Lecture 4: Basic Structure of the Earth

January 30, 2020

1) The Earth is approximately 4.5 billion years old. The Earth has changed enormously during its history and today it is very different than it was when during its "infancy."

2) Earth history has 2 basic periods, a period of formation followed by a period of differentiation.

3) Earth formation occurred over a period of roughly 500 million years by the gravitational accretion of cosmic material -- mostly through meteorite impacts.

• The Earth formed as an undifferentiated mass whose composition reflected the meteorites that accreted to form it.

• Impacts were frequent at first, and have tapered off as the primordial matter of the solar system has clustered into the planets and other bodies.

• Meteorite impacts still occur, but at a very low rate compared to the early history of the Earth.

• The study of the earliest history of the Earth, described above, is one of the realms of astronomers and planetary geologists.

4) Starting about 4 billion years ago, Earth became differentiated into successive envelopes of distinct compositional layers.

• This process of differentiation was driven largely by heat from short-lived radioactive isotopes (e.g., ²⁶Al), and from meteorite impacts.

• This process is ongoing - "The Living Earth".

• The study of the period of Earth differentiation is the realm of geologists and paleontologists.

5) The differentiation of Earth produced five basic compositional layers. From outer-most to inner-most, these are:

1. ATMOSPHERE - the outermost layer; the gaseous envelope that surrounds the Earth; this is the least dense layer of the Earth; the maximum thickness of the atmosphere is 96 km.

2. HYDROSPHERE - includes the oceans, seas, lakes, rivers, etc., which float on the denser solid crust and saturate many porous parts of the crust; also the porous outer parts of the crust; the depth ranges up to 6 km.

3. CRUST - the thin outer shell of the solid Earth, approximately 5 to 40 km thick.

4. MANTLE - surrounds the core; is less dense than the core; its radius is approximately 2600 km.

5. CORE - located at the center of the Earth; the densest region of the Earth; composed largely of iron and other dense metals; its radius is about 3500 km.

6) The physical properties of the inner layers of the earth change at different depths within the Earth, due to increases in heat and pressure.

• The LITHOSPHERE consists of the crust and the uppermost mantle, and averages 100 km in thickness.

• Below the lithosphere, the outer-most layer of the mantle is known as the ASTHENOSPHERE. This is a molten or partly molten layer which churns in slowly rolling convection currents whose motion is driven by heat moving outward from the core.

• The MESOSPHERE is beneath the asthenosphere and is a region of high temperature and rock strength.

• The metallic core is separated into two parts. The OUTER CORE is zone in which temperature and pressure are balanced in such a way that the iron is molten and exists as a liquid. Currents of liquid movement within the outer core produce a magnetic field whose flux reaches outwards beyond the Earth's surface.

• The INNER CORE is under such great pressure that the iron is solid despite the enormous temperature.

7) The lithosphere is not a smooth, continuous 'skin' around the Earth; it instead is formed from a series of semi-rigid tectonic plates.

• The continuous flow of heat outward from the core and inner mantle cause convection of the asthenosphere. This in turn causes movement and deformation of the overlying tectonic plates.

• Plate movement is driven by the extrusion of magma from spreading zones located in the floors of the oceans.

• In areas where crustal plates collide, the plate margins are deformed, often producing mountains and deep oceanic trenches that are associated with volcanoes and earthquakes.

8) The continued production of heat by radioactive decay in the core and inner mantle has resulted in continuous evolution of the Earth and change in the structure and composition of its layers.

9) Through tectonic movement, throughout the period of Earth differentiation, there has been continual remodeling of the surface of the Earth.

10) TECTONICS is the study of the formation of Earth's major structural features by thermal convection in the asthenosphere and subsequent deformation of the lithosphere.

11) The appearance of Life about 4 billion years ago has also changed many aspects of the crust, hydrosphere, and atmosphere.

12) Life history has been played out on the lithosphere, and in the hydrosphere and atmosphere. It is profoundly influenced by processes occurring in the asthenosphere and deeper layers of the mantle and core. Earth history and Life history are thus intricately linked.

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