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Unit 10 Earth Science
Chapter 29 Formation of Rocks
29.1 Volcanoes
 The eruption of Mount St. Helens in 1980
reduced the height of this mountain from
2,932 meters (9,677 feet) to 2,535 meters
(8,364 feet).
 Early in the morning of May 18, 1980, an
earthquake triggered a landslide that
caused the bulge to eject magma, water,
and gases.
 Solid rock melts and becomes magma
under certain conditions that lower the
melting point of the material.
 Volcanologists (scientists who study
volcanoes) use several indicators to
predict when a volcano will erupt. They
look for and/or monitor:
 Upward movement of magma
 Bulging of a volcano’s side
 Temperature of the ground water and the release of gases
 Liquid rock material below the Earth’s surface is called magma; liquid rock material at or
above the Earth’s surface is called lava.
 At subduction zones, water is the key for solid rock to melt and become magma.
 The cup-like depression at the top of a volcano where lava usually exits is called the crater.
 Most volcanic activity is found at the edges of tectonic plates, namely at divergent and
convergent plate boundaries, but does not occur at transform plate boundaries.
Unit 10—Earth Science
Chapter 29 Formation of Rocks
Unit 10—Earth Science
Chapter 29 Formation of Rocks
A large belt of mountain ranges and volcanoes surround the Pacific Ocean. Earthquakes are common
in this area known as the Ring of Fire.
 There are three main types of volcanoes: shield volcanoes, stratovolcanoes (or composite
volcanoes), and cinder cones.
 Low viscosity, fast-flowing lava, low
in silica content and made
principally from melted basalt, is
associated with shield volcanoes.
 Because this lava easily flows down
hill, shield volcanoes are gently
sloped and flattened.
 The gently-sloped, dome-shaped
Hawaiian Islands are examples of
hot-spot shield volcanoes, as are the
volcanoes of Iceland.
Unit 10—Earth Science
Chapter 29 Formation of Rocks
 High viscosity lava is associated with
stratovolcanoes (also called
composite volcanoes).
 High silica content can be found in
stratovolcano lava
 These volcanoes range in height
from 500 to 10,000 meters high.
 Mt. St. Helens is a stratovolcano.
 Cinder cone volcanoes are steep
stacks of loose pyroclasts (clumps
and particles of lava).
 Cinder cones are rarely higher than
300 meters.
 Crater Lake National Park (Oregon)
contains a cinder cone, Wizard
Island, which actually formed in the
caldera of a stratovolcano.
 Lava viscosity also determines how explosive an eruption will be.
 Explosive eruptions occur when the lava has a lot of water and dissolved gases.
 Gentle eruptions are associated with fast-flowing lava from oceanic crust.
Unit 10—Earth Science
Chapter 29 Formation of Rocks
 Volcanoes also form when an oceanic plate slides under another oceanic plate.
 Hydrothermal vents are deep sea, chimney-like structures that occur along midocean ridges.
Unit 10—Earth Science
Chapter 29 Formation of Rocks
 Some gemstones are also associated with volcanic activity.
For example, diamonds form at high temperatures deep underground when carbon
crystallizes inside rocks called kimberlites.
 Geothermal energy is the useful product of volcanic activity. Geothermal energy is energy
derived from the heat and steam associated with magma close enough to the earth’s surface
to harness.
 When steam from magma collects below
ground, it can be tapped just like water in a
 The pressurized steam can be used to
generate electricity.
 Volcanic activity results in the formation of
two kinds of igneous rocks:
 extrusive
 intrusive
 A batholith is a large underground rock that
formed when a mass of magma cooled
Unit 10—Earth Science
Chapter 29 Formation of Rocks
29.2 The Surface of Earth
 Earth’s surface is constantly
 Recall that earthquakes,
volcanoes, mountains, and the
construction of new lithosphere
are events that occur at plate
 These events are changing the
appearance of Earth’s surface all
the time.
 The features we see on Earth’s
surface represent the dynamic
balance between constructive
processes versus destructive
 Mountain-building is a major constructive process.
 Mountains form in three main ways:
 by folding at convergent plate boundaries (fold
 by movement of chunks of land at faults (faultblock mountains)
 by volcanic activity (volcanic mountains)
Unit 10—Earth Science
Chapter 29 Formation of Rocks
Topography and Erosion
 Topography refers to features and formations, like
bodies of water and mountains, that characterize
Earth’s surface.
 Observing hill slopes, stream patterns, and bedrock
structures would help you describe landscape
 Local climate and geology have the greatest influence
on the development of a natural landscape.
 Erosion (also known as weathering) is a major
destructive process.
 Weathering is defined as the collective processes in
which soil and rock are eroded as a result of climate and
seasonal changes.
 Erosion describes the continuous physical and chemical
events that cause land and rock to wear down.
