1. Ch. 18 – Volcanic Activity
Magma is a mixture of molten rock, suspended mineral grains, and dissolved gases deep beneath Earth’s surface.
These rocks start to melt a temperatures between 800°C and 1200°C.
This occurs and the base of the lithosphere and the asthenosphere.
The pressure and amount of water in the rocks pores also affect the temperature a which magma forms.

2. Types of Magma 1. Basaltic Magma – made mostly of basalt.
Mostly found in the Hawaiian Islands
2. Andesitic Magma – made mostly of andesite.
Mt. St. Helens and Tamobra in Indonesia
3. Rhyolitic Magma – made mostly of granite.
Dormant volcanoes in Yellowstone National Park.
Active Volcanoes  Pg. 473 (fig. 18-2)

3. Magma Composition TABLE 18-1 (Pg. 474)
Viscosity – the resistance to flow
The hotter the magma or lave the lower the viscosity. Why?
It moves and flows easier when it’s hot.
Rhyolitic magma has the highest viscosity and highest explosiveness.
Found in continental crust
Basaltic magma has the lowest viscosity and lowest explosiveness.
Found in both oceanic and continental crust.

4. Intrusive Activity
Magma is less dense than the rock that surrounds it, so it will eventually rise the Earth’s surface.
As the magma cools forms under the Earth’s surface plutons can form. These are igneous rock bodies.
The largest plutons are called batholiths. They cover over 100 km2 and take millions of years to form.
Most batholiths in N. America are composed mostly of granite.
Pg. 477 – Fig. 18-5

5. Stocks are smaller in size and irregular shaped plutons.
Laccoliths are smaller yet and have mushroom-shaped plutons with a round top and flat bottom.
Sills are plutons that form when magma intrudes parallel to layers of rock.
Dikes are plutons that cut across preexisting rocks.
Most plutons form from mountain building processes.
When the continental crust is forced down into the lower mantle it melts and eventually cools as batholiths.

6. Volcanoes What are some characteristics of a volcano?
A vent is the area in a volcano were lava can flow to the surface of a volcano.
The crater is the bowl-shaped depression near the top of the volcano.
Large craters are called calderas.
Ex. Crater Lake (pg. 481)

7. Types of Volcanoes
Shield Volcanoes are mountains with broad gentle slopping sides and a nearly circular base.
Forms from several layers of basaltic lava
They have low viscosity and low explosions.
Cinder-Cone Volcanoes form when material is ejected high into the air and falls back to the Earth and piles up around the vent.
Steep slopes & more explosiveness than shield volcanoes.

8. Composite Volcanoes form when layers of volcanic fragments alternate with lava.
Larger than cinder-cone volcanoes.
Violently explosive in nature.
Mt. St. Helens and Mt. Rainer
Tephra are rock fragments that are thrown into the air during the eruption.
Dust is the smallest fragments
Ash are the larger fragments, but still less than 2 mm in size.
Pyroclastic flow is rapidly moving volcanic material and may contain hot, poisonous gases.

9. Hot spots are regions away from plate boundaries were volcanoes form.
Volcanoes can occur in convergent boundaries (come together) and divergent boundaries (spread apart).
Fig (pg. 485)
Hot spots are regions away from plate boundaries were volcanoes form.
Hawaiian Islands have several hot spot volcanoes.
Volcanoes are proof that the Earth is dynamic planet.