Listening to Earth
Bob Dziak (zee-AK) studies plate boundaries in a surprising way: he listens to them. Dziak is a scientist who works for the National Oceanic and Atmospheric Administration (NOAA), a government department that studies the ocean and the atmosphere. Dziak and his team wanted to know more about what happens on the ocean floor, especially at plate boundaries.
How does Bob Dziak study plate boundaries?
Sending people to the bottom of the ocean is difficult, so Dziak and his team used hydrophones—powerful microphones that are built to travel deep under water. The team sent hydrophones 10.99 kilometers (6.83 miles) down into the deepest place in the ocean, an area known as Challenger Deep. Challenger Deep is an underwater canyon, part of a larger landform called the Mariana Trench.
What is a hydrophone?
Dziak and his team didn’t expect to hear very much noise so deep below the ocean’s surface. What they actually found surprised them—their hydrophones picked up sounds from many different sources! One type of sound collected by Dziak and his team was the sound of plate motion in the form of earthquakes. Earthquakes happen at plate boundaries all over the world—they are caused by the motion of plates.
What causes earthquakes?
This diagram shows how sound travels around deep trenches like the Mariana Trench. Here, sound is represented by red and yellow lines. If the source of a sound is directly over the trench, like it is in the middle panel, sound will easily travel into the deepest parts of the trench. However, if the source of the sound is not directly over the trench, most of the sound does not make it to the bottom of the trench.
The Mariana Trench lies on a convergent boundary where two plates are moving toward each other. When the two plates collide, or run into each other, one is forced underneath the other. This collision is very slow, but the forces are very strong. Over long periods of time, the plate that is on top can bend and fold. The plate on the bottom is shoved into the mantle, bending downward and forming a deep trench at the boundary.
A Convergent Boundary is when...
Divergent boundaries are the opposite of convergent boundaries: they are places where two plates are moving away from each other. As the plates move, hot material from the mantle comes up to fill the space between them. The hot material cools and hardens and adds new rock to the edge of each plate. Over time, this new rock forms a mountain range. When this happens in the ocean, we call the landform a mid-ocean ridge.
A Divergent Boundary is when...
Underwater mountain ranges formed at Divergent Boundaries are called...
By listening to the ocean around plate boundaries, Dziak's team can detect earthquakes and volcanic eruptions and can hear how many are taking place. The motion takes place at a rate of only centimeters per year, but since one side of the plate is being destroyed and the other keeps getting new plate material, they keep moving. Dziak’s research is helping us understand more about how plate motion happens—we just have to listen!
Plates move at a rate of......