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Chapter 2 Plate Tectonics

Digging Deeper

TOPOGRAPHY OF VOLCANIC REGIONS

Topographic Maps

In the Investigate, you made a topographic map of a model volcano.

You also compared this to the topographic map of an actual volcano.

Topographic maps have contour lines. These are curves that connect

all points at the same elevation. The contour interval is the difference

in elevation between contour lines that are next to one another. A

topographic map shows how steep or gentle a slope is. It also shows

the elevation and shape of the land. Relief is the difference in elevation

between the highest and lowest points on the map.

The following are some important points to consider when interpreting

topographic maps.

•

Contour lines never cross. (However, two or more can run together,

where there is a vertical cliff).

•

The closer together the contour lines, the steeper the slope.

•

Contour lines for closed depressions, such as a volcanic crater, are

marked with “tick marks.” (These are short lines at right angles to

the contour line.) The marks point downward into the depression.

•

On most topographic maps, every fifth contour line is darker. Its

elevation is always marked.

Magma Composition

Volcanoes are often pictured as cone-shaped mountains. However,

volcanoes come in many shapes and sizes. Ice, wind, and rain can

change the shape of a volcano. These changes can take place between

eruptions or after the volcano becomes dormant. A large eruption or

giant landslide can remove the top or side of a volcano. The chemical

composition of magma can have an even greater effect on the shape

the volcano takes as it forms.

Magma is melted rock (a liquid). Magma may also contain dissolved

gases. The most abundant chemical elements in magma are silicon and

oxygen. As the magma cools, minerals form. Silicon and oxygen are the

building blocks of the most common minerals. They are called silicate

minerals and form from magmas. One silicon atom and four oxygen

EarthComm

Plate Tectonics Questions