The interior of the earth is divided into crust, mantle, and core based on composition and depth. The crust is the uppermost shell of the earth, it is primarily (roughly 95%) made up of igneous rock and metamorphic rock with an overall composition between intermediate and felsic. The remaining 5% is made up of sedimentary rock, which is dominated by mudstone. The mantle, which is the second major part of the earth, includes several layers, all with the same overall ultramafic composition. The upper mantle extends from below the Mohrovicic discontinuity up to a depth of 1000 km and is typically composed of peridotite, a rock dominated by olivine and pyroxene.
The lower mantle extends from 1000 km to the core boundary (Gutenberg discontinuity) and it has a similar chemical composition, but different minerals because of the extreme pressures that are present. The core is the innermost part of the earth. Core constitutes around 17% of the volume and 34% of the mass of the earth. It is separated from the mantle by the Gutenberg discontinuity, and is composed of iron, with lesser amounts of nickel (about 5%) and several percent oxygen. It is extremely hot (roughly 3500° to 5000°C). The outer core is liquid while the inner core is solid—even though it is hotter—because the pressure is so much greater at that depth (Mahapatra, 2018).
The Seismic Structure of the Earth’s Interior
Crust: The seismic structure of the continental crust is variable, but it has an average thickness of 35 km and an average P-wave velocity of about 6.5 kms−1. The oceanic crust is thinner, 7–8 km thick, with an average P-wave velocity of more than 6 kms−1
Mantle: Since primary and secondary waves record a definite increase in their velocities with depth, it is logical that material of the mantle is denser than that of the crustal rocks. The normal P-wave velocity at the top of the mantle is 8.1 km s−1. The uppermost mantle is very heterogeneous, its structure being dependent upon plate processes and history. Standard velocity models vary in representation of the uppermost mantle depending upon the data used and the assumptions made.
Core: At the core–mantle boundary (Gutenberg discontinuity) the P-wave velocity drops from about 13.7 to about 8.1 km s−1, and also the S-wave velocity drops from about 7.3 km s−1 to zero. This structure is determined by the strong reflections and so on. The P-wave velocity increases slowly through the outer core until the boundary of the inner core.
The Temperature of Earth’s Interior
The internal temperature of the earth increases with depth. Thus, the rate of increase is not linear. The temperature gradient is around 15° to 30°C per km within the lithosphere (upper 100 kilometres); it then drops off dramatically through the mantle and increases more quickly at the base of the mantle, and then increases slowly through the core. The temperature is around 1000°C at the bottom of the crust, around 3500°C at the base of the mantle, and around 5,000°C at centre of the earth (inner core). The temperature gradient within the lithosphere is quite variable depending on the tectonic setting and are lowest in the central parts of continents, higher in the vicinity of subduction zones, and higher still at divergent boundaries (Steven Earle, 2019).
Sources of heat of Earth’s interior
The heat of interior of the earth comes from two main sources, each contributing about 50% of the heat. One of those is the frictional heat left over from the collisions of large and small particles that formed Earth in the first place, plus the subsequent frictional heat of redistribution of material within Earth by gravitational forces. The other source is radioactivity, specifically the spontaneous radioactive decay of the isotopes 235U, 238U, 40K, and 232Th, which are primarily present in the mantle. The total heat produced that way has been decreasing over time, and is now roughly 25% of what it was when Earth created. This means that the interior of the earth is slowly becoming cooler (Steven, 2019).
Steven Earle, (2019). Physical geology. BCcampus pub 2nd ed. ISBN 9781774200285.
Mahapatra G. B. (2018). The text book of Physical Geology. CBS pub. ISBN 8123901100.
Mr. Aliyu Mohammed Lawan is a Lecturer II at the Department of Geology, Skyline University Nigeria. He holds a Master’s degree in Applied Geophysics from Ahmadu Bello University Zaria, Nigeria.