be dated sufficiently precisely, a complete register of reversals could be traced against the geological record. By 1960, US scientists had refined the radiometric technique for dating rocks, deploying especially the potassium - argon method. A group at Berkeley developed a timescale of reversals for the Pleistocene era ; Australian scientists produced their own scale, based on the dating of Hawaiian lava flows. Oceanography was developing too. Here the work of William Maurice Ewing (1906 - 1974) was especially significant. Ewing ascertained that the crust under the ocean is much thinner than the continental shell. Ewing also demonstrated that mid - ocean ridges were common to all oceans. Ewing's work demonstrated that far from being ancient, ocean rocks were recent. The US geophysicist Harry Hess (1906 - 1969) played a key role in promoting the new theories, viewing the oceans as the major centre of activity. The new crust was produced in the ridges, whereas trenches marked the sites where old crust was subtended into the depths, completing the convection current's cycle. Carried by the horizontal motion of the convection current, continents would glide across the surface. Constantly being formed and destroyed, the ocean floors were young ; only continents - too light to be drawn down by the current - would preserve testimony of the remote geological past. Support came from J Tuzo Wilson (1908 - 1993), a Canadian geologist, who provided backing for the sea - floor spreading hypothesis . A dramatic new line of evidence, developed by Drummond Hoyle Matthews (1931 - ) and Fred Vine (1939 - 1988) of Cambridge University, confirmed sea - floor spreading. (1931 - ) and Fred Vine (1939 - 1988) of Cambridge University, confirmed sea - floor spreading. The majority of Earth scientists accepted the new plate tectonics model with remarkable rapidity. In the mid - 1960s, a full account of plate tectonics was expounded. The Earth's surface was divided into six major plates, the borders of which could be explained by way of the convection - current theory. Deep earthquakes were produced where one section of crust was driven beneath another, the same process also causing volcanic activity in zones like the Andes. Mountains on the western edge of the North and South American continents arose from the fact that the continental ` raft ´ is the leading edge of a plate, having to face the oncoming material from other plates being forced beneath them. The Alps and Himalayas are the outcome of collisions of continental areas, each driven by a different plate system. Geologists of the late 1960s and 1970s undertook immense reinterpretation of their traditional doctrines. Well - established stratigraphical and geomorphological data had to be redefined in terms of the new forces operating in the crust. Tuzo Wilson's A Revolution in Earth Science (1967) was a persuasive account of the plate tectonics revolution. Geology is remarkable for having undergone such a dramatic

file://C:\Documents and Settings\s h a f i q\Desktop\Geology.htm

3/22/2007

Geology

Page 7 of 14

and comprehensive conceptual revolution within recent decades. The fact that the most compelling evidence for the new theory originated from the new discipline of ocean - based geophysics has involved considerable revaluing of skills and priorities within the profession. Above all, the ocean floor now appears to be the key to understanding the Earth's crust, in a way that Wegener never appreciated. Satellite observations:-

In recent years Earth observation satellites have measured continental movements with unprecedented accuracy. The surface of the Earth can be measured using global positioning geodesy (detecting signals from satellites by Earth - based receivers), satellite laser ranging (in which satellites reflect signals from ground transmitters back to ground receivers), and very long - long - baseline interferometry, which compares signals received at ground - based receivers from distant extraterrestrial bodies. These techniques can measure distances of thousands of kilometres to accuracies of less than a centimetre. Movements of faults can be measured, as can the growth of tectonic plates. Previously, such speeds were calculated by averaging displacements measured over decades or centuries. The results show that in the oceanic crust, plate growth is steady : from 12 mm / 0.05 in per year across the Mid - Atlantic Ridge to 160 mm / 6.5 in per year across the East Pacific Rise. The major continental faults seem to be very irregular in their movement ; the Great Rift Valley has remained stationary for 20 years, when long - term averages suggest that it would have opened up about 100 mm / 4 in in that time. Feldspar:-

