Why an oceanic plate sinks under a continent
1 min 36 s. The words are spoken and shown in the picture; the player can also show them as subtitles.
For the Earth and Space Sciences Regents. Topic: Plate boundaries, with the past questions on it.
What Dr. Mars says (the words of the video)
Why does an oceanic plate sink under a continent?
Two blocks of the same size: basalt and granite.
Dr. Mars holds the balance level.
He lets go: the basalt side goes down.
Same size but heavier: basalt is denser.
Oceanic crust is basaltic, like the basalt block.
Continental crust is granitic, like the granite block.
Oceanic crust is denser and thinner than continental crust.
Densities in grams per cubic centimeter.
At a convergent plate boundary, the plates move toward each other.
The denser oceanic plate sinks under the continental plate.
The less dense continent stays on top.
This sinking is called subduction.
A deep trench forms where the plate bends down.
Deep below, rock melts and magma rises.
Volcanoes form on the continent, above the sinking plate.
Check yourself! Where do volcanoes form: A, B, or C?
Look for where the sinking plate is deep below.
Answer: C! Magma rises above the sinking plate.
Dr. Mars's balance showed why: basalt is denser, so its plate sinks.
Dr. Mars. Free practice for the New York Earth and Space Sciences Regents.