Name: __________________
World Book 2007 Study Guide
Define Volcano:
Volcanic eruptions result from magma (molten rock below the ground). Magma
usually forms ___________ miles (50 to 200 kilometers) beneath Earth's surface. It
rises because it is less __________ than the surrounding rock. Rising magma can collect
below or inside a volcano in a region called a magma ________. As the magma
accumulates, the pressure inside the chamber increases. When the pressure becomes
too great, the chamber breaks open, and magma rises in the volcano. If magma reaches
the surface, an ____________ occurs. The hole through which the magma erupts is
Volcanic eruptions
Volcanic eruptions differ in their violence and in the materials they produce. Some
eruptions involve calm outpourings of lava. Other eruptions produce powerful explosions
and large volumes of rock, ash, and gas.
Products of eruption. During an eruption, a variety of materials can come from a
volcanic vent. These include __________, __________s (rock fragments), and
__________.
Lava is magma that flows onto Earth's surface. As lava spreads from a vent, parts of
it begin to cool and harden. The resulting stream of lava and rock is called a
___________. Lava flows vary greatly in appearance depending on their viscosity,
temperature, and rate of advance.
Fluid lava flows spread easily from the vent. Two common types of fluid lava flows are
pahoehoe (pronounced PAH hoy hoy or pah HOY hoy) and aa (pronounced ah AH or ah
ah). _________ flows have smooth, glassy surfaces and wavy, ropelike ridges. They
form when hot, fluid lava advances relatively slowly. __________ flows have rough,
broken surfaces. They form when less fluid lava advances rapidly. Pahoehoe and aa are
Hawaiian terms adopted by most volcanologists.
Highly viscous lavas cannot flow easily. They pile up around the vent to form thick
mounds called lava __________ or short, stubby flows with rugged surfaces. These
domes and flows advance extremely slowly.
Pyroclasts, also called pyroclastics, form when __________ of magma are thrown into
the air by expanding gas. More explosive eruptions tend to produce finer pyroclasts.
Pyroclasts that settle to the ground can cement together to form a rock called tuff.
Volcanologists also classify pyroclasts by texture. __________, a lightweight
pyroclast, contains many tiny cavities left behind by gas bubbles in the magma. The
cavities trap air, enabling some pumice to float on water. Scoria (cinder), another
pyroclast, also has many tiny cavities, but it does not float on water. Pumice and scoria
come from vigorous eruptions that hurl magma fragments high into the air. They solidify
before landing, often forming a loose pile around the vent called a scoria _________ or
pumice __________.
The violence of an eruption depends on the amount of _________ dissolved in the
magma and the magma's __________. Magmas rich in gas develop many bubbles as
they rise to the vent. The bubbles increase the pressure in the vent, causing a more
explosive eruption. Viscous magmas resist the expansion of bubbles, leading to a
buildup of pressure in the magma. When the pressure of the bubbles finally overcomes
the magma's viscosity, an explosive eruption occurs. In more fluid magmas, the bubbles
expand without building up excess pressure. The resulting eruptions are relatively mild.
When external water, such as groundwater or sea water, mixes with magma, the
water rapidly turns to __________, expanding in the process. This increases the
violence of an eruption. Some volcanologists call eruptions involving external water
hydromagmatic eruptions.
As gases and pyroclasts erupt from a volcano, they draw in and heat some of the
surrounding air. The heated air, gases, and pyroclasts form an eruption _________. If
an eruption draws in enough air and heats it sufficiently, the eruption column becomes
lighter than the surrounding air. As a result, the column floats upward in a process
called __________ __________. Convective rise can carry erupted gas and ash high
into the atmosphere. Some eruptions do not draw in enough air or heat it sufficiently to
produce convective rise. In these eruptions, erupted materials remain closer to the
ground.
The dangers of volcanoes
Volcanoes can endanger people, wildlife, and property. Volcanic disasters are much
more difficult to avoid once an eruption begins. Instead, volcanologists and disaster
planners strive to identify and evacuate dangerous areas before eruptions occur. Define
each of the following terms and explain why each is dangerous.
(1) lava flows
(2) pyroclastic hazards
(3) lahars
(4) dangerous gases
(5) avalanches and landslides
(6) tsunamis.
Where volcanoes form
Volcanoes form above regions where __________ is produced in the mantle, the
rocky layer beneath Earth's crust. The theory of __________ __________ helps explain
why volcanoes form where they do. According to the theory, Earth's outer shell consists
of rigid pieces called plates that slowly move against one another. Nearly all volcanoes
form along the edges of plates at __________ zones and __________ boundaries.