large group of minerals composed of aluminosilicates of potassium, sodium, calcium, or occasionally barium. They occur as single crystals or as masses of crystals and form an important constituent of many igneous and metamorphic rocks, including granite, gneiss, basalt, and other crystalline rocks. Feldspars are the most abundant of all minerals and account for nearly half of the volume of the earth's crust. Although the feldspar minerals may belong to either the monoclinic or triclinic systems, they nevertheless resemble each other in crystal habit, methods of twining, and especially by having cleavage surfaces inclined to each other at an angle of nearly 90°. They have a hardness of 6 to 6.5 and a specific gravity ranging from 2.5 to 2.8. Feldspars have vitreous luster and vary in color from white or colorless to various shades of pink, yellow, green, and red. All the feldspars weather readily to form a type of clay known as kaolin. Orthoclase, a monoclinic feldspar with the formula KAlSi3O8, is one of the most common of all minerals. It is often white, gray, or flesh-red in color and sometimes occurs as colorless crystals. Orthoclase is used extensively in the manufacture of porcelain and glass. Adularia is a colorless, translucent to transparent variety of orthoclase. Microcline, which crystallizes in the triclinic system, is identical with orthoclase in chemical composition and virtually identical in physical properties. It occurs occasionally in the form of enormous single crystals. The industrial uses of microcline are similar to those of orthoclase. A green variety of microcline, amazonstone, is valued as a gemstone when highly polished. The plagioclase feldspars comprise an isomorphous series of triclinic mineral ranging from pure sodium aluminosilicate to pure calcium aluminosilicate (see Crystal). Pure sodium aluminosilicate is called albite, and oligoclase, andesine, labradorite, bytownite, and anorthite are minerals with increasing percentages of calcium. Anorthite is pure calcium aluminosilicate with the formula CaAl2Si2O8. The plagioclase feldspars are of lesser commercial importance than orthoclase and microcline. They sometimes show an attractive play of colors and are polished as semiprecious stones. Opalescent albite and iridescent labradorite are called moonstones. Oligoclase with included impurities that cause a sparkling effect is called sunstone.

file://C:\Documents and Settings\s h a f i q\Desktop\Geology.htm

3/22/2007

Geology

Page 8 of 14

Gabbro:- general name for a large group of granular igneous rocks, the intrusive equivalents of basalt, composed of plagioclase feldspar with a predominance of dark minerals, usually pyroxenes, hornblende, or olivine. The rocks are heavy, often greenish in color. Gabbros occur in the Adirondack Mountains, in the vicinity of Baltimore, Maryland, and in the highlands along the north shore of Lake Superior. Sismology: -

Study of earthquakes and how their shock waves travel through the Earth. By examining the global pattern of waves produced by an earthquake, seismologists can deduce the nature of the materials through which they have passed. This leads to an understanding of the Earth's internal structure. On a smaller scale, artificial earthquake waves, generated by explosions or mechanical vibrators, can be used to search for subsurface features in, for example, oil or mineral exploration. Earthquake waves from underground nuclear explosions can be distinguished from natural waves by their shorter wavelength and higher frequency.

plate tectonics:Theory formulated in the 1960s to explain the phenomena of continental drift and seafloor spreading, and the formation of the major physical features of the Earth's surface. The Earth's outermost layer, the lithosphere , is regarded as a jigsaw puzzle of rigid major and minor plates that move relative to each other, probably under the influence of convection currents in the mantle beneath. At the margins of the plates, where they collide or move apart, major landforms such as mountains , volcanoes , ocean trenches , and ocean ridges are created. The rate of plate movement is at most 15 cm / 6 in per year. The concept of plate tectonics brings together under one unifying theory many previously unrelated phenomena observed in the Earth's crust. The size of the crust plates is variable, as they are constantly changing, but six or seven large plates now cover much of the Earth's surface, the remainder being occupied by a number of smaller plates. Each large plate may include both continental and ocean crust. As a result of seismic studies it is known that the lithosphere is a rigid layer extending to depths of 50 100 km / 30 - 60 mi, overlying the upper part of the mantle (the asthenosphere ), which is composed of rocks very close to melting point, with a low shear strength. This zone of mechanical weakness allows the movement of the overlying plates. Each large plate may include both continental and ocean crust. As a result of seismic studies it is known that the lithosphere is a rigid layer extending to depths of 50 - 100 km / 30 - 60 mi, overlying the upper part of the mantle (the asthenosphere ), which is composed of rocks very close to melting point, with a low shear strength. This zone of mechanical weakness allows the movement of the overlying plates. The margins of the plates are defined by major earthquake zones and belts of volcanic and tectonic activity, which have been well known for many years. Almost all earthquake, volcanic, and tectonic activity is confined to the margins of plates, and shows that the plates are in constant motion. Constructive margins:-

