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Chapter 18- Volcanic Activity

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I. Magma

A. How magma forms

1. All volcanoes are fueled by magma deep beneath Earth’s surface.

 2. Magma forms when temperatures are high enough to melt the rocks involved.

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3. Pressure is one factor that determines whether rocks will melt to form magma.

 4. The presence of water also influences whether a rock will melt

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B. Types of magma  

1. There are three major igneous rock types- basalt, andesite and granite. These rocks form three types of magma.

 2. Basaltic magma, andestic magma and rhyolitic magma.

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Basalt

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Andesite

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Granite

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C. Viscosity

1. The viscosity of magma and of its counterpart, lava depends on both temperature and composition.

2. The hotter the magma or lava, the lower the viscosity.

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II. Intrusive Activity

A. Plutons 

1. Intrusive igneous rock bodies are called plutons, can be exposed at Earth’s surface as a result of uplift and erosion are classified based on their size, shape and relationship to surrounding rocks.

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2. The largest plutons are called batholiths.

3. Laccolith is a mushroom- shaped pluton with a round top and flat bottom.

 4. A sill is a pluton that forms when magma intrudes parallel to layers of rock.

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Batholiths

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Laccolith

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Sill

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5. A dike is a pluton that cuts across preexisting rocks.

 6. Many plutons are formed as the result of mountain-building processes.

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B. Tectonics

1. The Earth’s crust and rigid upper mantle are broken into enormous slabs called plates.

2. Tectonic plates move in different directions and at different rates over Earth’s surface.

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THE BAD LANDS

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More Bad Lands…..

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Needles Point

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Old Faithful

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Morning Glory

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Unique Land forms

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More Unique Land forms

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Acid Boiling lakes

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Yellowstone

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More Yellowstone….

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Continental Divide

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Continental Divide

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Grand Teton Mountains

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III. Volcanoes

A. Anatomy of a volcano 

1. At the top of the volcano, around the vent is a bowl shaped depression called a crater.

 2. Larger depression called caldera can form when the side of the volcano collapses into the magma chamber.

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Crater

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Caldera

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B. Types of volcanoes

1. A shield volcano is a mountain with broad, gently sloping sides and a nearly circular base.

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2. Cinder – Cone volcanoes forms when material ejected high into the air falls back to Earth and piles up around the vent.

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3. Composite Volcano- forms when layers of volcanic fragments alternate with lava.

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C. Volcanic material

1. Rock fragments thrown into the air during a volcanic eruption are called tephra.

 2. Some tephra cause tremendous damage and can kill thousands of people.

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D. Where volcanoes occur?

1. The distribution of volcanoes on Earth’s surface is not random. Most volcanoes form at plate boundaries.

 2. Most volcanoes form at plate boundaries.

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3. About 80% of all volcanoes are found along convergent boundaries and 15% are found along divergent boundaries.

 4. Only about 5% of volcanoes occur far from plate boundaries.

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5. The volcanoes associated with convergent plate boundaries form two major belts- the Circum- Pacific belt (Pacific ring of fire)

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6. Mediterranean belt.  

7. Some volcanoes are located far from plate boundaries, these volcanoes form as the result of hot spots, which are unusually hot regions of earth’s mantle where high-temperature plumes of mantle material rise toward the surface.

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THE END