Composition of earth

The crust is composed of two basic rock types granite and basalt. The continental crust is composed mostly of granite. The oceanic crust consists of a volcanic lava rock called basalt. Basaltic rocks of the ocean plates are much denser and heavier than the granitic rock of the continental plates. Because of this the continents ride on the denser oceanic plates. The crust and the upper layer of the mantle together make up a zone of rigid, brittle rock called the Lithosphere. The layer below the rigid lithosphere is a zone of asphalt-like consistency called the Asthenosphere. The asthenosphere is the part of the mantle that flows and moves the plates of the Earth.

The crust

• The mantle is approximately 2900 kilometers thick, making it Earth's largest layer. The mantle has a property called "plasticity" (where a solid has the ability to flow like a liquid). You might call the mantle "partially molten". Remember that the temperature of the mantle increases the deeper you go. This difference in temperature causes CONVECTION CURRENTS to form. This type of current forms when hot things rise and cooler things sink. These convection currents tumble throughout the mantle. They cause the Lithospheric plates floating on the mantle to move around. These currents cause our continents and oceans to change location slightly each year. The currents are the driving force for Plate Tectonics or Continental Drift, which we will discuss in more detail in a later section. The forces which drive continental drift seem to come from the mantle. The hot rock, which boils up at mid-ocean ridges, comes from the upper mantle. This rock spreads out forming new oceanic plates. When these meet the continents they plunge back down into the mantle, sometimes going down as far as the outer core.

• In addition there are hot spots, which start at the outer core and rise up through the mantle to form islands such as Hawaii or Iceland.

Convection Currents - Large convection systems in the mantle may carry along the plates of the lithosphere like a conveyor belt

Outer Core & Inner core

Inner core

The inner core of the Earth has temperatures and pressures so great that the metals are squeezed together and are not able to move about like a liquid, but are forced to vibrate in place as a solid. The inner core begins about 4000 miles beneath the crust and is about 800 miles thick. The temperatures may reach 9000 degrees F. and the pressures are 45,000,000 pounds per square inch. This is 3,000,000 times the air pressure on you at sea level!!!

Composition of earth

The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km):

• 0 - 40 Crust
• 40 - 400 Upper mantle
• 400 - 650 Transition region
• 650 - 2700 Lower mantle
• 2700 - 2890 D'' layer
• 2890 - 5150 Outer core
• 5150 - 6378 Inner core

Composition of earth

The internal structure of Earth

Model for Structure of Earth

Composition of earth

Layers of the earth based on composition

1) Outer Crust

· Outer layer of the Earth, extending from solid surface down to the first major discontinuity in seismic wave velocity in the lithosphere. Thickness of crust varies from about 8 km under the oceans to about 35 km under the continents.

· There are two kinds of earth crust classified according to two different kinds of rock they contained where each with its own general composition, thickness and density.

(a) Continent Crust: 35 - 60 km thick

relatively low density

granitic rock

average density: 2.8 g/cm3

(b) Oceanic Crust : thickness rarely exceed 5 km

denser material

basaltic composition

average density: 2.9 g/cm3

2) Mantle

• The next major compositional layer of the Earth which covers the core and this zone constitute 82% of its volume and 68% of mass of the Earth. The mantle is composed of iron and magnesium silicate rock, and it goes down to about 2900 km from surface of Earth. The earth's mantle is a heavy layer of rock which lies under the earth's crust. In places where the crust is broken, part of the upper mantle can melt and form a substance called magma, or molten rock. Magma can be forced through the earths crust. It flows out of volcanoes as lava.
• Average density: 4.5 g/cm3

3) Outer core

· It is speculated that the thickness is about 2250 km and it is made of molten iron and nickel. The outer core is so hot that the metals in it are all in the liquid state.

· Average density: 10.7 g/cm3

4) Inner core

· The thickness' is about 1300 km and probably consists of mostly iron and nickel. The inner core of the Earth has temperatures and pressures so great that the metals are squeezed together and are not able to move about like a liquid, but are forced to vibrate in place as a solid. The temperatures may reach 9000 degrees F. and the pressures are 45,000,000 pounds per square inch. This is 3,000,000 times the air pressure on you at sea level!!!

· Average density: 17.0 g/cm3