Where two plates are moving apart from each other, molten rock from the mantle wells up in the space between the plates and hardens to form new crust, usually in the form of an ocean ridge (such as the Mid - Atlantic Ridge ). The newly formed crust accumulates on either side of the ocean ridge, causing the seafloor to spread ; the floor of the Atlantic Ocean is growing by 5 cm / 2 in each year because of the welling - up of new material at the Mid - Atlantic Ridge. Destructive margins:-

file://C:\Documents and Settings\s h a f i q\Desktop\Geology.htm

3/22/2007

Geology

Page 9 of 14

Where two plates are moving towards each other, the denser of the two plates may be forced under the other into a region called the subduction zone . The descending plate melts to form a body of magma , which may then rise to the surface through cracks and faults to form volcanoes. If the two plates consist of more buoyant continental crust, subduction does not occur. Instead, the crust crumples gradually to form ranges of young mountains, such as the Himalayas in Asia, the Andes in South America, and the Rockies in North America. This process of mountain building is termed orogenesis

Conservative margins:-

Sometimes two plates will slide past each other - an example is the San Andreas Fault, California, where the movement of the plates sometimes takes the form of sudden jerks, causing the earthquakes common in the San Francisco - Los Angeles area. Most of the earthquake and zones of the world are found in regions where two plates meet or are moving apart.

Sea - floor spreading:-

New plate material is generated along the mid - ocean ridges, where basaltic lava is poured out by submarine volcanoes. The theory of sea - floor spreading has demonstrated the way in which the basaltic lava spreads outwards away from the ridge crest at 1 - 6 cm / 0.5 - 2.5 in per year. Plate material is consumed at a rate of 5 - 15 cm / 2 - 6 in per year at the site of the deep ocean trenches, for example, along the Pacific coast of South America. The trenches are sites where two plates of lithosphere meet ; the one bearing ocean - floor basalts plunges beneath the adjacent continental mass at an angle of 45 º , giving rise to shallow earthquakes near the coast and progressively deeper earthquakes inland. In places the sinking plate may descend beneath an island arc of offshore islands, as in the Aleutian Islands and Japan, and in this case the shallow earthquakes will occur beneath the island arc. The destruction of ocean crust in this way accounts for another well - known geological fact - that there are no old rocks found in the ocean basins. The oldest sediments found are 150 million years old, but the vast majority are less than 80 million years old. This suggests that plate tectonics has been operating for at least the last 200 million years. In other areas plates slide past each other along fault zones, giving rise to shallow earthquakes. Sites where three plates meet are known as triple junctions. Causes of plate movement:The causes of plate movement are all very hypothetical. It has been known for some time that heat flow from the interior of the Earth is high over the mid - ocean ridges, and so various models of thermal convection in the mantle have been proposed ; the geometry of the flow in any convective system must be complex, as there is no symmetry to the arrangement of ridges and trench systems over the Earth's surface. It seems likely that a plume of hot, molten material rises below the ridges and is extruded as basaltic lava. In zones of descending flow, at deep ocean trenches, the surface sediment is scraped off the descending plate onto the margin of the static plate, causing it to grow outwards towards the ocean, while the basaltic rocks of the descending plate, together with any remaining sediment, suffer partial fusion as they descend. This gives rise to large volumes of molten rock material, or magma, which ascends to form andesitic lavas and intrusions of diorite or granodiorite at the margin of the overlying continent. These theories of plate tectonics may provide explanations for the formation of the Earth's crust ; by partial fusion of the mantle, oceanic crust is generated at the mid - ocean ridges, while partial fusion below active continental margins generates the more silica - rich continental rocks. Ocean crust is continually produced by, and returned to, the mantle, but the continental crust rocks, once formed, remain on the surface and are not returned to the mantle.

