PLANET EARTH

OCEAN FLOOR

Just a century ago, the ocean floor was largely unknown. Now we know that the deep oceans have features such as mountains, deep valleys, and vast plains. Many of these are formed by the movement of the tectonic plates that make up Earth’s crust. Far below the ocean’s surface, volcanic mountain chains are rising in mid-ocean zones where plates pull apart. Elsewhere, deep trenches descend in subduction zones where plates collide and one dives below the other.

HYDROTHERMAL VENTS

In 1977, scientists used submersible vehicles to explore the seabed and discovered vents gushing dark plumes of superhot, mineral-rich water. These black smokers, are caused by volcanic activity at mid-ocean ridges. Water entering cracks in the crust is heated by magma and mixed with mineral sulphides, then belched forth in dark clouds.

ISLANDS

Islands are land masses entirely surrounded by water. They are found in oceans, seas, rivers, and lakes. Islands vary in size from tiny rock outcrops to vast areas such as Greenland, which covers 2.2 million sq km (840,000 sq miles). There are two main types of island: oceanic islands which are remote from land; and continental islands, which often lie close to the mainland. Many oceanic islands are volcanoes. Continental islands are often formed by changes in sea level.

CONTINENTAL ISLANDS

Continental islands, such as the British Isles, rise from the shallow waters of continental shelves, which fringe the world’s continents. Often these islands were once part of the mainland, but were cut off when sea levels rose to flood the land in between. Smaller islands, called barrier islands, sometimes form off coasts where ocean currents or rivers deposit sand or mud.

CORAL ISLANDS

Coral islands, such as the Maldives in the Indian Ocean, are composed of the limey skeletons of coral polyps. Large colonies of these anemone-like creatures thrive in the warm, shallow waters off tropical coasts or around seamounts. The polyps’ soft bodies are protected by cup-shaped shells, which grow on top of one another to form rocky reefs that eventually break the surface. If the seamount subsides, just a ring of coral, called an atoll, may be left.

ISLAND ARCS

Oceanic islands are often formed by volcanic eruptions when plates collide. As one plate is forced below another, its crust melts in the red-hot mantle below. This molten rock rises up again to burn through the crust and erupt on the sea floor. Over time, the erupted rock forms a tall seamount and eventually breaks the surface as an island.

RIVERS

Rivers drain the surrounding land, carrying water that falls as rain and snow down to the sea. As rivers flow, they erode (wear away) rock, breaking it into fragments, called sediments, that are carried downstream. Most erosion occurs when rivers flood after heavy rain or as mountain snows melt in spring. Over time, erosion creates valleys and waterfalls, and sediments form land areas called flood plains and deltas.

RIVER’S COURSE

A river has three main stages. In the first, the river is steep and narrow and its flow is rapid and rough. In the second stage, it is wider, less steep and flows more smoothly through flat-bottomed valleys. In the third stage, the river is broad and flows placidly across flat coastal plains to the sea, where it drops its sediment.

LIMESTONE CAVES

When water flows over some rocks, such as limestone, caves may be formed by a process called chemical weathering. Water seeps into cracks and gradually dissolves the rock, widening the cracks until, over thousands of years, the limestone becomes riddled with caves and passageways. Water flowing through caves forms underground streams, rivers, and pools (such as this one in Mexico). Surface rivers disappear into sink holes and reappear many kilometres away. Eventually a cave roof may fall in, creating a gorge.

Source:

factmonster.com

Other Links:

PLANET EARTH: EROSION

PLANET EARTH: SOIL