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ChAPTER 9 • Plate Tectonics
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Investigation 9.2
Plate Tectonics and Crustal Features
What is plate tectonics?
Many of Earth’s crustal features can tell you a lot about the shape, size,
and behavior of tectonic plates. In this investigation you will identify
crustal features on your bathymetric map to identify different kinds of
tectonic plate activity at plate boundaries.
Convection cells and moving tectonic plates
 Earth’s tectonic plates
are constantly moving.
This motion is caused by
convection cells in the mantle.
Material in the mantle becomes
heated close to the core, which
causes it to rise. Newly heated
material below it pushes it up
toward the lithosphere and
Earth’s tectonic plates.
4 Bathymetric map
Mid-ocean ridge
Rising convection current
Convection cell
in the mantle
Lithosphere
Subduction
zone
4 Pencil
Core
The heated material moves
under the plates in a zone
Mantle
called the asthenosphere and
drags the plates along with it
as it moves. The heated material
eventually cools enough to begin sinking
back toward the core, where the process repeats itself. Different
convection cells circulate in different directions, and this is the key to
what happens at tectonic plate boundaries.
The plates can move toward one another, causing a subduction zone,
or they can move away from one another which produces a mid-ocean
rise. These crustal features will appear at a plate boundaries and are
an excellent way to identify the direction of motion of two plates at
a boundary.
Mid-ocean ridges, deep-ocean trenches, and mountain ranges are
great indicators of tectonic plate motion. You will be identifying these
features on your bathymetric map in the following section.
Divergent plate boundaries
 Mid-ocean ridges are crustal features that occur at a divergent plate
boundary. A divergent plate boundary occurs when convection cells
cause two adjacent plates to move in opposite directions away from
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Unit 3 Earth’s Systems
4 Colored pencils
A Model of Large-cell
Convection in the Mantle
The tectonic plate moves over
the asthenosphere underneath.
Mantle material cools and
sinks backtoward the core.
Mantle material
expands and rises as
a convection current.
Mantle material
is heated by
the core.
Core
Hot, expanded, less dense,
more buoyant
Cool, contracted, denser,
less buoyant
ChAPTER 9 • Plate Tectonics
one another. When this happens material from the mantle comes to the
surface to fill in the space left behind from the two plates moving apart,
and can cause mid-ocean ridges, rises, island chains, and volcanoes
to form.
Divergent plate boundary
Mid-ocean
ridge
Ocean
crust
1. Chose one color of pencil to indicate mid-ocean ridges on your
bathymetric map.
2. Find examples of mid-ocean ridges on your bathymetric map and
color those areas with the colored pencil you have selected. Include
long mid-ocean rises as well. These are similar to ridges, just not
quite as big.
Convergent plate boundaries
 Magma
Plates moving in opposite directions
Converging plates produce mountain ranges, deep-ocean trenches,
and even islands. These trenches indicate a subduction zone as one
plate slides beneath another. See in the diagram below how this
takes place.
Subduction zone of one oceanic plate
and one continental plate
Andes Mountains, Peru
1. Select a color to indicate deep-ocean trenches on your
bathymetric map.
2. Find examples of deep-ocean trenches on your bathymetric
map and color those areas with the colored pencil you
have selected.
Thinking about what you have observed
 a. Examine your marked map. Can you begin to see the
boundaries of Earth’s seven major tectonic plates?
Lightly sketch in pencil on your map where you think the
boundaries are.
South American
Plate
Nazca Plate
Oceanic plate, basalt, denser,
less buoyant, sinks under the
continental plate
b. The Himalayan Mountains are a crustal feature that
Continental plate, andesite and granite,
demonstrates a special case in convergent plate
less dense, more buoyant, floats
above the oceanic plate
boundaries. Find them on
your map and color them
Subduction zone of two oceanic plates
in a dark color. Does this
Deep-ocean trench
help you in identifying
Island arc
tectonic plate boundaries?
c. Your teacher will show your
Sediments
(scraped off subducting plate)
class an image of the seven
Oceanic
major tectonic plate
crust
boundaries. How did the
Oc
ea
A spot here will move closer
boundaries you found on
n
(ol ic lit
to the trench as the plate subducts.
de ho
your map compare to the
r p sp
lat he
e)
re
boundaries shown in this image?
Asthenosphere
Volcano
Oceanic lithosphere
(younger plate)
Region of melting
Magma and water released
from subducted crust
Investigation 9.2 Plate Tectonics and Crustal features
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CHAPTER 9 • Plate Tectonics
d.Label on your map the obvious convergent and divergent plate boundaries. Label any areas you are not
sure about “difficult” boundaries.
e. What made the difficult boundary areas challenging to label?
f. We have identified seven major tectonic plates based on obvious convergent and divergent plate
boundaries. Would you be surprised to find out there are several smaller plates that are responsible
for some of the most active geologic activity on Earth’s surface? Using your map predict the general
location of these smaller and dynamic plates.
g. We discussed seven major tectonic plates in this investigation but a case made for eight major plates. If
you had to predict where the boundary would be that creates the eighth major plate, where would you
predict it to be?
h.There is a third type of boundary. Research the term transform fault boundary. Describe what it is, what
makes it different from convergent and divergent boundaries, and give two examples of a transform
fault boundary.
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Unit 3 Earth’s Systems