file://C:\Documents and Settings\s h a f i q\Desktop\Geology.htm

3/22/2007

Geology

Page 10 of 14

Development of plate tectonics theory:-

The concept of continental drift was first put forward in 1915 in a book entitled The Origin of Continents and Oceans by the German meteorologist Alfred Wegener, who recognized that continental plates rupture, drift apart, and eventually collide with one another. Wegener's theory explained why the shape of the east coast of the Americas and that of the west coast of Africa seem to fit together like pieces of a jigsaw puzzle ; evidence for the drift came from the presence of certain rock deposits which indicated that continents have changed position over time. In the early 1960s scientists discovered that most earthquakes occur along lines parallel to ocean trenches and ridges, and in 1965 the theory of plate tectonics was formulated by Canadian geophysicist John Tuzo Wilson ; it has now gained widespread acceptance among earth scientists who have traced the movements of tectonic plates millions of years into the past. The widely accepted belief is that all the continents originally formed part of an enormous single land mass, known as Pangaea. This land was surrounded by a giant ocean known as Panthalassa. About 200 million years ago, Pangaea began to break up into two large masses called Gondwanaland and Laurasia, which in turn separated into the continents as they are today, and which have drifted to their present locations. In 1995 US and French geophysicists produced the first direct evidence that the Indo - Australian plate has split in two in the middle of the Indian Ocean, just south of the Equator. They believe the split began about 8 million years ago.

Earthquake:-

Abrupt motion that propagates through the Earth and along its surfaces. Earthquakes are caused by the sudden release in rocks of strain accumulated over time as a result of tectonics . The study of earthquakes is called seismology . Most earthquakes occur along faults (fractures or breaks) and Benioff zones . Plate tectonic movements generate the major proportion : as two plates move past each other they can become jammed. When sufficient strain has accumulated, the rock breaks, releasing a series of elastic waves ( seismic waves ) as the plates spring free. The force of earthquakes (magnitude) is measured on the Richter scale , and their effect (intensity) on the Mercalli scale . The point at which an earthquake originates is the seismic focus or hypocentre ; the point on the Earth's surface directly above this is the epicentre . The Alaskan (USA) earthquake of 27 March 1964 ranks as one of the greatest ever recorded, measuring 8.3 to 8.8 on the Richter scale. The 1906 San Francisco earthquake is among the most famous in history. Its magnitude was 8.3 on the Richter scale. The deadliest, most destructive earthquake in historical times is thought to have been in China in 1556. In 1987, a California earthquake was successfully predicted by measurement of underground pressure waves ; prediction attempts have also involved the study of such phenomena as the change in gases issuing from the crust , the level of water in wells, slight deformation of the rock surface, a sequence of minor tremors, and the behaviour of animals. The possibility of earthquake prevention is remote. However, rock slippage might be slowed at movement points, or promoted at stoppage points, by the extraction or injection of large quantities of water underground, since water serves as a lubricant. This would ease overall pressure. Most earthquakes happen at sea and cause little damage. However, when severe earthquakes occur in highly populated areas they can cause great destruction and loss of life. A reliable form of earthquake prediction has yet to be developed, although the seismic gap theory has had some success in identifying likely locations. The San Andreas fault in California, where the North American and Pacific plates move past each other,

file://C:\Documents and Settings\s h a f i q\Desktop\Geology.htm

3/22/2007

Geology

Page 11 of 14

is a notorious site of many large earthquakes. Earthquakes have been responsible for moving the North Pole towards Japan at a rate of about 6 cm / 2 in every 100 years. This is because most major earthquakes occur along the Pacific Rim, and tend to tilt the pole towards their epicentres. Volcano:-

