Chapter 8: Earthquakes and Volcanoes Section 2: Volcanoes

A. Volcanoes - cone-shaped hills or mountains formed by magma eruptions

1. When magma flows onto Earth’s surface through a vent, it is called lava.

2. Tephra – bits of rock or solidified lava dropped from the air after an explosive eruption

3. Some volcanoes form where Earth’s plates collide. One plate subducts, or is forced underneath, the other. b. Part of the plate that is forced underneath melts, forming magma chambers.

Avalanches of hot, glowing molten rock that flow on cushions of hot gases down a side of a volcano are called pyroclastic flows. Montserrat, 1996

5. How forceful an eruption is depends on the composition of the magma More silica makes magma thicker and more resistant to flow. High viscosity

b. More iron and magnesium make magma flow more easily.

Water vapor trapped in the magma becomes steam and creates pressure.

6. The type of lava and gases contained in the lava determine the type of volcano that forms

B. Four types of volcanoes:

1. Shield volcanoes – basaltic lava, which flows easily a. Forms a broad volcano with gently sloping sides b. Largest type of volcano Mauna Loa, Hawaii

c. Form where Earth’s plates are separating and magma is forced upward between the plates

2. Cinder cone volcanoes - high gas content in the magma Explosive, but short-lived, eruptions b. Form a small cone of volcanic material from tephra

3. Composite volcanoes – made of alternating layers of lava and tephra Steep-sided mountains Form where Earth’s plates are colliding and being forced underneath each other, or subduction zones. Mt Ranier in Washington

4. Fissure eruptions – magma that is very fluid Oozes from cracks in the Earth’s surface. Magma flows freely across the land, as flood basalts. Most of Earth’s crust beneath the ocean is flood basalts.