The History of the Earth

Origin of the Universe

The universe began about 14.4 billion years ago The Big Bang Theory states that, in the beginning, the universe was all in one place All of its matter and energy were squeezed into an infinitely small point, a

singularity Then it exploded

The tremendous amount of material blown out by the explosion eventually formed the stars and

galaxies After about 10 billion years, our solar system began to form.

The Nebular Hypothesis

In cosmogony, the Nebular Hypothesis is the currently accepted argument about how a Solar

System can form.

• A large gas cloud (nebula) begins to condense
• Most of the mass is in the center, there is turbulence in the outer parts

• The turbulent eddies collect matter measuring meters across
• Small chunks grow and collide, eventually becoming large aggregates of gas and solid chunks
• Gravitational attraction causes the mass of gas and dust to slowly contract and it begins to rotate
• The dust and matter slowly fall towards the center

The Sun

After sufficient mass and density were achieved in the Sun, the temperature rose to one million °C, resulting in thermonuclear fusion.

H atom + H atom = He atom + energy

Birth of the Solar System

                                 

Global Chemical Differentiation

This global chemical differential was completed by about 4.3 billion years ago, and the Earth had developed an inner and outer core, a mantle and crust

The Age of the Earth

Earth is ~ 4,570,000,000 years old

• •Meteorites give us access to debris leftover from the formation of the solar system
• •We can date meteorites using radioactive isotopes and their decay products

Earth is unique among the known planets: it has an abundance of water. Otherworlds —

including a few moons — have atmospheres, ice, and even oceans, but only Earth has the

right combination to sustain life. Earth's oceans cover about 70 percent of the

planet's surface with an average depth of 2.5 miles (4 kilometers). Freshwater exists in liquid

form in lakes and rivers and as water vapor in the atmosphere, which causes much of Earth's

weather.

Earth has multiple layers.

1. Crust:

The ocean basins and the continents compose the crust, the outermost layer. Earth's crust is

between three and 46 miles (five and 75 km) deep. The thickest parts are under the

continents and the thinnest parts are under the oceans.

Earth's crust is made up of several elements: iron, oxygen, silicon, magnesium, sulfur, nickel, and trace amounts of calcium, aluminum, and other elements. The crust is divided into huge plates that float on the mantle, the next layer. The plates are constantly in motion; they move at about the same rate as fingernails grow. Earthquakes occur when these plates grind against each other. Mountains form when the plates collide and deep trenches form when one plate slides under another plate. Plate tectonics is the theory explaining the motion of these plates

      2. Mantle

The mantle under the crust is about 1,800 miles deep (2,890 km). It is composed mostly of silicate rocks rich in magnesium and iron. Intense heat causes the rocks to rise. They then cool and sink back down to the core. This convection — like a lava lamp — is believed to be what causes the tectonic plates to move. When the mantle pushes through the crust, volcanoes erupt.

      3. Core

At the center of the Earth is the core, which has two parts. The solid, inner core of iron has a radius of about 760 miles (about 1,220 km). It is surrounded by a liquid, outer core composed of a nickel-iron alloy. It is about 1,355 miles (2,180 km) thick. The inner core spins at a different speed than the rest of the planet. This is thought to cause Earth's magnetic field. When charged particles from the solar wind collide with air molecules above Earth's magnetic poles, it causes the air molecules to glow, causing the auroras — the northern and southern lights.

CHEMICAL AND MINERALOGICAL COMPOSITION OF THE EARTH'S CRUST

About 100 chemical elements are known to exist in the earth's crust. Considering the possible combinations of such a large number of elements, it is not surprising that some 2,500 minerals have been recognized. Relatively few elements and minerals, however, are of great importance in soils.

Minerals are natural inorganic compounds, with definite physical and chemical properties, that are

so conspicuous in many granitic rocks. They are broadly grouped into primary and secondary minerals. Primary minerals have not been altered chemically since their crystallization from molten lava.

Some of the elements released in weathering react to form secondary minerals. A secondary mineral

results from the decomposition of a primary mineral or from the precipitation of the decomposition

products of minerals. Because of the generally small particle size of secondary minerals,

they dominate the clay fraction of soils

Chemical Composition of the Earth's Crust

Approximately 98 percent of the mass of the earth's crust is composed of eight chemical elements. In

fact, two elements, oxygen and silicon, compose 75 percent of it.

Many of the elements important in the growth of plants and animals occur in very small quantities. Obviously, these elements and their compounds are not evenly distributed throughout the earth's surface. In some places, for example, phosphorus minerals (apatite) are so concentrated that they are mined; in other areas, there is a deficiency of phosphorus for plant growth.