Plate interactionsAs the tectonic plates move about on the asthenosphere, they

interact with one another at their boundaries. There are three types of boundaries: (1) divergent, where plates move apart from each other, (2) convergent, where plates move toward each other, and (3) transform, where plates slide alongside each other.

Three types of plate boundaries

Constructive margins (diverging boundaries)

Where two plates are moving apart from each other, molten rockfrom the mantle wells up in the space between the plates and hardens to form new crust, usually in the form of an ocean ridge (such as the Mid-Atlantic ridge). The newly formed crust accumulates on either side of the ocean ridge, causing the seafloor to spread; the floor of the Atlantic Ocean is growing by5cm or 2 inches each year because of the welling-up of new material at the Mid-Atlantic Ridge.

Destructive margins (Converging boundaries)

Where two plates are moving towards each other, the denser of the two plates may be forced under the other into a region calledthe subduction zone. The descending plate melts to form a body ofmagma, which may then rise to the surface through cracks and faults to form volcanoes. If the two plates consist of more buoyant continental crust, subduction does not occur. Instead, the crust crumples gradually to form ranges of young mountains, such as Himalayas in the Asia, the Andes in South America, and the Rockies in North America. This process of mountain building is termed orogenesis.

Conservative margins (Transform Fault)

Sometimes two plates will slide past each other –an example is the San Andreas Fault, California, where the movement of the plates sometimes takes the form of sudden jerks, causing the earthquakes common in the San Francisco- Los Angeles area.Most of the earthquake and zones of the world are found in regions where two plates meet or are moving apart.

Source: Fundamentals of Earth science

Internal Structure of the earthThe Earth’s crust. Continents and ocean basins (lands

beneath the oceans) are part of a rocky “skin” that surrounds themain body of the earth. This skin is called the earth’s crust.

The thickness of the crust varies from 8 km under the oceans to about 40 km under the continents. Temperatures within the deepestparts of the crust may reach 870 degrees Celsius- hot enough to melt rocks.

The crust is made up of three kinds of rock; igneous, sedimentary, and metamorphic. Igneous are formed when melted rockdeep inside the crust cools and hardens- or erupts at the surfaceas lava. Sedimentary rocks develop from materials that once were part of older rocks or of plants or animals. These materials wereworn away from the land. They then collected in low places, layerupon layer, and hardened into rock. Many sedimentary rocks contain shells bones other remains of living things. Such remains, or the impressions o remains in sedimentary rocks, are called fossils. Metamorphic rocks are formed deep in the crust when igneous and sedimentary rocks are changed by heat and the weight of the crust pressing on them. Rock formation is a slow process that has been occurring throughout geologic time.

All rocks on the earth’s surface are made of minerals, the most common solid materials found on the earth. Minerals are themselves made up of basic chemical substances called elements. Rocks in the earth’s crust consist mostly of two elements- silicon and oxygen. The next most common elements in the earth’s crust are aluminum, iron, calcium, sodium, and magnesium- in thatorder.

The earth’s crust consists of the continental crust and the oceanic crust. The continental crust makes up the continents. It is thick, and it consists largely of granite, a type of hard, coarsely crystalline rock. The oceanic crust forms the oceanic floors. It is thin, and it is made up of several igneous rocks, including a hard, black, and finely crystalline rock called basalt. The bottom of the earth’s entire crust is called Mohorovičić discontinuity or Moho. It marks the boundary between the crust and the inner parts of the earth.

Beneath the earth’s crust are the mantle, the outer core, and the inner core. Scientists learn about the inside if the earth by studying how waves from earthquakes travel through the earth.

Beneath the crust, the earth is a sphere of hot rock metal. By studying the records of earthquake waves, scientists have learned that the inside of the earth is divided into three parts:the mantle, the outer core, and the inner core.

The mantle is a thick layer of rock below the crust. It goesdown about 2,900 km. the rock in the mantle is made of silicon,

oxygen, aluminum, iron, and magnesium. The upper part of the mantle has a temperature of about 870°C. This temperature gradually increases down through the mantle to about 4400°C wherethe mantle meets the next lower section, the outer core.

The outer core begins about 2,900 km below the earth’s surface. Scientists believe the outer core is about 2,250 km thick and is made of melted iron and nickel. The temperature of the outer core ranges from about 4500°C in the uppermost parts toabout 6300°C in the deepest parts.

The ball shaped inner core lies within the outer core and makes up the center of the earth. The boundary between the outer and inner cores is about 5150 km below the earth’s surface. The center of the inner core is about 1300 km below the earth’s surface. Scientists believe the inner core consists of solid ironand nickel. The temperature there may be as high as 7000°C. At normal pressures, these metals would be vapours.

Source: The world book encyclopedia