Some volcanoes appear above locations called __________ that can be far from plate
boundaries.
Define Subduction zones:
Bands of subduction zone volcanoes often form along the edges of continents where
an __________ plate subducts beneath a continental plate. These include the volcanoes
of the Andes Mountains in South America and the Cascade Range in North America.
Other bands of volcanoes occur where two oceanic plates meet and one __________
beneath the other. These include the volcanoes of Japan and Indonesia. Subduction
zones border most of the __________ Ocean, creating a region of volcano and
earthquake activity called the __________ of __________.
Define Divergent boundaries:
Divergent boundaries usually produce fluid magmas that contain few gases.
Accordingly, eruptions at divergent boundaries tend to be less __________ than
eruptions at subduction zones. Most divergent boundaries involve two oceanic plates.
For this reason, most eruptions at divergent boundaries occur ___________.
Define Hot spots:
Types of volcanoes
Define each of these types of volcanoes by telling how each forms, where each is
found, and an example of a famous volcano for each type.
(1) shield volcanoes
(2) stratovolcanoes
(3) silicic caldera complexes
(4) monogenetic fields
(5) mid-ocean ridges
(6) flood basalts.
Studying volcanoes
The study of volcanoes offers many benefits. Erupting magma can carry material from
deep underground to the surface, providing scientists with samples from Earth's interior.
Hardened lava and ash deposits preserve evidence of major changes in Earth's history.
Studying ancient volcanoes also helps scientists find new deposits of ores. The fluids
moving through volcanoes can concentrate valuable metals in deposits called veins.
Perhaps most importantly, scientists study volcanoes to learn how to predict
eruptions. This information can help reduce the damage and loss of life eruptions cause.
Predicting eruptions. To determine whether or when a particular volcano will erupt,
volcanologists monitor __________ activity (earthquakes) near the volcano. They also
watch for changes in the volcano's ___________ caused by pressure from magma
below. They analyze the types and amounts of __________ coming from vents.
__________ may signal that an eruption is coming.
Most volcanologists consider any volcano that has erupted in the last __________
years or so to be active. Some of them use the term __________ to describe an active
volcano that is not currently erupting or showing signs of a coming eruption.
Volcanologists label a volcano __________ if there is strong evidence it will never erupt
again.
Describing eruptions. Volcanologists sometimes rate the explosive power of an
eruption using a scale called the Volcanic Explosivity Index (VEI). The index ranks
eruptions according to the volume of magma erupted, the amount of energy released,
and the height of the eruption cloud. Most eruptions have VEI ratings from 0, for
__________, to 8, for extremely __________. Each number on the index represents a
tenfold increase in explosive power or the volume of material erupted. For example, a
VEI rating of 5 represents 10 times more power or eruption volume than a rating of 4.
Another common method for describing eruptions is based on how they produce
__________. In this system, both Plinian and Hawaiian eruptions produce pyroclasts
almost continuously. __________ eruptions produce convective rise that can carry huge
plumes of fine pyroclasts into the atmosphere. Hawaiian eruptions do not cause
convective rise. Their pyroclasts are larger and stay closer to the ground, often landing
while still molten. Vulcanian and Strombolian eruptions produce pyroclasts in bursts
separated by periods of relative calm. __________ eruptions produce convective rise,
but __________ eruptions do not. Some volcanologists use classification schemes that
have additional categories of eruptions.
Volcanoes in the solar system
In addition to Earth, planetary scientists have identified evidence of volcanic activity
on the __________ and on __________, __________, and __________, a large
satellite of Jupiter. Much of the activity on these planets resembles volcanic activity on
Earth. Scientists have identified __________, a rock commonly erupted on Earth, in
samples retrieved from the moon and in space probe observations of Mars. Many
volcanoes on Mars and Venus resemble giant versions of __________ volcanoes found
on Earth. The shield volcano __________ _______ on Mars ranks as the largest volcano
in the solar system. It measures more than 370 miles (600 kilometers) in diameter and
rises about 16 miles (25 kilometers) above the surrounding plain.
__________ ranks as by far the most volcanically active body in the solar system.
Space probes and telescopes have identified hundreds of volcanoes on Io, many of them
active. Some eruptions on Io measure at least 350 Fahrenheit degrees (175 Celsius
degrees) higher than the hottest eruptions on Earth. Other eruptions on Io involve sulfur
rather than molten rock. Probes have recorded eruptions shooting sulfur as high as 310
miles (500 kilometers) above Io's surface.