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What is a Rock?

 Scientists define rock as a naturally occurring solid
mixture of one or more minerals and organic matter.
 It may be hard to believe, but rocks are always
changing.
 The continual process by which new rock forms from
old rock material is called the rock cycle.
The Value of Rock

 Rock has been an important natural resource as long
as humans have existed.
 Early humans used rocks as hammers to make other
tools.
 They discovered that they could make arrowheads,
spear points, knives, and scrapers.
Value of Rocks

 Rock has also been used for centuries to make
buildings, monuments, and roads.
 Figure 1 shows how rock has been used as a
construction material by both ancient and modern
civilizations.
 Buildings have been made out of granite, limestone,
marble, sandstone, slate, and other rocks.
Question

 Which rock would be the weakest building material?




A.
B.
C.
D.
Granite
Sandstone
Slate
Marble
 Why?

Processes That Shape the
Earth

 Certain geological processes make and destroy rock.
 These processes shape the features of our planet.
 These processes also influence the type of rock that is
found in a certain area of Earth’s surface.
Weathering, Erosion, and
Deposition

 The process in which water, wind, ice, and heat
break down rock is called weathering.
 Weathering is important because it breaks down
rock into fragments.
 These rock and mineral fragments are the sediment
of which much sedimentary rock is made.
Process of Weathering

 The process by which sediment is removed from its
source is called erosion.
 Water, wind, ice, and gravity can erode and move
sediments and cause them to collect.
 Figure 2 shows an example of the way land looks
after weathering and erosion.
Bryce Canyon in Utah

Can you think of other landmarks that were
shaped by weathering & erosion?
More Weathering

 The process in which sediment moved by erosion is
dropped and comes to rest is called deposition.
 Sediment is deposited in bodies of water and other
low-lying areas.
 In those places, sediment may be pressed and
cemented together by minerals dissolved in water to
form sedimentary rock.
W. E. D.

Heat and Pressure

 Sedimentary rock made of sediment can also form
when buried sediment is squeezed by the weight of
overlying layers of sediment.
 If the temperature and pressure are high enough at
the bottom of the sediment, the rock can change into
metamorphic rock.
 In some cases, the rock gets hot enough to melt. This
melting creates the magma that eventually cools to
form igneous rock.
How the Cycle Continues

 Buried rock is exposed at the Earth’s surface by a
combination of uplift and erosion.
 Uplift is movement within the Earth that causes rocks
inside the Earth to be moved to the Earth’s surface.
 When uplifted rock reaches the Earth’s surface,
weathering, erosion, and deposition begin.
Question

 A rock at the Earth’s surface is primarily affected by
forces of




A.
B.
C.
D.
Heat and pressure
Pressure only
Weathering and Erosion
Cooling
Question

 A rock deep underground is primarily affected by
forces of




A.
B.
C.
D.
Extreme heat and pressure
Cooling
Weathering and Erosion
Heat only
Illustrating the Rock
Cycle

 You have learned about various geological processes,
such as weathering, erosion, heat, and pressure, that
create and destroy rock.
 The diagram on the next slide illustrates one way
that sand grains can change as different geological
processes act on them.
 In the following steps, you will see how these
processes change the original sand grains into
sedimentary rock, metamorphic rock, and igneous
rock.

Round and Round It Goes

 You have seen how different geological processes can
change rock.
 Each rock type can change into one of the three types of
rock.
 For example, igneous rock can change into sedimentary
rock, metamorphic rock, or even back into igneous rock.
 This cycle, in which rock is changed by geological
processes into different types of rock, is known as the
rock cycle.

Hmm…
What would form?

 Sediment + Compaction & Cementation =________
 Metamorphic Rock + heat=_______________
 Sedimentary Rock + heat & pressure =_________
 Igneous Rock + heat=__________________
Rock Classification

 You have already learned that scientists divide all
rock into three main classes based on how the rock
formed: igneous, sedimentary, and metamorphic.
 But did you know that each class of rock can be
divided further?
Rock Classification

 These divisions are also based on differences in the
way rocks form.
 For example, all igneous rock forms when magma
cools and solidifies. But some igneous rocks form
when magma cools on the Earth’s surface, and others
form when magma cools deep beneath the surface.
Composition

 The minerals a rock contains determine the
composition of that rock, as shown in Figure 4.
 For example, a rock made of mostly the mineral
quartz will have a composition very similar to that of
quartz.
 A rock made of 50% quartz and 50% feldspar will
have a very different composition than quartz does.

Texture

 The size, shape, and positions of the grains that make up
a rock determine a rock’s texture.
 Sedimentary rock can have a fine-grained, mediumgrained, or coarse-grained texture, depending on the size
of the grains that make up the rock.
 The texture of igneous rock can be fine-grained or coarsegrained, depending on how much time magma has to
cool.
 Based on the degree of temperature and pressure a rock is
exposed to, metamorphic rock can also have a finegrained or coarse-grained texture.
Sedimentary Rock Texture

Think about it!

 Scientists classify rocks using what important
criteria?




A.
B.
C.
D.
Composition & texture
The depth at which they form
Elevation
The pressure under which they formed
Clues on how the Rock
Formed

Think about it!

 What factors can affect the texture of an igneous
rock?




A. The length of time the magma had to cool
B. The size of the rock
C. The minerals that cement the rock together
D. The pressure and temperature the rock was
exposed to