Crack in the Earth's crust through which hot magma (molten rock) and gases well up. The magma is termed lava when it reaches the surface. A volcanic mountain, usually cone shaped with a crater on top, is formed around the opening, or vent, by the build - up of solidified lava and ashes (rock fragments). Most volcanoes arise on plate margins (see plate tectonics ), where the movements of plates generate magma or allow it to rise from the mantle beneath. However, a number are found far from plate - margin activity, on ` hot spots ´ where the Earth's crust is thin. There are two main types of volcano : Composite volcanoes:- such as Stromboli and Vesuvius in Italy, are found at destructive plate margins (areas where plates are being pushed together), usually in association with island arcs and coastal mountain chains. The magma is mostly derived from plate material and is rich in silica. This makes a very stiff lava such as andesite, which solidifies rapidly to form a high, steep - sided volcanic mountain. The magma often clogs the volcanic vent, causing violent eruptions as the blockage is blasted free, as in the eruption of Mount St Helens , USA, in 1980. The crater may collapse to form a caldera . Shield volcanoes :- such as Mauna Loa in Hawaii, are found along the rift valleys and ocean ridges of constructive plate margins (areas where plates are moving apart), and also over hot spots. The magma is derived from the Earth's mantle and is quite free - flowing. The lava formed from this magma - usually basalt - flows for some distance over the surface before it sets and so forms broad low volcanoes. The lava of a shield volcano is not ejected violently but simply flows over the crater rim. The type of volcanic activity is also governed by the age of the volcano. The first stages of an eruption are usually vigorous as the magma forces its way to the surface. As the pressure drops and the vents become established, the main phase of activity begins, composite volcanoes giving pyroclastic debris and shield volcanoes giving lava flows. When the pressure from below ceases, due to exhaustion of the magma chamber, activity wanes and is confined to the emission of gases and in time this also ceases. The volcano then enters a period of quiescence, after which activity may resume after a period of days, years, or even thousands of years. Only when the root zones of a volcano have been exposed by erosion can a volcano be said to be truly extinct. Many volcanoes are submarine and occur along mid - ocean ridges. The chief terrestrial volcanic regions are around the Pacific rim (Cape Horn to Alaska) ; the central Andes of Chile (with the world's highest volcano, Guallatiri, 6,060 m / 19,900 ft) ; North Island, New Zealand ; Hawaii ; Japan ; and Antarctica. There are more than 1,300 potentially active volcanoes on Earth. Volcanism has helped shape other members of the Solar System, including the Moon, Mars, Venus, and Jupiter's moon Io. There are several methods of monitoring volcanic activity. They include seismographic instruments on the ground, aircraft monitoring, and space monitoring using remote sensing satellites. Lava:- Molten rock (usually 800 - 1,100 º C / 1,500 - 2,000 º F) that erupts from a volcano and cools to form extrusive igneous rock . It differs from magma in that it is molten rock on the surface ; magma is molten rock below the surface. Lava that is viscous and sticky does not flow far ; it forms a steep sided conical composite volcano . Less viscous lava can flow for long distances and forms a broad flat

file://C:\Documents and Settings\s h a f i q\Desktop\Geology.htm

3/22/2007

Geology

Page 12 of 14

shield volcano . The viscosity of lava, and thus the form of volcano they form, depends on silica content, temperature, and degree of solidification upon extrusion. It is often said that viscosity increases with silica content because silica polymerizes, but this rule can be misleading. Lavas having the composition of basalt , which is low in silica content, tend to flow easily and form broad flat volcanoes as in the Hawaiian Islands. But some very silica - rich lavas of rhyolite composition can also flow readily. Lavas that are especially viscous are often of andesite composition and intermediate in silica content. Andesite lavas can therefore give rise to explosive volcanoes like the island of Montserrat, West Indies. The viscosity of lava was once ascribed to whether a lava was acidic or basic . These terms are misleading and no longer used. (See acid rock and basic rock ). Rock;-

