Landforms
If you look at the Earth’s surface and take away the water in the oceans, you will see that the surface has two distinctive features, continents and the ocean basins. The continents are large land areas extending from high elevations to sea level. The ocean basins extend from the edges of the continents down steep slopes to the ocean floor and into deep trenches.
Both the continents and the ocean floor have many features with different elevations. Some areas of the continents are high. These are the mountains we have already talked about. Even on the ocean floor there are mountains! Let’s discuss each.
Continents
Continents are relatively old (billions of years) compared to the ocean basins (millions of years). Because the continents have been around for billions of years, a lot has happened to them! As continents move over the Earth’s surface, mountains are formed when continents collide. Once a mountain has formed, it gradually wears down by weathering and erosion.
Every continent has mountain ranges with high elevations (Figure 2.8). Some mountains formed a very long time ago and others are still forming today:
- Young mountains (< 100 million years) – Mountains of the Western United States (Rocky Mountains, Sierra Nevada, Cascades), Mountains around the edge of the Pacific Ocean, Andes Mountains (South America), Alps (Europe), Himalayan Mountains (Asia)
- Old mountains (> 100 million years) – Appalachian Mountains (Eastern United States), Ural Mountains (Russia).
Mountains can be formed when the Earth’s crust pushes up, as two continents collide, like the Appalachian Mountains in the eastern United States and the Himalayas in Asia. Mountains can also be formed by a long chain of volcanoes at the edge of a continent, like the Andes Mountains in South America.
Over millions of years, mountains are worn down by rivers and streams to form high flat areas called plateaus or lower lying plains. Interior plains are in the middle of continents while coastal plains are on the edge of a continent, where it meets the ocean.
As rivers and streams flow across continents, they cut away at rock, forming river valleys. The bits and pieces of rock carried by rivers are deposited where rivers meet the oceans. These can form deltas, like the Mississippi River delta and barrier islands, like Padre Island in Texas. Our rivers bring sand to the shore which forms our beaches.
Ocean Basins
The ocean basins begin where the ocean meets the land. The names for the parts of the ocean nearest to the shore still have the word “continental” attached to them because the continents form the edge of the ocean. The continental margin is the part of the ocean basin that begins at the coastline and goes down to the ocean floor. It starts with the continental shelf, which is a part of the continent that is underwater today. The continental shelf usually goes out about 100 – 200 kilometers and is about 100-200 meters deep, which is a very shallow area of the ocean.
From the edge of the continental shelf, the continental slope is the hill that forms the edge of the continent. As we travel down the continental slope, before we get all the way to the ocean floor, there is often a large pile of sediments brought from rivers, which forms the continental rise. The continental rise ends at the ocean floor, which is called the abyssal plain.
The ocean floor itself is not totally flat. Small hills rise above the thick layers of mud that cover the ocean floor. In many areas, small undersea volcanoes, called seamounts rise more than 1000 m above the seafloor. Besides seamounts, there are long, very tall (about 2 km) mountain ranges that form along the middle parts of all the oceans.
They are connected in huge ridge systems called mid-ocean ridges. The mid-ocean ridges are formed from volcanic eruptions, when molten rock from inside the Earth breaks through the crust, flows out as lava and forms the mountains.
The deepest places of the ocean are the ocean 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. 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 midocean 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.
EmoticonEmoticon