 The substance that has the greatest effect on the rate at
which rocks weather is water.
Unit 10—Earth Science
Chapter 29 Formation of Rocks
 The rate of erosion of mountains (previous page) is related to the height and steepness of the
mountain—the steeper the mountain is, the faster it erodes because it is easier to push
material down a steep slope than a gradual slope.
 A soil profile is a cross-section
that shows the different layers of
soil in the ground. It shows the
weathered products of rock.
 It takes a long time and a lot of
weathering for soil to have all
the layers.
 Young soil does not have each of
these layers.
 Topsoil, in Horizon A, is
important for growing crops. It
contains the most nutrient-rich
soi. The topsoil can increase in
thickness when biological activity
increases since more roots will
hold soil in place and plants will serve as a barrier to erosion at the surface.
 Horizon D is rock or bedrock. This is the horizon where the least amount of weathering has
taken place.
 A glacier is a huge mass of ice on land that can be
many kilometers thick and thousands of kilometers
wide. Glaciers erode the land as they move.
 Glaciers at the poles are a frozen form of about 2
percent of all the water on Earth.
 Glaciers are formed from the accumulation of snow
over hundreds or thousands of years.
Unit 10—Earth Science
Chapter 29 Formation of Rocks
 Geologic hazard maps indicate the location of faults where earthquakes occur, areas where
volcanoes are active, and where landslides, avalanches, floods, or other natural hazards are
 The term urban sprawl refers to how living areas around a city “sprawl” as they grow instead
of concentrate near facilities that serve the people of the community.
 Building roads changes the land.
 Roads and parking lots prevent water from slowly seeping into the ground to replenish the
water supply in aquifers.
 Increased pollution and loss of open space are typical results of urban sprawl.
 The most practical way individuals can help control the adverse effects associated with urban
sprawl is by becoming aware of the problems associated with urban sprawl and carefully plan
the use of our environment.
Unit 10—Earth Science
Chapter 29 Formation of Rocks
29.3 Rocks and Minerals
 A mineral is a solid, naturally-occurring, in
organic object with a defined chemical
 Minerals are inorganic and have a
crystalline structure.
 Minerals are the building blocks of a rock.
 Granite, at the right, is a type of
rock. The minerals horneblende,
quartz, mica, and feldspar are
found in this rock.
 Faulted rock systems (areas where rocks are exposed) would be the best place to start
prospecting for precious minerals like gold and silver .
 Graphite and diamonds are two different minerals that are made of pure carbon. Diamond is
the hardest naturally occurring substance, while graphite is quite soft (pencil “lead”). Their
very different properties arise from the fact that the carbon atoms in the diamond are bonded
or connected differently than the carbon atoms in the graphite.
 A mineral is a material that is naturally occurring, inorganic, and crystalline.
 There are more than 3,000 minerals on Earth.
 About 20 minerals make up Earth’s crust.
Some Common Minerals
Name and
chemical formula
Silver, Ag
Jewelry, electrical wire,
Corundum, Al2O3 Sandpaper,gems (ruby,
Quartz, SiO2
Glass making, gems
(onyx, amethyst)
Used to make Plaster of
 Mohs hardness scale was developed in 1812 by
Friedrick Mohs (an Austrian mineral expert) as a
method to identify minerals.
 The hardness of a mineral can be determined by
using it to scratch other solid material of known
Unit 10—Earth Science
Chapter 29 Formation of Rocks
 Most minerals (except metals) have one or more cleavage planes that also help in determining
their identity. Cleavage planes are flat surfaces that minerals break along when struck.
 A rock is a naturally formed solid usually made of one or more minerals.
 The terms igneous, sedimentary, and metamorphic refer to the origin of a rock, how a rock
was formed.
 Igneous rocks form by the cooling and solidification of magma or lava. Igneous rocks
usually contain crystals since the cooling often occurs underground where the process
takes a long time so that crystals can form. A crystal is a solid with a definite internal
structure of atoms arranged in a regular, repeating pattern.
 Sedimentary rocks are found in many areas on most continents. Sedimentary rocks
are formed from the compression and cementing of weathered particles (called
sediment). Sediment must be compressed and cemented before it can change to
sedimentary rock. Preserved animal remains are most commonly found in
sedimentary rock.
 Metamorphic rocks are formed by intense pressure or heat applied to an existing rock.
 The rock cycle is the set of processes leading to the formation and recycling of various kinds of
rock. The rock cycle is depicted on the next page and illustrates the formation and recycling of
rocks by geological processes.
Unit 10—Earth Science
Chapter 29 Formation of Rocks
 Identifying Rocks--Observe and ask questions:
 What does the rock look like?
 Examine the grain of a section.
 What is the rock's composition?
 Determine what minerals are in it.
 Where was the rock found?
 Mountain
 Stream or river
 Volcano
 Ocean floor