Constituent of the Earth's crust composed of minerals or materials of organic origin that have consolidated into hard masses as igneous , sedimentary , or metamorphic rocks . Rocks are formed from a combination (or aggregate) of minerals, and the property of a rock will depend on its components. Where deposits of economically valuable minerals occur they are termed ores . As a result of weathering , rock breaks down into very small particles that combine with organic materials from plants and animals to form soil . In geology the term ` rock ´ can also include unconsolidated materials such as sand , mud, clay , and peat . Igneous rock is formed by the cooling and solidification of magma , the molten rock material that originates in the lower part of the Earth's crust, or mantle , where it reaches temperatures as high as 1,000 º C. The rock may form on or below the Earth's surface and is usually crystalline in texture. Larger crystals are more common in rocks such as granite which have cooled slowly within the Earth's crust ; smaller crystals form in rocks such as basalt which have cooled more rapidly on the surface. Because of their acidic composition, igneous rocks such as granite are particularly susceptible to acid rain Sedimentary rocks are formed by the compression of particles deposited by water, wind, or ice. They may be created by the erosion of older rocks, the deposition of organic materials, or they may be formed from chemical precipitates. For example, sandstone is derived from sand particles, limestone from the remains of sea creatures, and gypsum is precipitated from evaporating sea water. Sedimentary rocks are typically deposited in distinct layers or strata and many contain fossils . Metamorphic rocks are formed through the action of high pressure or heat on existing igneous or sedimentary rocks, causing changes to the composition, structure, and texture of the rocks. For example, marble is formed by the effects of heat and pressure on limestone, while granite may be metamorphosed into gneiss , a coarse - grained foliated rock. Rock studies:-

The study of the Earth's crust and its composition fall under a number of interrelated sciences, each with its own specialists. Among these are geologists, who identify and survey rock formations and determine when and how they were formed, petrologists, who identify and classify the rocks themselves, and mineralogists, who study the mineral contents of the rocks. Palaeontologists study the fossil remains of plants and animals found in rocks. Applications of rock studies:Data from these studies and surveys enable scientists to trace the history of the Earth and learn about the kind of life that existed here millions of years ago. The data are also used in locating and mapping

file://C:\Documents and Settings\s h a f i q\Desktop\Geology.htm

3/22/2007

Geology

Page 13 of 14

deposits of fossil fuels such as coal, oil, and natural gas, and valuable mineral - containing ores providing metals such as aluminium, iron, lead, and tin, and radioactive elements such as radium and uranium. These deposits may lie close to the Earth's surface or deep underground, often under oceans. In some regions, entire mountains are composed of deposits of iron or copper ores, while in other regions rocks may contain valuable nonmetallic minerals such as borax and graphite, or precious gems such as diamonds and emeralds. Rock as construction material and building stone:-

In addition to the mining and extraction of fuels, metals, minerals, and gems, rocks provide useful building and construction materials. Rock is mined through quarrying, and cut into blocks or slabs as building stone, or crushed or broken for other uses in construction work. For instance, cement is made from limestone and, in addition to its use as a bonding material, it can be added to crushed stone, sand, and water to produce strong, durable concrete, which has many applications, such as the construction of roads, runways, and dams. Among the most widely used building stones are granite, limestone, sandstone, marble, and slate. Granite provides one of the strongest building stones and is resistant to weather, but its hardness makes it difficult to cut and handle. Limestone is a hard and lasting stone that is easily cut and shaped and is widely used for public buildings. The colour and texture of the stone can vary with location ; for instance, Portland stone from the Jurassic rocks of Dorset is white, even textured and durable, while Bath stone is an oolitic limestone that is honey - coloured and more porous. Sandstone varies in colour and texture ; like limestone, it is relatively easy to quarry and work and is used for similar purposes. Marble is a classic stone, worked by both builders and sculptors. Pure marble is white, streaked with veins of black, grey, green, pink, red, and yellow. Slate is fine - grained rock that can be split easily into thin slabs and used as tiles for roofing and flooring. Its colour varies from black to green and red. Rock identification:-

