Chapter 2 Plate Tectonics
TOPOGRAPHY OF VOLCANIC REGIONS
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
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.
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