1. Earth’s Core:
The core has about 33% of earth’s entire mass, but only a sixth of its volume. It is divided into two regions – inner core and outer core – divided by a transition zone of several hundred kilometers. The estimated temperatures of the core range from 3000°C to about 6650°C.
(i) The Inner Core:
The inner core is thought to be solid iron. It remains solid despite its high temperature because of tremendous pressure exerted on it. Its density ranges from 13.3 to 13.6. It extends from a depth of 5,150 km to the centre of the earth. This inner core was first noted by the lady seismologist Lehmann of Denmark in 1936.
(ii) A transition zone between the inner and outer cores, whose thickness is estimated at about 450 km or so.
(iii) The Outer Core:
Its density ranges from 10 to 12.3. It extends from a depth of 2900 km. to about 4950-5150 km. It is thought to be liquid metallic; the metallic constitutions comprise iron and nickel.
Lighter substances like silicon are also present, though in small proportion. The fluid outer core generates at least 9-0% of the earth’s magnetic field and the magnetosphere that surrounds and protects the earth from the solar wind.
The junction between the outer core and the lower mantle, located at a depth of about 2,900 km. has been called Gutenberg or Oldham Discontinuity. Though R.D. Oldham had marked this discontinuity in 1906, B. Gutenberg confirmed the same in 1954.
It is divisible into two zones:
It represents about 80% of earth’s total volume. The mantle is rich in oxides and silicates of iron and magnesium, and is denser at depth, becoming less dense toward the surface.
(i) Lower Mantle:
It extends from a depth of about 2900 km. to 700-1000 km. or so. It comprises about 50% volume of the mantle. It is composed of rock that has never been at the earth’s surface.
(ii) Upper Mantle:
The upper mantle is separated from the lower mantle by a broad transition zone of several hundred kilometers. It extends from a depth of about 700-1000 km. to the Moho or Mohorovicic discontinuity occurring at a depth of about 30 to 40 km.
It may be noted that Moho occurs at an average depth of 35 km below the continents and at about 10 km below the oceans. The density ranges from 3.3 to 4.6 or so. The upper mantle is composed of dense ultramafic rocks consisting of pyroxene and olivine minerals.
The upper mantle has been divided into three distinct layers. The outermost layer lies just below the crust. The thickness of this high velocity zone is roughly 45-70 km.
The seismic waves are transmitted rapidly through this rigid, cooler layer. It is to be noted that this uppermost layer of the mantle along with the crust, makes up the lithosphere.
Below the lithosphere, from about 70 km down to 250 km is the plastic layer known as the asthenosphere. It contains pockets of increased heat from radioactive decay.
Slow convective currents are produced in these hotter and less dense materials. It is also known as the plastic layer, the least rigid portion of the earth’s interior.
It is interesting to note that the asthenosphere is the least rigid region of the mantle. It contains almost all the hot spots. It is in this zone that the rate of temperature rise overcomes the effect of increasing pressure.
Hot spot is an individual point of upwelling material originating in the asthenosphere. Such a point tends to remain fixed relative to migrating plates. Some 100 hot spots are identified the entire world over.
Hot spots are exemplified by Yellow-Stones National Park, Hawaii, and Iceland. Hot spots represent the localities on the earth’s surface where greater than average thermal activity takes place, often leading to volcanic eruptions. These points are closely related to the mechanisms of plate tectonics.
In asthenosphere partial the occurs melting and flow.
3. Earth’s Crust:
The crust consists of two layers: the sima and sial. The crust together with the uppermost part of the mantle forms a single rigid unit above the asthenosphere that is called the lithosphere.
This composite layer i.e. the lithosphere averages 100 km. thick increasing to over 300 km. thick beneath some continents and decreasing to only 45 km thick beneath some ocean ridges.
The boundary between the crust and the high-velocity portion of the lithosphere upper mantle is another discontinuity is called Moho. The depth of Moho ranges from 5 to 7 km beneath the oceanic crust and 45 to 70 km from the surface in the folded mountain areas.
Crustal areas below mountain masses extend farther downward to 50-60 km, whereas the crust beneath continental interior averages about 34 km in thickness and oceanic crust averages only 11 km.
Continental crust is basically granite. It is crystalline and high in silica, aluminum, potassium, calcium and sodium. Continental crust is called sial (shorthand for silica and aluminium). Oceanic crust is basalt. It is granular and high in silica, magnesium and iron. Oceanic crust is called sima – silica and magnesium.
In the crust density ranges from 2.7 to 2.9. Siliceous rocks are predominant in this layer.
The crust makes up only 1 percent of the volume of the earth. The chemical composition of the shallow portion of the earth’s crust can be investigated directly.
The eight elements, i.e. oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium makeup 98.6 percent of the earth’s crust by weight. The remaining naturally occurring g elements makeup only 1.4 percent of the crust.
Temperatures throughout the crust increase at a rate of l°C/30m of depth. Some of this heat is released due to the tremendous pressure of rocks at great depths, but most of this heat is produced by the radioactive decay of unstable elements.