Ch. 18 – Volcanic Activity

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. • Plutons are classified by their size, shape, and relationship to surrounding rocks. • 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. • Depression that forms when the top or side of a volcano collapses into the magma chamber. • Ex. Crater Lake (pg. 481) • Landscape features (exterior) include: vents, craters, and calderas.

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. • Forms as tephra ejected high into the air fall back to the Earth and pile around the vent.

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 • Made from layers of tephra and lava. • Largest of the volcanoes. • 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. Volcanoes can occur in convergent boundaries (come together) and divergent boundaries (spread apart). • Fig. 18-16 (pg. 485) • Mainly convergent boundaries. • Hot spots are regions away from plate boundaries were volcanoes form. • Hawaiian Islands have several hot spot volcanoes. • Volcanism that occurs under water along mid-ocean ridges are rift volcanism • Volcanoes are proof that the Earth is dynamic planet.