Volcanoes, Magma & Plutons

Volcanoes, Magma & Plutons Earth Science and the Environment (4th ed) Thompson & Turk

8.1 Magma ► Creation

of magma – rocks melt more easily with:  Increasing temperature  Decreasing pressure  Addition of water

8.1 Magma ► Environments

of magma formation

 At the axis of Mid-Ocean Ridges ►

e.g. East Pacific Rise, Mid-Atlantic Ridge

 Above Mantle plumes ►

e.g. Hawaii, Iceland, Yellowstone

 Above Subduction zones ►

e.g. Andes Mountains, Marianas Islands

At a mid-ocean ridge the crust and lithosphere are thin, which means that hot asthenosphere is nearer to the surface, which is therefore at a lower pressure than normal. The over-lying crust is fractured, which allows water to seep down and come in contact A spreading center, therefore, has all with the hot the conditions necessary to melt rock below. rock: High heat, low pressure and

The lithosphere above a mantle plume (hot spot) is subjected to high heat, which cause it to bulge upwards and thin. This dynamic force lowers the pressure on the underlying asthenosphere and induces melting.

The crust atop the descending lithosphere in a subduction zone is heated and releases water as it sinks, which eventually leads to

Although most (about 80%) of the volcanism around the world is submarine and occurs along the mid-ocean ridge system, volcanoes forming above subduction zones account for most of the subaerial volcanism. The subduction zones that lie along the margins of the Pacific Ocean produce about 75% of the subaerial volcanoes, which are collectively referred to as the “Ring of Fire”.

8.4 Magma behavior ► Magma

 

rises

Cooling solidifies Lower pressure keeps it liquid

► Magma

composition differences

 Granitic – 70% silica, up to 10% water  Basaltic – 50% silica, 1-2% water

8.4 Magma behavior ► Effects

of silica

 High silica content increases magma viscosity ► Effects

of water

 Increased water lowers solidification temperature (dry magma solidifies quicker)  Rising magma loses water, hardens quicker

Intrusive Structures ► Pluton

– a bulbous mass of magma that intrudes into and solidifies within the crust. ► Intrusive structures:  Batholith – a collection of plutons that form a body the size of a mountain range, exposed by erosion, typically bigger than 100 km2  Stock – an irregularly shaped intrusive body that is similar in size to a pluton (1 to 10 km2)  Dike – a discordant tabular intrusive body  Sill – a concordant tabular intrusive body

Batholiths form above subduction zones as plutons of magma collect and cool within the crust beneath a volcanic arc. The plutonic rock of a batholith is exposed after subduction and volcanism cease and the overlying volcanic rocks have eroded away. A batholith is presently

Plutons of the Sierra Nevada batholith are exposed by glacial erosion here in Yosemite Valley. The bright granite face of the El Capitan pluton is over

A pluton or stock may supply magma to a variety of smaller intrusive structures such as dikes and sills, as well as being the reservoir for magma that erupts at the surface to form a volcano or lava flow.

Dike

Dikes and Sills are tabular intrusive bodies, which are easily recognized by their relationship with surrounding rocks. Generally dikes form as walls, and sills as layers, such that the former cuts across layers and the latter lies between layers of sedimentary rock.

8.6 Volcanoes ► Volcanic

Vents are places where magma and

related fluids erupt onto the surface

 Vents typically occur as tubes and fissures ► Volcanoes form where repeated eruptions occur from a central vent.

 A volcano is a conical pile of erupted materials

A volcanic crater is a circular depression formed around a central vent, usually at the peak of a volcano. In the foreground below are two small craters sitting within a larger crater, which itself sits within an even larger crater whose walls are the background.

8.6 Volcanoes ► Effusive

(Calm) Eruptions Produce Lava

Lava – magma at the surface, flowing or solid Basalt – the most common type of lava has two varieties:  Pahoehoe – ropy surface, easy flowing lava  Aa – rubbly surface, viscous (stiff) lava

► Explosive

(Violent) Eruptions Produce Pyroclastics Pyroclastics – fragmental and glassy materials   

Volcanic Ash Cinders Volcanic Breccia (Dominated by Rock Fragments)

8.6 Volcanoes ► Fissure

eruptions – low viscosity lava exuding from cracks

► Flood

basalt – very large, rapid, fissure eruption

► Lava

(basalt) plateau – many cubic kilometers sized event

The Columbia Plateau The plateau formed around 15 million years ago by repeated eruptions of flood basalt that covers an area of about 200,000 Km2 and in places is up to to 3 Km thick.

