Download Questions For Review KEY

GEOLOGY 12
CHAPTER 4 VOLCANOES
QUESTIONS FOR REVIEW
3.
4.
Describe the nature of fissure eruptions and give an example.
•
Fissure eruptions are those in which lava emerges from a CRACK or FISSURE
rather than from a pipe or vent.
•
The seafloor spreading ridges are examples; so are the Columbia River flood
basalts.
What is a shield volcano, and from what kind of lava are shield volcanoes usually
built?
•
5.
•
A shield volcano is broad, flat, and gently sloping. The shield volcano is built of
very fluid (typically basaltic) lava.
A volcanic dome is more compact and steep-sided dome. The volcanic dome
is made of more viscous, silica-rich magmas, andesitic to rhyolitic in
composition.
What causes the eruption of pyroclastics, and with what type of lava are they more
often associated?
•
•
7.
A shield volcano is a low, broad, gently sloping one, typically built of many thin
sheets of very fluid BASALTIC LAVA.
Compare and contrast a volcanic dome with a shield volcano. What causes the
differences in form between these two kinds of volcanic structures?
•
6.
ANSWERS
Pyroclastics are erupted when trapped gases in the magma build up sufficient
pressure to cause explosive eruption.
They are more often associated with the intermediate-to-silicic magmas, which
are more viscous and, therefore, trap gases more readily.
Lava is usually more of a hazard to property than to lives. Why? Describe one
strategy for protecting property from an advancing lava flow.
•
•
Most lavas flow slowly enough to be outrun; moreover, their flow paths are
predictable (downhill).
An advancing lava flow may be slowed or stopped by spraying or pouring water
on it to chill and solidify it, or by constructing barriers to block or divert the flow.
8.
On what kind of a volcano is a volcanic mudflow most likely to develop? Name a
recent example.
•
•
9.
•
•
The emission of a nuee is not usually immediately preceded by any special
precursors. Certain volcanoes, however, have a history of eruption of nuees
ardentes. A wise precaution may be to evacuate in the early stages of an
eruptive phase of such a volcano.
Water seeping into the ground close to hot magma may be heated to steam,
which causes an explosion.
A classic example is Krakatoa.
Volcanic eruptions may alter climate. How?
•
12.
A nuee ardente is a hot, glowing cloud of gas and ash that is so dense that it
flows downhill at high speeds.
What causes a phreatic explosion? Cite an example.
•
11.
Mount St. Helens is an example.
What is a nuee ardente? What strategy can be used to minimize the loss of life
from eruption of nuees ardentes?
•
10.
These mudflows (LAHARS) develop on snow-capped volcanoes that erupt hot
ash and cinders, which melt some of the snow.
Volcanoes may emit fine ash and sulfurous gases that remain in the
atmosphere for some time, blocking sunlight and causing temporary global
cooling.
Describe two kinds of precursors used in predicting volcanic eruptions.
(Various answers are possible.)
•
An INCREASE IN SEISMIC ACTIVITY may be caused by stresses from the
progress of magma rising in the crust.
•
There may be CHANGES IN THE TYPE OF SEISMICITY.
•
The GROUND MAY TILT OR BULGE as a subsurface magma chamber fills.
•
If magma is quite close to the surface WARM AREAS OF GROUND may be
detected.
•
There may be CHANGES IN THE COMPOSITION OF GASES EMITTED.
Geology 12
Chapter 4
Questions For Review
Page 2