Plate Tectonics Questions (Page 3 of 4) in pdf

Plate Tectonics Questions Page 3

Chapter 2 Plate Tectonics

Types of Volcanic Landforms

When low-silica magma erupts, lava tends to flow freely and far. If it

Geo Words

erupts from a single opening (vent) or closely spaced vents, it forms a

broad shield volcano, as shown in Figure 1.

shield volcano: a

broad, gently sloping

volcanic cone with

a flat-dome shape,

usually several tens or

hundreds of square

miles in extent.

Figure 1 Volcanoes such as these are called shield volcanoes because they somewhat

resemble a warrior’s shield. They are formed when low-silica magma erupts.

Figure 2 The

eruption of low-

silica magma along

long, narrow

openings in the

Columbia Plateau

flowed over a

vast area. The

result was a broad

lava plateau that

makes up the cliffs.

Silica-rich magmas are far less fluid. They often stop moving before

they reach the surface. If they do reach the surface, they ooze slowly,

like toothpaste squeezed out of a vertical tube. The viscous lava forms

volcanic domes with steep slopes, as shown in Figure 3. If the volcano’s

vent gets plugged, gases cannot escape and pressure builds up. The

pressure can be released in a violent eruption that blasts pieces of lava

and rock (pyroclastics) into the atmosphere.

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Plate Tectonics Questions Page 3