Figure 2.12: Map of the mid-ocean ridge system (yellow-green) in the Earth’s oceans. ( 23 )The deepest places of the ocean are theocean trenches. There are many trenches in the
world’s oceans, especially around the edge of the Pacific Ocean. The Mariana Trench, which
is located east of Guam in the Pacific Ocean, is the deepest place in the ocean, about 11
kilometers deep (Figure2.13). To compare the deepest place in the ocean with the highest
place on land, Mount Everest is less than 9 kilometers tall. In these trenches, the ocean floor
sinks deep inside the Earth. The ocean floor gets constantly recycled. New ocean floor is
made at the mid-ocean ridges and older parts are destroyed at the trenches. This recycling
is why the ocean basins are so much younger than the continents.
The Earth’s surface is constantly changing over long periods of time. For example, new
mountains get formed by volcanic activity or uplift of the crust. Existing mountains and
continental landforms get worn away by erosion. Rivers and streams cut into the continents
and create valleys, plains, and deltas. Underneath the oceans, new crust forms at the mid-
ocean ridges, while old crust gets destroyed at the trenches. Wave activity erodes the tops
of some seamounts and volcanic activity creates new ones. You will explore the ways that
the Earth’s surface changes as you proceed through this book.
Lesson Summary
- Earth scientists must be able to describe the exact positions or locations of features
on the Earth’s surface. - Positions often include distances and directions. To determine direction, you can use
a compass, which has a tiny magnetic needle that points toward the Earth’s magnetic
North Pole. Once you have found north, you can find east, west and south, using your
compass for reference.