Volcanoes and Volcanism - The Changing Earth: Exploring Geology and Evolution - page 113

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Figure 5.6 Columnar Jointing Columnar jointing is seen mostly in mafi c lava fl ows and related
intrusive rocks.

b Columnar joints in a basalt lava fl ow at Devil's Postpile National
Monument in California. The rubble in the foreground is collapsed columns.

c Surface view of the columns from (b). The straight lines and polish resulted
from abrasion by a glacier that moved over this surface.

a As lava cools and contracts, three-pronged cracks form
that grow and intersect to form four- to seven-sided columns,
most of which are six-sided.

of a solidifi ed crust of a lava fl ow. Because of their size, lapilli,
bombs, and blocks are confi ned to the immediate area of an
eruption.

their danger and obvious connection to Earth's interior, they
have been held in awe by many cultures.
In Hawaiian legends, the volcano goddess Pele resides in
the crater of Kilauea on Hawaii. During one of her frequent
rages, Pele causes earthquakes and lava flows, and she may
hurl fl aming boulders at those who offend her. Native Ameri-
cans in the Pacifi c Northwest tell of a titanic battle between the
volcano gods Skel and Llao to account for huge eruptions that
took place about 7000 years ago in Oregon and California.
Most volcanoes have a circular depression known
as a crater at their summit, or on their flanks, that forms
by explosions or collapse. Craters are generally less than
1 km across, whereas much larger rimmed depressions on

WHAT ARE THE TYPES
OF VOLCANOES?
Simply put, a volcano is a hill or mountain that forms
around a vent where lava, pyroclastic materials, and gases
erupt. Although volcanoes vary in size and shape, all have
a conduit or conduits leading to a magma chamber beneath
the surface. Vulcan, the Roman deity of fi re, was the inspira-
tion for calling these mountains volcanoes, and because of