Ch#19 Plate Techtonics

CH# 19 TECHTONICS Tectonics is the study of the origin and arrangement of the broad structural features of the earth surface including folds, faults, and mountain belts, continents and earthquake belts. CONTINENTAL DRIFT: Continental drift is the idea that continents move freely over the earth surface, changing their position relative to one another. SEA FLOOR SPREADING: Sea floor spreading is the hypothesis that the sea floor forms at the crest of mid oceanic ridge, then move horizontally away from the ridge crest towards an oceanic trench. EARLY CASE FOR CONTINENTAL DRIFT: Continents can be made to fit together a German Meteorologist; Alfred Wegener noted that South America, Africa, India and Australia had almost the identical late Paleozoic rocks and fossils. He resembled that all continents to form a giant supercontinent Pangaea. Pangaea initially separated into two parts Laurasia and Gondwanaland. Laurasia was northern supercontinent containing North America and Eurasia. Gondwanaland was southern supercontinent, composed all present-day India. Wegner determined the position of North and South poles for each geologic period. He found that ancient poles were in different positions then the present poles. This apparent movement of poles is called polar wandering. PALEOMAGNETISM and REVIVAL OF CONTINENTAL DRIFT: The study of ancient magnetic field is called Paleomagnetism. Many rocks record the strength and direction of the earth’s magnetic field at that time when rock formed, e.g. Magnetite in a cooling basaltic lava flow acts like a tiny compass needle preserving a record of earth’s magnetic field when lava cools. Iron stained sedimentary rocks such as red sand-stone can also record earth magnetism. The inclination of the magnetic alignment preserved in a magnetic mineral in the lava flows can be used to determine the distance from a flow to the pole at the time that flow formed. Old pole position can be determined from the magnetism of old rocks. The magnetic alignment preserved in magnetic minerals points to the poles, and the dip of the alignment tells how far away from the pole was. SEA FLOOR SPREADING: Harry Hess in 1962 proposed that the sea floor might be moving too. This proposal sharply contrasted with the earlier ideas of Wegner, who thought that the ocean floor remained stationary as the continents plowed through it. This proposal called sea floor spreading According to this concept the sea floor is moving like a conveyor belt away from the crest of mid oceanic ridge, down from the flanks of ridges, and across the deep ocean basins, to disappear finally by plunging beneath a continent or island arc. The sliding of the sea floor beneath a continent or island is called subduction. The sea floor move 1 to 6 cm/per year.

HESS DRIVING FORCE: The sea floor spreading is driven by deep mantle convection. Convection is the circulation pattern driven by the rising of hot material and or sinking of cold materials. THE MID OCEANIC RIDGE: If convection drives sea-floor spreading, then hot rocks in mantle must be rising under the mid-oceanic ridge. As hot rocks continues to rise beneath the ridge crest, the circulation pattern splits diverges near the surface. Mantle rocks move away horizontally from the ridge crest on each side of the ridge. This movement creates tension at the ridge crest, cracking open the oceanic crust to form the rift valley and it create shallow focus earthquakes. OCEANIC TRENCHES: As the mantle rocks move away from the ridge crest, it carries sea floor with it. As hot rocks moves sideways, it cool and become denser, sinking beneath the ocean surface. Hess thought that it would become cold and denser enough to sink back into the mantle. This downward plunge of cold rocks accounts for existence of oceanic trenches as well as their low heat flow values. AGE OF THE SEA FLOOR: New sea floor is continuously being formed by basalt eruption at the ridge crest. This basalt is then carried sideways by convection and is sub-ducted into the mantle at an oceanic trench. Thus old sea floor is continuously being destroyed at the trenches, while new sea-floor is formed at ridge crest. So youngest sea floor should be at the ridge crest, with the age of sea floor becoming progressively older towards a trench.

MARINE MAGNETIC ANOMALIES: (THE VINE-MATTHEWS HYPOTHESIS) Two British geologists, Fred Vine and Drummond, recognized that the pattern of magnetic anomalies on one side of the mid-oceanic ridge was a mirror image of the pattern on the other side. They noticed that the same pattern of magnetic anomalies exists over different parts of mid-oceanic ridge. The most important observation made was that the pattern of magnetic anomalies at the sea matches the pattern of magnetic reversals already known from study of lava flow on the continents. 1) DIVERGENT PLATE BOUNDARIES: These are the boundaries at which plates move away from each other. This can occur in the middle of the oceans or middle of a continent. As a result a new ocean basin is opened or created. When a supper continent Pangaea breaks up, the divergent boundary can be found in the middle of the continent. This is marked by rifting, basaltic volcanism and uplift. During rifting the continental crust is stretched and thinned. This produces shallow focus earthquakes on normal faults. These faults act as pathways for basaltic magma, which rise from mantle and to erupt on the surface.

TRANSFORM BOUNDRIES: These are boundaries where one plate slides horizontally past another plate. Plate motion can be taken on a single fault or on a group of parallel faults. These boundaries are marked by shallow focus earthquakes in a narrow zone for single fault, or in a broad zone for a group of parallel faults. 2) CONVERGENT PLATE BOUNDRIES: These are the boundaries at which two plates move towards each other. In this density difference play a major role. The dense oceanic plate sub-ducts under the continental plate. i.

OCEAN-OCEAN CONVERGENCE;

When two plates capped by sea floor converge, one plate sub-ducts under the other. The subducting plate bends downwards and form outer wall of oceanic trench. ii.

OCEAN-CONTINENT CONVERGENCE;

When a plate capped by oceanic crust is sub-ducted under the continental lithosphere, magnetic arc are formed in continents and trenches are formed in oceans. iii.

CONTINENT-CONTINENT CONVERGENCE;

When two continents approach and collide, mountain ranges are formed. e.g. Himalayas are formed in this way.