Rocks can often be identified by their location and appearance. For example, sedimentary rocks lie in stratified, or layered, formations and may contain fossils ; many have markings such as old mud cracks or ripple marks caused by waves. Except for volcanic glass, all igneous rocks are solid and crystalline. Some appear dense, with microscopic crystals, and others have larger, easily seen crystals. They occur in volcanic areas, and in intrusive formations that geologists call batholiths, laccoliths, sills, dikes, and stocks. Many metamorphic rocks have characteristic bands, and are easily split into sheets or slabs. Rock formations and strata are often apparent in the cliffs that line a seashore, or where rivers have gouged out deep channels to form gorges and canyons. They are also revealed when roads are cut through hillsides, or by excavations for quarrying and mining. Rock and fossil collecting has been a popular hobby since the 19th century and such sites can provide a treasure trove of finds for the collector. igneous rock:Rock formed from cooling magma or lava, and solidifying from a molten state. Igneous rocks are largely composed of silica (SiO 2 ) and they are classified according to their crystal size, texture, method of formation, or chemical composition, for example by the proportions of light and dark minerals. Igneous rocks that crystallize from magma below the Earth's surface are called plutonic or intrusive , depending on the depth of formation. They have large crystals produced by slow cooling ; examples include dolerite and granite . Those extruded at the surface from lava are called extrusive or volcanic . Rapid cooling results in small crystals ; basalt is an example. metamorphic rock:Rock altered in structure and composition by pressure, heat, or chemically active fluids after original

file://C:\Documents and Settings\s h a f i q\Desktop\Geology.htm

3/22/2007

Geology

Page 14 of 14

formation. (If heat is sufficient to melt the original rock, technically it becomes an igneous rock upon cooling.) The term was coined in 1833 by Scottish geologist Charles Lyell (1797 - 1875). The mineral assemblage present in a metamorphic rock depends on the composition of the starting material (which may be sedimentary or igneous) and the temperature and pressure conditions to which it is subjected. There are two main types of metamorphism. Thermal metamorphism , or contact metamorphism, is brought about by the baking of solid rocks in the vicinity of an igneous intrusion (molten rock, or magma, in a crack in the Earth's crust). It is responsible, for example, for the conversion of limestone to marble. Regional metamorphism results from the heat and intense pressures associated with the movements and collision of tectonic plates (see plate tectonics ). It brings about the conversion of shale to slate, for example. sedimentary rock:Rock formed by the accumulation and cementation of deposits that have been laid down by water, wind, ice, or gravity. Sedimentary rocks cover more than two - thirds of the Earth's surface and comprise three major categories : clastic, chemically precipitated, and organic (or biogenic). Clastic sediments are the largest group and are composed of fragments of pre - existing rocks ; they include clays, sands, and gravels. Chemical precipitates include some limestones and evaporated deposits such as gypsum and halite (rock salt). Coal, oil shale, and limestone made of fossil material are examples of organic sedimentary rocks. Most sedimentary rocks show distinct layering (stratification), caused by alterations in composition or by changes in rock type. These strata may become folded or fractured by the movement of the Earth's crust, a process known as deformation . Isostasy:-

The theoretical balance in buoyancy of all parts of the Earth's crust , as though they were floating on a denser layer beneath. There are two theories of the mechanism of isostasy, the Airy hypothesis and the Pratt hypothesis, both of which have validity. In the Airy hypothesis crustal blocks have the same density but different depths : like ice cubes floating in water, higher mountains have deeper roots. In the Pratt hypothesis , crustal blocks have different densities allowing the depth of crustal material to be the same. There appears to be more geological evidence to support the Airy hypothesis of isostasy. During an ice age the weight of the ice sheet pushes that continent into the Earth's mantle ; once the ice has melted, the continent rises again. This accounts for shoreline features being found some way inland in regions that were heavily glaciated during the Pleistocene period

file://C:\Documents and Settings\s h a f i q\Desktop\Geology.htm

3/22/2007