N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 6
Changing Earth
Spring 2006
CHANGING EARTH
In this 8th unit the student will know that cycles exist in Earth Systems. The
student is expected to analyze and predict the sequence of events in the lunar and
rock cycles. Students were exposed to both of these cycles in previous grades,
including the lunar cycle in 7th grade and the rock cycle in 6th grade. In addition,
the 8th grade student will predict land features resulting from gradual changes
such as mountain building, beach erosion, land subsidence, and plate tectonics.
These should be taught as processes that are ongoing and make the Earth the
dynamic planet that it is today.
The Lunar Cycle
Students should be able to relate information about this cycle with emphasis on
what stage may come next in a sequence that depicts the cycle. The following lists
the information students should know in regards to the lunar cycle:
 The lunar cycle occurs about every 28 days.
 No matter the phase, the sun always lights one half of the moon.
 The reflection of sunlight is the reason the moon is visible.
 The gravitational pull of the moon is what causes the tides on earth.
 Spring tides are tides with the largest daily range.
 Spring tides occur during the new and full moon phases.
 Neap tides are tides with the smallest daily range.
 Neap tides occur during the first and last quarter moon phase.
 The lunar cycle can be broken down into 8 repeating phases:
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Moon Phase
new moon
Diagram
Unit 6
Changing Earth
Spring 2006
Explanation
no moon is visible as the sunlit portion does
not face Earth
waxing
crescent
seen as a small crescent / slice of the moon
as the sunlit portion is increasing in size from
day to day
first quarter
seen as one half of the moon as the sunlit
portion continues to increase in size from day
to day
Note: The first quarter moon looks like a
right half of a circle.
seen as a greater than quarter but less than
a full moon as the sunlit portion is increasing
in size from day to day
waxing gibbous
full moon
the entire sunlit side of the moon is visible
waning gibbous
seen as greater than a quarter but less than
a full moon as the sunlit portion begins to
decrease in size from day to day
last quarter
seen as one half of the moon is lit as the
sunlit portion is decreasing in size from day
to day
Note: The first quarter moon looks like a
left half of a circle.
seen as a tiny slice of the moon as the sunlit
portion continues to decrease in size from
day to day
waning
crescent
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 6
Changing Earth
Spring 2006
The Rock Cycle
All rocks are made of minerals. Students should know that these minerals combine
to form three basic rock types: igneous, sedimentary and metamorphic. The 8th
grade student should know how each type is formed and be able to predict how one
rock may change into another through the processes that are a part of the rock
cycle. Students should not be responsible for matching the name of a rock with its
classification, but they should be able to look at a sample and hypothesize how it
was formed through the processes that are a part of the rock cycle.
In order for students to understand the origins of rocks, it would be appropriate
to review the basic layers of the earth. The earth has three basic layers: the
crust, mantle and core. The crust is the thin, outermost layer made of rock. The
mantle is the largest layer and contains magma. The core is the Earth’s hot, dense
center layer. The outer core is liquid while the inner core is solid from intense
pressure. The primary elements in the core are iron and nickel. This is because
they are very dense and pulled towards the center by gravity. Students have
learned about these layers in the past, but they may not be familiar with the sublayers including the lithosphere, asthenosphere and mesosphere. The lithosphere
consists of the crust and the uppermost, somewhat rigid portion of the mantle. It
is divided into the tectonic plates. The asthenosphere is made of the magma upon
which the tectonic plates slowly move. The mesosphere is the solid, lower part of
the mantle that lies above the core.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 6
Changing Earth
Spring 2006
Igneous rocks form from either magma or lava. Both of these are molten rock.
Magma is found underground. When it comes to the surface through a fissure or
volcanic eruption, it undergoes a chemical change as it is exposed to the
atmosphere. At this point, the molten rock is referred to as lava. If the igneous
rock forms from magma that has cooled and hardened underground, it is an
intrusive igneous rock. Rocks that form when magma cools slowly will have large
crystals as they are afforded time to grow. Granite is a good example of a slow
cooling, intrusive igneous rock. Rocks that form when magma cools quickly will have
small crystals. Basalt is a good example of a fast cooling, intrusive igneous rock.
Sedimentary rocks form from the weathered sediments of other rocks.
Weathering occurs through forces such as wind, water, ice, organisms, etc.
Erosion carries the resulting sediment from one place to another. Deposition of
the sediment occurs in layers. Over time, the pressure of overlying layers causes
the sediment to become compacted and cemented together. As a result of this
process, many sedimentary rocks appear to have strata. Sedimentary rocks are
made from the compaction and cementation of different materials. There are
different methods through which sedimentary rocks my form. Some are formed
by mineral and rock fragments, such as conglomerate, sandstone, and shale.
