Section 4 is all about seafloor spreading, evidence of sea-floor spreading, and the terrain under the sea.
On the floor of the deep ocean, there are hot water vents. The water there is heated by geothermal energy, which means that the hot water is heated by the heat inside the earth. The water gets heated near the mantle. Then, the water spurts up.
Mid-ocean ridges are undersea mountain chains. Of course, people cannot travel down that deep because of the water pressure. Therefore, they use sonar, a device that bounces ultrasonic sound waves onto a landscape and records the time it takes for the sound to bounce back.
Sea-floor spreading happens when new crust is added. It spreads apart on both sides of the mid-ocean ridge. Spreading begins in the middle of the ridge, which forms a crack in the oceanic crust. Molten material in the ridge rises, and erupts. Then, the older rock, farther away from the ridge, moves outward. The rock that has formed most recently is closer to the ridge.
There is much evidence for sea-floor spreading. The submarine Alvin found rocks shaped like pillows and toothpaste. That meant that those rocks had cooled and hardened quickly underwater. More evidence came from the magnetic stripes in the rock of the sea-floor. Those rocks contained iron. The iron lined up to the magnetic poles. After it cooled and hardened, the iron bits were locked into place. The poles could then reverse themselves, and the iron in the new rock lined up to the new poles.
The Glomar Challenge, a drilling ship, found more evidence. They found out that the newest rock was nearest the ridge, and the oldest rock was furthest away from the ridge.
Subduction trenches are where the ocean floor eventually goes. The oceanic crust sinks beneath the continental crust at the trenches. The ocean floor turns too dense and sinks at the trenches. Also, gravity helps pull it down. The ocean floor sinks towards the mantic. The Pacific Ocean is shrinking because there are many trenches and few ridges. The Atlantic Ocean, on the other hand, is growing, for it has few trenches and many ridges.