The Continents

The most abundant source of sediment is the terrigenous material. The biogenous sediment is next, but it takes a thousand years to get about 1 centimeter of this material on the floor of the ocean. Hydrogenous and cosmogenous sediments are much less common. The hydrogenous sediments may be from manganese and other salts that don't dissolve well in certain types of water. Cosmogenous sediments come from asteroids or possibly comet remains.

THE CONTINENTS

There are seven continents now, these are considered the major continuous landmasses around earth. These are permanent as far as we are concerned but were not very permanent in the past. You’ve studied 7 continents, but Europe and Asia are often combined to form Eurasia when it comes to geology. Australia is part of a larger region of islands collectively called Oceania, which includes the Pacific Islands. About 29 percent of the earth is covered with the continents. There is more continental earth above the equator than below it. Australia is confusing; it is a big island and a small continent, at least 4 times bigger than Greenland. Greenland is our largest island, part of North America.

Continental crust is different from oceanic crust. It has more granite in it that contains a lot of aluminum silicate material. Oceanic crust is mostly basalt or hardened lava-like material. As mentioned, the continental crust is less dense overall and is older than the crust in the oceans. Expect the crust in the continents to be about 40 kilometers thick compared to only 6 kilometers thick in the ocean depths.

The lithosphere is divided into plates that began much smaller. The continents are the older centers of these plates. Most plate boundaries are not near the edges of the continents themselves. The study of plate tectonics is how these plates move, shift, and subduct over time.

All continents have flattened areas and mountainous ones. Shields are a major part of Africa but not as much as you'll find in Asia, which is more mountainous. Each continent has its own unique climate that affects everything else about it.

Cratons are the oldest nuclei or beginnings of each of the major continents. These areas are also called basement rocks dating from the Precambrian periods. There are actually numerous cratons that formed at nearly the same time. Look for them near the interior parts of continents and expect few earthquakes in these regions.

Cratons have shields and platforms as part of them. Shields are exposed areas of ancient rock, such as you see in the Canadian Shield. Look for the oldest rocks here. There is also the Amazonian shield in South America, the Indian Shield and Angaran shields in Asia, the Baltic Shield in Europe, the African Shield in Africa, and one in Antarctica called the Antarctic Shield. Figure 8 shows Precambrian rocks on the Canadian shield:

Figure 8.

In North America, the Canadian shield is located along its eastern half to include Ontario, parts of Manitoba, Quebec, and just the extreme northern parts of the US central states of Minnesota and Michigan. While the shields are sloping in general, the platforms are the plains next to them. There is younger sedimentary rock overlying the

older basement rock. Some of this sediment occurred when the areas were covered with shallow seas millions of years ago.

Mountain belts are also parts of most continents. These are narrow bands of mountains, such as you'll see in Andes and Rocky Mountain Ranges. These are areas where the crust has crumbled up and folded. Most are seen near plate boundaries and not in the middle of the continent. The mountains come when two plates slowly collide with one another. Larger mountains come where two areas of continental crust collide versus when continent collides with oceanic crust. The mountains around the world have different ages; some have eroded already, such as the Appalachians, while the Himalayans are still being built.

Fractures can develop where crust falls into spaces that open up in the existing crust. You might see these called rift valleys, existing across any of the great shields. The Great Rift Valley in Kenya and similar rift valleys show some areas where there is upward movement of lava through the crust, forming flattened fields of lava. You can actually get eruptions in these areas as well. Figure 9 shows this Great Rift Valley:

Figure 9.

Volcanic Plateaus are flat basaltic rock areas where you'll actually see these vast lava flows in the middle of nowhere, such as the Deccan Traps of India and the Ethiopian Plateau located in Africa. Volcanic belts are narrow regions where there are a lot of volcanoes forming cone-shaped mountains like you might see near the edges of some continents or in island arcs in the ocean that form island chains. Mount St. Helens, Mount Fuji, Mount Hood, and Mount Rainier are all formations made from volcanoes rather than pushed up tectonic plates.

Continents will extend below the surface of the ocean as well. Some continental margins are very large – hundreds of thousands of kilometers long and comprising 15 percent of the earth's ocean surface area. You can expect a few to exist 2 kilometers below the ocean surfaces. The continental shelf, the continental rise, and the continental slope are all parts of the continents. The official edge of a continent is then usually at the continental rise.

The continents probably started as a chemical change in the earth's crust. Basalt from volcanoes is much denser than the crust of the continents. There were chemical changes that occurred to cause the crust to be of a structure with decreased density so it would float much higher on the mantle compared to oceanic crust. The continental crust started to be more stable around 4 billion years ago. Pieces of land accreted to other pieces. Accretion is just another term for sticking together of rocks. Africa itself was made from 5 separate cratons.

Subduction zones are where the oceanic crust sinks below other rocky layers or plates. These are necessary because lava or magma comes up to make new ocean floor. The edges of the ocean floor must go somewhere to make room for the new floor. Ocean basins are young while the edges are older. Continental crust is too stable to be lost during subduction. Figure 10 shows a theoretical subduction zone in what you would call a convection current of magma and rock:

Figure 10.

Isostasy is a geologic term used to describe the fact that the lighter and bigger parts of the earth's crust rise, while the denser and thinner parts sink. Mountains are large partly because they erode from the top and not the bottom. This causes them to eventually become flat plains. Isostasy means that there is a balance between the upper weight of the continental crust and the lower weight beneath the earth's surface.

When Pangaea finally broke up, it made Laurasia to the north and Gondwana to the south. Laurasia made most of the northern hemisphere, except the Arabian area and India. Gondwana formed the southern hemisphere continents. Gondwana fractured 94 million years ago, and Laurasia fractured 50 million years ago.

We will talk more about continental drift in a later chapter. It was proven to be true by the fact that magnetic patterns frozen in rocks are different in orientation from other rocks. The magnetic parts of rocks are aligned at the time they were first solidified. In today's time, our current GPS systems can detect this drift.