Age, Structure and Composition of the Earth
Geology is the study of the Earth, its rocks and minerals and the dynamic processes involving their formation and change. Geological events affect us all to a greater or lesser extent even if they occur halfway around the world.
The Earth, the Sun, and the rest of the solar system, was formed four and a half billion years ago by the settling of a rotating mass of dust and gas. The immense amount of heat energy released from gravitational energy and the decay of radioactive elements melted the entire planet.
It is still cooling. Denser materials like iron sank into the core of the Earth, while lighter compounds, and water rose near the surface. The oldest Earth rocks have been determined to be 3.96 billion years old. Older ages have been calculated for moon rocks (4.5 to 3.2 billion years) and meteorites (4.5 billion years}.
Meteorites fall into three groups. Iron meteorites are composed of an iron-nickel alloy. These comprise 10% of all meteorites and give us hints as to the composition of the core of the earth. These grade composition-wise through stony-iron to stony meteorites depending on the ratios of the nickel-iron complex to lighter silicate minerals
Size of Earth: Radius = 6370 km Diameter = 12,740 km
Structure and Composition
The earth is divided into four main layers: the inner core, outer core, mantle, and crust. The outer core (2270 km thick) is composed mostly of iron and lesser amounts of nickel and sulphur. It is extremely hot and molten.
The inner core also made up of iron and nickel (1216 km radius) is under such great pressure that it remains solid. Evidence for an iron-nickel core comes from a combination of the content of meteorites, measurements of the earth’s density, and the seismic velocity of the earth’s interior.
Most of the Earth’s mass is in the mantle (2885 km thick), which is composed of iron, magnesium, aluminum, silicon, and oxygen silicate compounds. The mantle is solid, but plastic and consists of several concentric layers. The outermost 100 km is called lithosphere. It is thinner beneath the oceans than under the continents.
The next layer down in the mantle is called the asthenosphere. Here the rocks are closer to melting than those both above and below. The base of the asthenosphere is between 200 and 300 km down in the earth.
Next is the mesosphere, which extends from the base of the asthenosphere down to the mantle-core boundary at 2830 km. The crust is the thinnest layer, and is composed of low-density calcium and sodium aluminum-silicate minerals.
The continental crust consists of granite and other igneous, metamorphic, and sedimentary rocks with an average thickness of 30 to 50 km. The crust is rocky and brittle, so it can fracture. The boundary between the crust and mantle is called the Moho.
Isostasy is a balance or equilibrium between adjacent blocks of less dense brittle crust, which float on a denser plastic upper mantle. Rocks of the crust will move vertically to reach isostatic equilibrium in a concept called isostatic adjustment.
As a consequence, thicker areas of the crust will sink deeper into the mantle than adjacent lowland areas. This will produce a balance between the weight of each segment of the crust and adjacent segments because the weight of each column of crust is equal to the weight of the mantle it displaces.
Hence, a heavy segment, such as occupied by a glacier or mountain range can bend the lithosphere down into the asthenosphere, which is displaced causing the heavier segment to sink until it is buoyant.
If the weight of a heavier segment is reduced through erosion or melting of the ice, that segment will rise slowly over thousands of years. This is the process of isostatic rebound.
The temperature increases with depth into the earth. This is called the geothermal gradient. The average increase is about 25°C/km, a rate that decreases to 1°C/km within the mantle.
Heat flow explains the small amount of heat that is lost gradually through the earth’s surface. One source heat of this heat may be from the earth originally being formed as a hot mass that is cooling very slowly. The flow of heat could also be as a result of radioactive decay of isotopes.
A third possible source may be due to the rising of hot mantle material along plate boundaries under the ocean crust.