Teacher answer key · Grades 6–10 · about 15 minutes

Answer key: Plate Boundaries Activity

For teachers. This page is hidden from search engines and not linked from student pages, but it is not password-protected, so please don’t share the link with students. Student worksheet: Plate Boundaries Activity.

Goal: Compare plate boundaries in a model, using landforms, earthquake depths and volcanoes as evidence of how plates move.

Connects to NGSS MS-ESS2-3 (Analyze and interpret data on the distribution of fossils and rocks, continental shapes, and seafloor structures to provide evidence of the past plate motions.); HS-ESS2-1 (Develop a model to illustrate how Earth’s internal and surface processes operate at different spatial and temporal scales to form continental and ocean-floor features.)

Completed results table

Answers match the explorer’s Evidence panel; accept any landform it lists. Earthquake depths: shallow is less than 70 km, intermediate 70–300 km, deep 300–700 km.

Boundary	Main landforms	Deepest earthquakes	Volcanoes?
Divergent: ocean–ocean	Mid-ocean ridge	Shallow	Yes
Divergent: continent–continent	Rift valley	Shallow	Yes
Convergent: ocean–continent	Trench, volcanic mountains	Deep	Yes
Convergent: ocean–ocean	Trench, volcanic island arc	Deep	Yes
Convergent: continent–continent	High mountain range	Intermediate	No
Transform: continent–continent	Long straight fault	Shallow	No

Answers vary: distance = speed × 20 × 10. For example, 5 cm per year gives 5 × 20 × 10 = 1000 km, as the explorer shows.

Answers

Which two boundaries had the deepest earthquakes? Why can earthquakes there be so deep?
Ocean–continent and ocean–ocean convergent boundaries (subduction zones). The sinking slab stays cold enough for rock in it to break, so earthquakes follow it down to about 700 km.
Near a coast, earthquakes get deeper toward the east, under a line of volcanic mountains. What boundary is this, and which way does the slab sink?
An ocean–continent convergent boundary (a subduction zone), like the Andes or the Cascades. The ocean plate sinks toward the east, under the continent, and the volcanoes stand above the slab.
Why do volcanoes form at ridges and subduction zones, but not at transform faults like the San Andreas?
At a ridge, rising mantle rock partly melts as the pressure drops; at a subduction zone, water from the slab helps the mantle above it melt. At a transform fault the plates only slide past, so nothing rises or sinks and no magma forms.
Suppose the Mid-Atlantic Ridge spreads 2.5 cm per year. How far apart do its plates move in 10 million years? (1 cm per year = 10 km per million years.)
2.5 cm per year = 25 km per million years, so in 10 million years the plates move 25 × 10 = 250 km apart.
A ridge spreads 6 cm per year in total (3 cm per year each way). How old is the seafloor 300 km from it?
Each plate moves 3 cm per year = 30 km per million years. Age = distance ÷ speed = 300 ÷ 30 = 10 million years.
A classmate says, “The plates float on a sea of liquid magma.” Correct this statement.
The mantle is almost all solid rock, hot enough to flow very slowly, a few centimeters a year. Magma forms only where rock partly melts, such as under ridges and above sinking slabs.

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