Igneous Rocks and Volcanoes

Volcanoes

Igneous Rocks

Structure

Magma/Lava

• Magma: molten rock that occurs below the surface of the earth
• When magma rises along a fault and pours out onto the surface, it becomes lava.
• melting & crystallization
• The cooling process is a series of chemical reactions.

Intrusive Rocks

Extrusive Rocks

• crystallized from liquid magmas that reached the surface & were vented as lava

• crystallized from magma that did not reach the surface but instead moved upward into cracks & voids deep in the crust

• Volcanoes are hills or mountains that form around a vent of liquid magma; the process, or venting of liquid magma through the crust, is called volcanism or volcanic activity.
• Pyroclasts/pyroclastic debris: pieces of rock blown out of a volcano
• Pyroclastic flows: dense, cloudlike mixtures of hot gas & pyroclastic debris that flow down a volcano's side like an avalanche
• Crater: circular depression at top of volcano caused by explosion erosion of central vent
• Caldera: MUCH larger depression (1-100 km) that forms at the top of a volcano when the summit is destroyed during eruption or when the crater flow collapses into the magma chamber below
• Crater-Lake type calderas - stratovolcanoes - explosive eruption
• Basaltic calderas - shield volcanoes - extraction of lava from magma chambers

Bowen's Reaction Series

Types

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ultramafic

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mafic

Shield Volcanoes

Composite volcanoes/stratovolcanoes

Cinder Cone/Pyroclastic Cone

• broad, cone-shaped, formed from cooled lava flows
• gently dipping, shallow sides
• lavas have low viscosity & spread quickly after eruption
• mafic lava
• A spatter cone forms when gas & lava are ejected through a small vent, resulting in a steep-sided cone that resembles an appendage. Usually develops on a cooling lava flow from a shield volcano.
• effusive eruption: outpouring of lava onto ground
• pahoehoe lava (smooth flows with ropey texture) vs. a'a flows (dense, viscous, slow moving, blocky flows)
• Hawaiian islands (Mauna Loa, Kilauea), Galapagos islands, Iceland, East Africa

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• composed of pyroclastic material (not lavas) ejected from a vent

intermediate

• alternating layers of lava & pyroclastic debris
• steep slopes
• long periods of dormancy (inactivity)
• lava has high amounts of silica & dissolved gas, hence the violent, explosive eruptions (felsic magma)
• lava usually cools and hardens before spreading far due to high viscosity
• located along circum-Pacific belt ("Ring of Fire") and Mediterranean belt
• Mt. St. Helens, Vesuvius, Krakatoa

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felsic

Other Formations

• Thick, viscous magma that doesn't easily flow may form a volcanic dome. They are steep & form near vents, creating a plug that traps gases, builds up pressure, and leads to violent explosions.
• Lava floods/plateau basalts are nonvolcanic lavas that pour from deep cracks/rifts in continental crust.

Basaltic lava

• high percentage of ferromagnesian minerals
• 25-50% silica
• dark green, gray, black

The principal constituent of most granites is:

Types of Extrusive Rocks (Lava)

W) feldspar

X) muscovite

Y) calcite

Z) dolomite

Igneous Rocks & Volcanoes

• Basaltic
• Andesitic
• Rhyolitic

All of the following are basaltic igneous rocks except:

Andesitic

Rhyolitic

Although scientists are unable to obtain samples by drilling, they believe the predominant mineral in the mantle is:

W) gabbro

X) granite

Y) basalt

Z) scoria

• more silica & plagioclase feldspar and less ferromagnesian minerals than basalt
• lighter greenish-gray

• most silicious of the extrusive rocks
• contains over 65% silica in feldspar and quartz and very few ferromagnesian minerals
• tan, pink, or cream color

Dacite

W) iron pyrite

X) potassium feldspar

Y) quartz

Z) olivine

• composition between andesite and rhyolite
• has slightly less potassium feldspar and quartz and slightly more ferromagnesian minerals than rhyolite
• light grayish-green
• difficult to distinguish from rhyolite in the field (and probably not going to show up in a question)

An example of a typical shield volcano is

W) Washington's Mt. Rainier

X) Hawaii's Mauna Loa

Y) Ecuador's Mount Cotopaxi

Z) Japan's Mount Fuji

Which of the following are thought to form the mantle of the Earth

Magma that squeezes into vertical cracks and hardens forms a

W) ultramafic rocks

X) felsic rocks

Y) silicic rocks

Z) mafic rocks

W) dike

X) batholith

Y) rill

Z) volcanic neck

Intrusive Rocks

Extrusive Rocks

• Most extrusive rocks are fine-grained (less than 1 mm. in diameter).
• Lava flow rocks usu. have a chilled margin that is fine grained (aphanitic). Grain size increases toward the center of the flow so that thicker flows have medium- to coarser-grained centers.
• Porphyritic rocks have coarser-grained crystals (phenocrysts) in a matrix/groundmass of finer-grained minerals.

Under the sea

Textures

Flood basalts cool and contract to form columnar structures or jointing, vertical, parallel, six-sided columns. Submarine lava flow cools to form pillow structures, blobs of lava that harden in cold water.

Classifying Rocks & Lavas

• When magma is released, the gas it contains "bubbles" out to create holes called vesicles.
• Pyroclastic material has a variety of fragmental textures: (from smallest to largest) dust/ash, cinders (pea-sized), lapilli (walnut-sized), bombs/blocks
• Blocks are pieces of hardened lava. Bombs are semimolten pieces of lava that solidify while falling.
• A tuff is made of fine-grained pyroclastic material and is named after its primary component, ex. ash tuff, crystal tuff, welded tuff, tuff breccias/agglomerates

Geothermal Gradient

Intrusions are exposed to uplift, mountain-building, and erosion for millions or billions of years. Country rock is the surrounding rock that the magma invades. Contact separates cooled intrusive rock from country rock. Xenoliths are fragments of country rock torn away as magma invades.

Grain size increases from the chill zone to the center.

Intrusive rocks are also classified as mafic, felsic, or intermediate.

• Mafic: Gabbro ~ Basalt
• Intermediate: Diorite ~ Andesite
• Felsic: Granite - Rhyolite
• Ultramafic: Peridotite

2.5 degrees Celsius per 100 meters in upper crust

• Mafic rocks have about 50% silica and HIGH amounts of iron, magnesium, and calcium. They are dark in color, ex. basalt.
• Ultramafic rocks consist almost entirely of ferromagnesian minerals and have no feldspars or quartz. They have less than 45% silica. Ultramafic magma is the least viscous due to its low silica content. The lava is thought to originate from the mantle, ex. komotiite (olivine, pyroxene)
• Felsic (silicic) rocks are rich in silica, potassium, sodium, aluminum, and poor in iron, magnesium, and calcium, ex. dacite, rhyolite. Felsic magmas are the most viscous due to their high silica content.
• Intermediate lava has silicate percentages between mafic and felsic lava and makes up the edges of continents, ex. andesite.