Chapter 7
Earthquakes
Section 1:
 Volcano:
is a weak spot in the crust where
molten material, or magma comes to the
surface
 Magma:
is a molten mixture of rockforming substances, gases, and water
from the mantle
 Lava:
when magma reaches the surface
Section 1:

Ring of Fire: formed by the many volcanoes that
rim the Pacific Ocean

Volcanic belts form along the boundaries of
Earth’s plates
 At
plate boundaries, huge pieces of the crust diverge
(pull apart), or converge (push together). As a result,
the crust often fractures, allowing magma to reach
the surface

Island arc: an volcano that is created in a string
of island
Section 2
Section 1

Hot Spot Volcanoes
 Hot
spot: is an area where material from deep within
the mantle rises and then melts, forming magma
A
volcano forms above a hot spot when magma erupts
through the crust and reaches the surface
 Some
hot spots lie in the middle of plate far from any
plate boundaries, but other hot spots occur on or near
plate boundaries
A
hot spot in the ocean floor can gradually form a
series of volcanic mountains
Section 2

Physical and Chemical Properties
 Element:
is a substance that cannot be broken down
into other substances
 Carbon,
hydrogen, and oxygen are examples of elements
 Compound:
is a substance made of two or more
elements that have been chemically combined
 Each
substance has a particular set of physical and
chemical properties. These properties can be used to
identify a substance or to predict how it will behave.
Section 2
 Physical
property: is any characteristic of a substance
that can be observed or measured without changing the
composition of the substance
 Examples
of physical properties include density, hardness, melting
point, boiling point, and whether a substance is magnetic
 Chemical
Properties: is any property that produces a
change in the composition of matter

Examples of chemical properties include a
substance’s ability to burn and its ability to
combine, or react, with other substances
Section 2
 Viscosity:
the physical property of liquids
 Because
liquids differ in viscosity, some liquids
flow more easily than others
 The
greater the viscosity of a liquid, the slower it
flows
 Example:
honey is thick, sticky liquid with high viscosity;
honey flows slowly
 The
lower the viscosity, the more easily a liquid flows
 Viscosity
of magma depends upon its silica
content and temperature
Section 2
 Silica:
is made up of particles of the elements
oxygen and silicon
 Silica
is one of the most abundant materials in
Earth’s crust
 The
amount of silica in magma helps to
determine its viscosity
 The
less silica magma contains, the lower its
viscosity
Section 2
 Different
types of lava
 Pahoehoe:
viscosity
is fast-moving, hot lava that has low
 The
surface of a lava flow formed from pahoehoe looks like a
solid mass of wrinkles, billows, and ropelike coils
 aa:
lava that is cooler and slower-moving
 Aa
has higher viscosity than pahoehoe
 When
aa hardens, it forms a rough surface consisting of
jaded lava chunks
Section 3
Section 3
Vent: molten rock and gas leave the volcano
through an opening
 Lava flow: is the area covered by lava as it pours
out of a vent
 Crater: is a bowl-shaped area that may form at
the top of a volcano around the central vent
 When a volcano erupts, the force of the
expanding gases pushes magma from the
magma chamber through the pipe until it flows
or explodes out of the lava

Section 3
 Geologists
classify volcanic eruptions as quiet
or explosive
 Quiet
eruptions: a volcano erupts quietly if its
magma is low in silica; low silica magma has low
viscosity and flows easily; the gases in the magma
bubble out gently; lave with low viscosity oozes
quietly from the vent and can flow for many
kilometers
 Explosive eruptions; a volcano erupts explosively if
its magma is high in silica; high silica magma has
high viscosity, making it thick and sticky; the highviscosity magma does not always flow out of the
crater
Section 3

Pyroclasic flow: occurs when an explosive
eruption hurls out a mixture of hot gases, ash,
cinders, and bombs

Volcano hazards
 During
a quiet eruption, lava flows from vents, setting
fire to, and then burying, everything in its path
 During
an explosive eruption, a volcano can belch out
hot clouds of deadly gases as well as ash, cinders, and
bombs
Section 3
 Geologists
often use the terms active,
dormant, or extinct to describe a volcano’s
stage of activity
 Dormant:
or sleeping volcano is likely to
awaken in the future and become active
 Extinct:
again
or dead volcano is unlikely to erupt
Section 4
Section 4

Volcanic eruptions create landforms made of
lava, ash, and other materials. These landforms
include shield volcanoes, cinder cone volcanoes,
composite volcanoes, and lava plateaus.

Shield volcanoes: lava that flows gradually build a
wide, gently sloping mountain
 Shield
volcanoes rising from a hot spot on the ocean
floor created the Hawaiian Islands
Section 4

Cinder cone: these materials build up around the
vent in a steep, cone-shaped hill or small
mountain
 Example:
Paricutin in Mexico erupted in 1943 in a
farmer’s cornfield

Composite volcanoes: are tall, cone shaped
mountains in which layers of lava alternate with
layers of ash
 Examples:
Mount Fuji in Japan, and Mount St. Helens in
Washington State
Section 4

Caldera: the huge hole left by the collapse of a
volcanic mountain

How does a caldera form: enormous eruptions
may empty the main vent and the magma
chamber beneath a volcano

Features formed by magma include volcanic
necks, dikes, and sills, as well as bathliths and
dome

Batholiths: is a mass of rock formed when a large
body of magma cools inside the crust
Section 4

Dome mountain forms when uplift pushes a
batholiths or smaller body of hardened magma
toward the surface

Geothermal activity: magma a few kilometers
beneath Earth’s surface heats underground water

Hot springs and geysers are types of geothermal
activity that are often found in areas of present or
past volcanic activity
Section 4
 Hot
springs forms when groundwater is
hearted by a nearby body of magma or
by hot rock deep underground.
 Geyser:
is a fountain of water and steam
that erupts from the ground