Organic material may also combine to form sedimentary rocks. A common example
would be limestone which is formed from the shells of once living sea creatures.
Limestone, however, may also form when minerals precipitate out of solution,
collect and harden.
Metamorphic rocks form from other rocks being heated and put under tremendous
pressure. Students should understand rocks that undergo metamorphosis are not
melted. In order for a rock to change, it must first be buried. Exposure to a heat
source and the weight of overlying rock layers then begins the metamorphic
process. Marble is an example of a metamorphic rock that forms from limestone.
The rock cycle describes how the form and structure of rocks is constantly
changing. It is comprised of the following processes:
 Melting: Heating within the Earth causes rock to melt and form magma.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science




Unit 6
Changing Earth
Spring 2006
Cooling and Hardening: Magma rises towards/to the surface, cools and
hardens.
Weathering, Erosion and Deposition: Weathering is the breakdown of rock
into sediment. Erosion is the movement of that sediment. Deposition is the
process by which the sediment is dropped in a new location.
Compacting and Cementing: Pressure compacts sediment together. Water
between particles evaporates, leaving minerals that cement sediments
together.
Heat and Pressure: Heat comes from either direct contact or nearby
contact with magma. Pressure from overlying rock layers squeezes minerals
in rocks. These two processes may work to actually change the identity of a
rock.
The following is a diagram of the rock cycle. Students should be very familiar with
a diagram such as this. It is essential that they are able to predict the processes
involved in a sequence which may lead to change and/or formation of a rock.
Students also need to understand that any type of rock may change into another
type of rock.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
5
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 6
Changing Earth
Spring 2006
Most of the process appearing in the rock cycle are gradual. Over time such Earth
processes can result in land features such as mountains, beaches and areas of
subsidence, just to name a few. Movement of tectonic plates is a major force in
the changing of the Earth’s surface.
Plate Tectonics
Alfred Wegener wrote about theory of continental drift in 1912. He observed that
the eastern edge of South America fits uniquely into the western edge of the
Africa. He also noted similar fossils dating back to the same age were found in
both South America and Africa. Wegener suggested that at one time in the
history of the development of earth all the continents were actually only one large
continent called Pangaea which means “all land.” Similar rock types, climate clues,
and fossils were the main evidence that supported the theory of continental drift.
Over time, science advanced, the ocean floors could be studied in more detail and
seafloor spreading was discovered. Magnetic reversals in the bands found on
either side of a diverging oceanic plate provided support for Wegener’s theory.
Today, the theory that the lithosphere is divided into tectonic plates that move on
top of the mantle’s asthenosphere is referred to as “plate tectonics.” The term
“continental drift” is no longer used as it is misleading in regards to the actual
method and process of what occurs.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
6
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 6
Changing Earth
Spring 2006
The 8th grade student has been exposed to the term “tectonic plate,” but has not
previously been taught the mechanism of plate tectonics or been given a thorough
lesson on types of plate boundaries and the earth features that occur at each one.
Students should apply the principle that hot materials expand and rise and then
become cooler and more dense, leading them to sink again. A lava lamp is a good
model to help students make this connection. The cyclical rising and sinking of
molten material in the asthenosphere is the driving force behind the movement of
tectonic plates. The convection currents created act like a conveyor belt that
actually will pull plates along. This process is known as slab pull, referring to a
tectonic plate as the “slab.”
Tectonic plates collide at convergent plate boundaries. Different processes occur
at these boundaries depending on the composition of the plates. When two
continental plates collide, they will buckle and crust will be pushed upwards. A
good example is where the Indian Plate collided with the Eurasian Plate. The result
of this collision was the formation of the Himalayan Mountains. When continental
and oceanic plates collide, the oceanic crust will slide under the continental crust in
a process known as subduction. This occurs because the oceanic crust is more
dense. The region where the process happens is referred to as a subduction zone.
As the oceanic crust sinks lower into the mantle, it will melt and form magma. This
may create a “hot spot” where magma rises. A subduction zone exists at the
boundary between the South American Plate and Nazca Plate. The result is the
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 6
Changing Earth
Spring 2006
formation of the Andes mountains, volcanoes, and frequent earthquakes. When
two oceanic plates collide, the one that denser will be subducted. Volcanic islands,
such as the Hawaiian and Aleutian Islands, are created by this process.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
8
N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 6
Changing Earth
Spring 2006
Divergent boundaries exist where two plates are moving away from one another.
