The Composition of the Earth: A Summary

The reasons that earthquakes and volcanic eruptions take place depend on the nature of the earths interior. This post will hopefully outline how the different elements of the earth are arrange, and a bit about the different sections of structure of the earth too.

Layers of the Earth

The Core

The globe is made up of a core surrounded by concentric shells. The core itself comprises nearly one third of the global mass. The inner core, 5,100-6,400km (3,169 – 3,977 miles) deep has a temperature of about 4,300oC/ It is apparently composed of iron, kept in solid in spite of its heat by the tremendous pressures confining it.

The Outer Core
The surrounding outer core, 2,885-5,100km (1,793-3,170 miles) deep is also composed largely of iron, but with some nickel and a few lighter elements. This outer core is fluid and convection currents keep it in slow but constant motion. These currents and the rotation of the globe make the outer core a self-exciting dynamo that has developed the Earth’s magnetic field. The Earth is a dipole, i.e it has two magnetic poles, North and South. During the planet’s history, however, the poles have, at different times, become reversed. These magnetic reversals have been used to date rocks and have proved to be vitally  important in demonstrating the movement of crustal plates.

The Mantle
The outer core is enveloped by the mantle which comprises the lower mantle and the upper mantle. The lower mantle is 2,235 (1,390 miles) thick and 650-2,885 km (404-1,793 miles) deep. It is solid, hot and held under great pressure. Nevertheless, stresses and strains and convection currents and creep have developed within it. Surrounding the lower mantle is a shell about 500km (311 miles) thick, lying between 60-150km (37-93 miles) and 650 km (404 miles) deep. This shell is composed of a transition zones and the more plastic layers of the upper mantle. Together, they form the asthenosphere (from the Greek asthenos meaning ‘weak’) which reacts to stresses and strains in a fluid way, although it is only partly molten. Convection currents keep the asthenosphere in slow, but continuous, motion and they are thus probably a major driving force behind the movement of the plates across the Earths surface.

The Lithosphere
The outermost shell of the globe is the lithosphere (from the Greek lithos meaning ‘stone’), which can be a s little as 60km (37miles) thick below the oceans, but can reach as much as 150km (93 miles) thick beneath the continents. Like the other shells, the lithosphere is divided into two layers: the lower layer is the solid upper mantle; the upper, outermost layer is the Earth’s crust. The thickness of the crust varies from about 70m (43.5 miles) under the main mountain ranges to only 3km (1.75 miles) along the crests of the mid-ocean ridges. It is an average of 40km (25 miles) thick on the continents, where it is pale in colour, generally granitic in character, with a low density. Under the oceans, on the other hands, it is dense, dark coloured, and basaltic, and averages no more than 6km (3.75 miles) in thickness. This crust that seems so firm to us is thin, brittle and fragile. It accounts for just one thousandth of the volume of the globe.  There is a constant interchange and interaction between the global shells – and especially between the outermost layers. The mains results are the growth, movement and consumption of the global plates, and their most obvious manifestations are earthquakes and volcanic eruptions.

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