The Earth has a layered structure, including the core, mantle and crust. The crust and upper mantle are cracked into large pieces called tectonic plates. These plates move slowly, but can cause earthquakes and volcanoes where they meet.
The Earth’s atmosphere has changed over billions of years, but for the past 200 million years it has been much as it is today.
The structure of the Earth
The Earth is almost a sphere. These are its main layers, starting with the outermost:
crust – relatively thin and rocky
mantle – has the properties of a solid, but can flow very slowly
outer core – made from liquid nickel and iron
inner core – made from solid nickel and iron
Note that the radius of the core is just over half the radius of the Earth. The core itself consists of a solid inner core and a liquid outer core.
Detail of each layer of the Earth is given here under;
The outermost part of the earth, this is what we walk around on is made of cold, brittle and relatively light material. The crust ranges from 5 to 70 km in depth. Under the continents the crust averages about 30-40 km thick (more under tall mountains, somewhat less in other areas) and under the ocean, it averages about 5 to 6 km thick. The thin parts are oceanic crust composed of dense (mafic) iron magnesium, silicate rocks and underlie the ocean basins. The thicker crust is continental crust, which is less dense and composed of (felsic) sodium, potassium, aluminium, silicate rocks. The crust-mantle boundary occurs as there is a discontinuity in the seismic velocity, which is known as Mohorovicic discontinuity or Moho.
Types of Crust:
There are two types of Crust. The continental and the oceanic crust.
a. Continental Crust:
It averages about 35 km thick; 60 km in mountains ranges. Continental crust is on average older, more silica-rich and thicker than oceanic crust. The oldest part of the continental crust, now as ‘shields’ or ‘cratons’, includes some rocks that are nearly 4 billion years old.
b. Oceanic Crust:
It averages from 5- 12 km. Oceanic Crust underlies most of the two-third of the earth’s surface, which is covered by the oceans. The ocean floor is the most dynamic part of the earth’s surface. As a result, no part of the oceanic crust existing today is more than 200 millions years old., Which is less than 5% of the age of the Earth itself. New oceanic crust is constantly being generated by sea-floor spreading at mid-oceanic ridges., while other parts of the oceanic crust are being recycled in the mantle at subduction zones.
Earth’s mantle extends to a depth of 2900 km, making it the largest layer of the Earth. The mantle is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust. Although solid, the temperature within the mantle causes of silicate material to be sufficiently ductile that it can flow on very long time and low speed. Convection of the mantle is expressed at the surface through the motions of tectonic plates. The melting point and viscosity of a substance depends on the pressure it is under. As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of the mantle flows less easily than does the upper part of the mantle. The mantle makes up of about 70% of earth’s mass and about 45% of its radius. The bulk of the lower mantle is termed the mesosphere and is stronger than the Asthenosphere. Following is the division of mantle.
a. Upper mantle:
Upper mantle is subdivided into two main parts namely :
Lithosphere:::Outermost 100 km of Earth. Is consists of the crust, Solid in state, plus the outermost part of the mantle which has got same physical state as that of overlying crust.
Asthenosphere:::Low velocity zone at 100 -670 km deep in earth (Seismic wave velocity decreases). Rocks are at or near melting point. Magma is generated here. Solid that flows as plastic behavior. Convention in this layer moves tectonic plates. It has got viscous state because of molten magma.
b. Lower Mantle (Mesosphere):
A large fraction of earth’s interior, from a depth of about 670 km down to 2900 km, consists of the lower mantle, which is composed chiefly of magnesium- and iron-bearing silicate. It is the largest continuous region of the earth’s interior and make up around half of the earth. It has got 2200 to 3700 Degree Celsius temperature range.
The average density of our earth is 5515 kg/cubic meter, making in the densest planet in the solar system. Since the average density of the surface material is only around 300kg/cubic meter, we must conclude that denser material exist within earth’s core. Further evidence for the high density core come from the study of seismology.
Seismic measurements show that the core is divided into two parts, a solid inner core with a radius of ~1220 km and a liquid outer core extending beyond it to a radius of ~3470 km. In early stage of the earth’s formation, about 4.5 million years ago, melting would have caused denser substance to sink toward the center in a process called planetary differentiation while less-dense material would have migrated to the crust. The core is thus believed to largely be composed of iron (80%), along with nickle and one or more light elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or tend to bind to lighter element and thus remain in the crust. The density of inner core and outer core combined has been calculated as 12.5g/cubic cm, which compensates the lightness of the crust (2.8 g/cubic cm) and accounts for density of the earth as a whole which is 5.5 g/cubic cm.
a. Outer Core (2250 km thick):
S-waves cannot pass through outer core, therefore, we know the outer core is liquid (molten). The outer core is a layer of molten metal that surrounds the inner core. The outer core lies at a depth from 2900 km to 5150 km. It comprises 30.8 percent of earth’s mass. With temperature ranging from 400 degree celsius to 6100 degree celsius near the boundary with the inner core, it is composed of Molten Fe(85%) with some Nickle and some lighter elements such as Silicon, Sulfur, Carbon and oxygen. Convection in liquid outer core plus spin of solid inner core generates earth’s magnetic field.
b. Inner Core (1220 km Radius):
The inner core is the center of planet and is the hottest part of the planet. The inner core was discovered Inge Lehmann in 1929. Inge Lehmann was studying a large New Zealand earthquake. The vibration that Inge Lehmann was studying seemed to be moving across something solid in the center of the planet. She wrote about this inner core for many years but it was not proved to exist until 1970, when studies were more exact. The inner core is believed to be made of a molten iron and nickel, but the earth is so heavy that the inner core does not move like a liquid. Scientists believe that the inner core may be hotter than the surface (adge) of the sun.