Most of these boundaries are located in oceans. The mid-ocean ridge in the
Atlantic is an example of where this is occurring. As plates diverge at the ridge,
new magma rises, cools and hardens. This forms new oceanic crust in a process
referred to as sea-floor spreading. The Great Rift Valley in Africa is an example
of a divergent boundary on land. Seas and volcanoes are the result of tectonic
activity in this area. At some point in the future, the rift may be large enough to
separate the land masses completely, creating a new mid-ocean ridge.
Rift Zone
As convergent and divergent boundaries are discussed in the classroom, an
important connection can be made to previous units of study. The constant
creation and subduction of crust is an example of the application of the Law of
Conservation of Mass. These tectonic processes create a state of equilibrium, a
concept first introduced at the 7th grade level in regards to volcanic eruptions.
Transform boundaries occur where two tectonic plates slide past one another. The
grinding and slipping at these boundaries often causes earthquake activity.
California is located on two different plates, the North American and Pacific. The
movement of these two plates along the San Andreas fault is responsible for the
high number of earthquakes that occur in this region.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 6
Changing Earth
Spring 2006
Transform
Boundary
Mountain Building
The lifting of land by forces in the Earth’s crust, or uplift, is one of the major
forces that creates mountains. Different types of plate movements are
responsible for the different types of mountains. There are three basic mountain
types: Volcanic, Folded and Fault-block. Volcanic mountains can occur at either
convergent or divergent boundaries as magma rises to the surface as a result of
subduction or sea-floor spreading. Folded mountains form from the folding, or
bulging, that occurs when crust collides and is pushed upwards at convergent
boundaries. The Rockies, Himalayas, Andes and Alps are all examples of folded
mountains. Fault-block mountains form when large pieces of crust fall and rise
alongside each other as plates diverge. Fault-block mountains are very common
near mid-ocean ridges. Examples on land include the Tetons and Sierra Nevadas.
Land Subsidence
Subsidence is essentially the opposite of uplift. Land subsidence is the sinking of
land that occurs either as a result of geologic processes or human activities.
Tectonic activity can cause subsidence in two ways. As plates moving in different
directions pull apart, a rift zone will form. The crust in the center of the rift zone
will subside, leaving a fissure. In areas where sea-floor spreading occurs, new hot
crust is added to the ocean floor. As the spreading continues, this crust cools, and
sinks because it now takes up less space/volume. Subsidence can also happen as a
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 6
Changing Earth
Spring 2006
result of human activity. Sinkholes often occur where man-made voids, such as
tunnels, wells and quarries, have a weak roof of earth above them. This same form
of subsidence may also take place over natural caves. Another common cause of
subsidence due to human activity is from excessive withdrawal of underground
resources, including petroleum and water. The following is an example of how
groundwater removal may result in sediment compaction and the subsidence of the
ground surface:



Step 1: The weight of the overlying material is supported by both sediment
grains and the water in the pore spaces between these grains.
Step 2: Pressure from water on the surrounding grains keeps the pore space
open.
Step 3: When the water is extracted, sediment grains may collapse inward on
the pore space.
Topographic Maps
The 2005 TAKS Information Booklet states the following: “Students should be
able to interpret and evaluate graphs, charts, and maps. Maps may include
topographic contour maps, in which each line on the map represents a change in
elevation and differences in spacing between lines indicate relative steepness of
slopes.” The 8th grade lessons on mountain building and land subsidence provide a
natural connection to reading topographic maps. Changes in elevation on a
topographic map may represent hills, mountains, valleys or areas of subsidence.
Students should be able to interpret a topographic map to determine the major
landforms depicted. In addition to understanding elevation lines and spacing as an
indicator of steepness, the student needs to be able to interpret information given
about the map, such as contour interval and scale measurements. Students should
also know that certain features, such as rivers, may cut across contour lines.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ric t
8th Grade Science
Unit 6
Changing Earth
Spring 2006
Example of a topographic
map depicting a hill
Beach Erosion
A beach is any area of the shoreline made up of material deposited by waves.
Shorelines made of sand, coral, or rock can be washed away over time through the
action of waves, wind, storms, currents, and development by human activity. This is
a land feature generally not affected by tectonic activity. There have been many
attempts by man to stop the erosion; examples would be building jetties, building
sea walls, and dumping more sand where the sand has washed away. It is not
known, however, if these attempts will actually help or do more harm.
The 8th grade student should be exposed to examples of coastal landforms. They
should not be required to memorize the names of specific beach features, but they
should have experience in looking at a feature and formulating a hypothesis as to
how it was created, drawing on their prior knowledge. Primary emphasis will be on
wave erosion which is responsible for features such as sea stacks, sea caves, sea
arches, terraces, etc.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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