Individual flow layers of the Columbia River Basalt are between 15 and 100 meters thick. Good exposures are found where the Columbia River and its tributaries have eroded deep canyons through this jointed rock. FLOOD BASALTS OF THE COLUMBIA PLATEAU

COLUMNAR JOINTING IS COMMON IN LAVA FLOWS

The Devil’s Post Pile, a formation located in the Sierra Nevada, dramatically illustrates the phenomenon of columnar jointing, which forms as a lava flow cools and shrinks.

A view from above reveals their hexagonal shape

8.6 Volcanoes ► Shield

  

volcanoes

Built by effusive eruptions of basalt Gently sloped The largest of volcanoes

► e.g.

Mauna Loa, Mauna Kea, Iceland

Shield volcanoes form the largest and tallest volcanoes on earth, despite their gentle slopes. They form from copious amounts of basalt lava erupted from a summit crater as well as from fissures around their flanks. Two common types of basalt lava (Pahoehoe and Aa) erupt from Hot Spot volcanoes like this one (Mt. Skjoldbreidier, Iceland). The two are differentiated by their relative viscosities and surface textures.

Pahoehoe is thin, gas-rich basalt lava. It advances quickly as a narrow stream, often within tubes, and has a smooth, ropey surface.

Aa is stiff, gas-poor basalt lava. It advances slowly as a thick sheet with a steep front and has a rough, blocky surface.

8.6 Volcanoes ► Cinder

cones

 Formed of pyroclastic fragments (ash & cinders)  Often steep, small and symmetrical  May from abruptly (hours or days) ► Eruptions

gases

are driven by escaping

This cinder cone in Bolivia has a symmetrical shape and a well formed crater (circular depression) at its peak.

8.6 Volcanoes ► Composite

cones

aka: stratovolcanoes Layers of lava and pyroclastics accumulate from both effusive and explosive eruptions  Relatively steep-sided  Associated with subduction zones  

Mount Rainier is a classic example of a composite volcano. Its presence is a constant reminder to the residents of Seattle of their precarious location above a subduction zone where tsunamis, earthquakes, volcanic eruptions and mudslides have occurred in the past and are surely to occur at anytime in the near future.

Ash flows and Calderas ► Ash

Flow – a cloud of pyroclastics that flows along and buries the surface  Nueè ardente – “glowing cloud”  Hot, Fast (200 km/h), Far-reaching (100 km)

► Caldera

– collapsed roof of magma chamber

 Large (10+ km) circular depression, steep sides  Associated with catastrophic eruptions of Ash

CALDERA: The sequence of events leading to the formation of a caldera involves creation of “ring” fractures in the crust that lies above a rising pluton of magma. The circular depression of the caldera is formed during the eruption as magma is ejected through the fractures and the unsupported crust sinks. This catastrophic eruption is driven by escaping gases and produces a huge

Crater Lake in southern Oregon fills a caldera that formed about 7000 years ago by the eruption of Mt. Mazama. The caldera is about 10 km across.

8.7 Volcanic explosions: ► Yellowstone

– 3 calderas

 Last eruptions (1.9 mya) – 2500 km3 of pyroclastic materials (in 1980 Mt St Helens erupted only 1 km3)  (0.6 mya) – 1000 km3 of ash & debris ► Long

Valley Caldera (Bishop Tuff ~ 0.8 mya)

 170x larger than 1980 Mt St Helens eruption  Hot Springs and CO2 releases denote continued magma activity today

Fig. 8.30, p.199

8.7 Volcanic explosions: ► Mt

Vesuvius – a composite volcano near Naples, Italy  Buried the cities of Pompeii & Herculaneum during its 79 A.D. eruption  5-8 meter thick ash flow deposits  Intermittent activity in early – mid 1900s  Magma still underlies Vesuvius

Mount Vesuvius erupted in 79 AD

Excavations at Pompeii revealed casts of humans buried in the ash flow deposits

Plateaus and Calderas are not volcanoes. But they do represent end members of the spectrum of eruption styles from most gentle (effusive) to most violent (explosive), respectively. Although the magma that erupts during formation of a caldera can be called “granitic” it is more correct to refer to the erupted material as Rhyolite. The true volcanoes are accumulations of materials erupted from a central vent. From largest to smallest they are the shield volcano, composite volcano, and cinder cone.