chapter 18 volcanic activity l.
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Chapter 18- Volcanic Activity. 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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i magma
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.

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

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.

c viscosity
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.

ii intrusive activity
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.

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.

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

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


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.

iii volcanoes
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.

b types of volcanoes
B. Types of volcanoes

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

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

d where volcanoes occur
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.

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.

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