Constructing a Plate Diagram
Objective: To construct a diagram of different types of plate boundaries showing the features of
the crust that are formed.
Discussion:
The less dense lithosphere, composed of the crust and solid upper mantle, floats on the denser
asthenosphere. The asthenosphere is solid but can flow and move under the extreme heat and
pressure inside the Earth. The asthenosphere flows by convection currents. As the
asthenosphere flows along, it pulls the lithosphere with it. The solid lithosphere is broken into
large plates that meet along plate boundaries.
There are three types of plate boundaries: convergent, divergent, and transform. A
convergent boundary is where the two plates move toward each other. A divergent boundary is
where the two plates move away from each other. A transform boundary is where the two plates
slide past each other. Each type of boundary can have different types of plates; any combination
of ocean and continental plates.
When two ocean plates converge, one is subducted (pushed under the other) forming a trench.
The area of subduction is known as a subduction zone. The subducted plate is melted and the
magma, being less dense, flows to the surface forming a line of volcanoes/volcanic islands along
the trench. These volcanic islands are called a Volcanic Island Arc. The Japanese Islands, the
Philippines, and the Aleutian Islands are examples of Volcanic Island Arcs.
When a continental plate and an ocean plate converge, the ocean plate is subducted forming a
trench. The collision of the two plates causes the continental plate to fold up forming folded
mountains. Also the subducted ocean plate melts and the magma flows to the surface to form
volcanic mountains among the folded mountains. The Andes Mountains in South America and
the Cascade Mountains in the Northwestern United States are examples of this type of
mountains.
When two continental plates converge, neither is subducted and both push up forming huge
folded mountains. The Himalaya Mountains and the Alps are examples of this type of mountains.
When two plates diverge, a Rift Valley is formed. If two continental plates diverge (continental
rifting), the rift valley will have faults from the stretching and breaking of the crust, hot springs
from the magma pushing up toward the surface, and possibly volcanoes. The rift valley will widen
until it is finally flooded with sea water and a new ocean basin is formed. As the new ocean
basin continues to widen, continental rifting becomes seafloor spreading. As the two ocean
plates diverge a mid-ocean ridge forms with a rift valley in the center. Magma will push up
through cracks in the rift valley forming new seafloor. When Pangaea broke apart, a rift valley
formed. As the Americas moved away from Europe and Africa, the Atlantic Ocean was formed
with the Mid-Atlantic Ridge in the center.
Directions:
1. Each of the two diagrams has small arrows (→) near the upper and lower margin at one
edge of the paper. Cut one of the end margins off and glue the two diagrams together so
the margin lines and the arrows line up.
2. Cut out the diagram pieces and glue them in the proper position. This must be done in A,
B, C order.
3. Draw arrows showing convection currents in the asthenosphere.
4. Color the diagram. (Hint: Look at the diagram in chapter 2.)
5. Label the diagram. Terms you should use are: ocean lithosphere, continental lithosphere,
asthenosphere, convection current, ocean plate, continental plate, volcanic Island arc,
trench, subduction zone, volcano, folded mountain, convergent boundary, divergent
boundary, mid-ocean ridge, magma, magma chamber, and rift valley. (Hint – Look at the
diagrams in chapter 2 of the textbook.) Some of these terms may be used multiple
times.
6. On the back of you diagram, write three paragraphs explaining what is happening at each
of the plate boundaries shown in the diagram (convergent ocean – ocean, convergent
ocean – continental, and divergent ocean- ocean). This should be a detailed description
of how the plates move, what is happening to them and what features are formed. Do not
write “Convergent comes together.” That is not enough!
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