Answer: D
Explanation:
Cause If you head to the middle of the ocean and dive down, down, down to where the Earth's plates meet, you might find an undersea mountain split by a deep trench. It's called a mid-ocean ridge. Up through such trenches, brand-new molten rock is bubbling. That's the newest rock on Earth! How does the ocean floor make room for all that new rock? Simply by pushing the old rock aside. In fact, the ocean floor is actually spreading right at this minute, out away from the deep trenches where new rock is being formed. The oldest rock can be found farthest away from the trenches, right up near the continents. It's all part of a process called seafloor spreading. It's been going on for millions of years (at the rate of a few centimeters per year), but scientists are just now getting to know more about it. In fact, because evidence of seafloor spreading was first discovered in the 1940s and 50s, your great-grandparents may have never heard of it!
