San Francisco Unified School District
EXPLAIN
In the Explore lesson, we began to investigate the formation and breakdown of different types of rocks. This lesson will help you understand why there are different types
of rocks and soil in different parts of San Francisco. It will also help you to figure
out the connection between the type of rocks in an area and how the area would be
affected by an earthquake. In this lesson, you will read an article that will help you
gather evidence to answer these questions.
Groupwork Norms
• Listen and pay attention to what others are saying.
• Rephrase and add on to others’ ideas.
Part 1: “Formation of Rocks” Article
This article will help you answer these questions: How does the type of ground under
a building impact its safety during an earthquake? How are different types of rocks
formed? How are rocks connected to earthquakes?
While reading the article, use A Protocol to Support Close Reading that can be
found in Appendix A.
Formation of Rocks
The Three Types of Rocks
Geologists group rocks based on how they were formed. In this article you will
learn about three types of rocks:
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Igneous rocks
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Sedimentary rocks
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Metamorphic rocks
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7th Grade Science
Igneous rocks form when magma
cools below Earth’s surface or
lava cools at the surface. The
lava in the photo will harden into
an igneous rock.
Image via Wikimedia Commons [Public Domain]
Sedimentary rocks form when sediments are compacted and cemented
together. These sediments may be
composed of gravel, sand, silt, or
clay. Sedimentary rocks often have
pieces of other rocks in them. Some
sedimentary rocks are formed from
the solid minerals left behind after
a liquid evaporates. Sandstone,
shown in the photo is a sedimentary rock.
Image by Kevin Walsh via Fickr.com [CC BY 2.0]
Metamorphic rocks form when an
existing rock is changed by energy
transfer (heat) or pressure. The
minerals in the rock change, but
do not melt. Metamorphic rocks
experience these changes within
the Earth. Mica schist, shown in
the photo is an example of a metamorphic rock.
Image by Charles de Mille-Isles via Fickr.com [CC BY 2.0]
Any of these types of rock can change to become another type of rock. Changes
from one type of rock to another usually happens over a long period of time.
Sometimes rocks change into another type above ground and other times this
process takes place below Earth’s surface.
(Continued)
Subunit 3: Earth’s Surface and Earthquakes
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San Francisco Unified School District
(Continued )
These changes are all part of the rock cycle. The diagram of the rock cycle that
follows shows how these three rock types are related to each other. The arrows
within the circle show how each type of rock becomes another type of rock.
The Rock Cycle
Image by Woudloper via Wikimedia Commons [Public Domain]
Stop and Check
1. Describe the rock cycle. How are the three types of rocks formed in the
rock cycle?
Geoscience Processes of the Rock Cycle
There are different processes that can change rock. These processes include:
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Cooling and Crystallization: Deep within Earth, temperatures can get hot
enough to create magma (melted rock). As magma cools, crystals grow,
forming igneous rock. The crystals grow larger if the magma cools slowly, as
it does if it remains deep within Earth. If the magma cools quickly as it does
at the Earth’s surface, the crystals will be very small. The process of crystals
forming from magma is called crystallization.
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Metamorphism: This word means “to change form.“ A rock undergoes
metamorphism if it is exposed to extreme heat and pressure within the crust.
In this process, the rock does not melt all the way. A metamorphic rock may
have a new mineral composition and/or texture than before.
168 Unit 2: Geoscience Processes and Earth’s Surface
7th Grade Science
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Weathering and Erosion: On Earth’s surface, water, wind, ice, and even plants
and animals wear down rocks. Over time, they can break large rocks into
smaller pieces through a process called weathering. Water, wind, and glaciers
carry these pieces from one place to another. This is called erosion.
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Sedimentation: When small pieces of rock created during weathering and erosion
are deposited somewhere, the process is called sedimentation. Sediments can be
compacted and cemented together to form a sedimentary rock.
The rock cycle really has no beginning or end. The processes involved in the rock
cycle take place over hundreds, thousands, or even millions of years. So even
though it seems that rocks remain the same, they are slowly changing.
Adapted from:
Earthquakes and Rocks
Adapted from the United States Geological Survey
Seismologists are scientists who study earthquakes. Seismologists have found
that ground shaking is the primary cause of earthquake damage to buildings,
roads, bridges, and other objects built by humans. When there is a lot of ground
shaking, buildings can be damaged or destroyed, which may harm the people
who are inside or near them.
Seismologists have analyzed and found patterns in data from areas that have
experienced many earthquakes, such as San Francisco. For example, during an
earthquake, areas of land made up of soft soil experience more shaking than
areas that have a harder soil. Soft soil is usually made up of sedimentary rocks
like sand, mud, and gravel. Soft soil amplifies, or increases, the ground shaking
caused by an earthquake. In areas with soft soil, more energy is transferred from
the earthquake to buildings, so there is generally more damage caused compared
to areas with harder soil types. Soil made mostly of metamorphic rock is harder
than soil made of sedimentary rock. Areas of land made up of soil that is mostly
metamorphic rock experience less shaking than areas of soil made of sedimentary rock. Soil made mostly of igneous rocks causes the least amount of ground
shaking. Soil composition is only one variable that determines how ground shaking occurs during an earthquake. Another variable that determines the amount
of ground shaking is distance from the location where the earthquake occurred.
Stop and Check
1. What is the relationship between the type of soil and the level of ground
shaking in an earthquake?
Subunit 3: Earth’s Surface and Earthquakes
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