What is a mid-ocean ridge?
Long chains of mountains that rise up from the ocean floor
What is sonar?
A device that uses sound waves to measure the distance to an object
What forms the longest mountain chains on Earth?
Mid-ocean ridges
What is sea-floor spreading?
The process that adds more crust to the ocean floor by continually adding new material
What happens to older strips of rock as new material is added to the ocean floor?
They move outward from either side of the mid-ocean ridge
What is a deep-ocean trench?
Underwater canyons where subduction occurs
Where does sea-floor spreading happen?
Mid-ocean trenches
Where does subduction occur?
Deep-ocean trenches
What is subduction?
The process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle again
What causes subduction?
Cool, dense oceanic crust collides with less dense continental crust. The oceanic crust sinks because it is more dense.
What are two processes that can change the shape and size of the oceans?
Sea-floor spreading and subduction
How often is the ocean floor renewed?
Every 200 million years
What happens to an ocean surrounded by many trenches?
It shrinks because subduction occurs in more places.
