The origin of the Earth

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Sonja Radmilovic                  Earths History                 17/4/02  

The origin of the Earth

The age of the Earth was once, and still is, a matter great debate. In 1650 Archbishop Usher used the Bible to calculate that the Earth was created in 4004BC. Later on in the mid-nineteenth century Charles Darwin believed that the Earth must be extremely old because he recognized that natural selection and evolution required vast amounts of time.

It wasn't until the discovery of radioactivity that scientists began to put a timescale on the history of the Earth. Rocks often contain heavy radioactive elements which decay over long periods of time, the decay is unaffected by the physical and chemical conditions and different elements decay at different rates (These rates are slow and half-life’s of several hundred million years are not uncommon)

Throughout this century the race has been on to discover the oldest rocks in the world. The oldest volcanic rock found so far has been dated at 3.75 billion years old, but this is not the whole story. Meteorites created at the same time as the Earth hit us all the time, radioactive dating shows that they are about 4.55 billion years old.

THE EARLY ATMOSPHERE

The atmosphere wasn't like this when the Earth was created over 4½ billion years ago.

THE FIRST BILLION YEARS

The Earth's surface was originally molten, as it cooled the volcanoes belched out massive amounts of carbon dioxide, steam, ammonia and methane. There was no oxygen. The steam condensed to form water, which then produced shallow seas.

Evidence points to bacteria flourishing 3.8 billion years ago so this means that life got under way about 700 million years after the Earth was created. Such early forms of life existed in the shallow oceans close to thermal vents, these vents were a source of heat and minerals.

THE NEXT BILLION YEARS

These primitive life forms then took the next evolutionary step and started to PHOTOSYNTHESISE (using sunlight to convert carbon dioxide and water to food energy and oxygen). This was an important turning point in Earth history because the carbon dioxide in the atmosphere was being converted to oxygen.

These green plants went on producing oxygen (and removing the CO2).

Most of the carbon from the carbon dioxide in the air became locked up in sedimentary rocks as carbonates and fossil fuels. Carbon dioxide also dissolved into the oceans.

The ammonia and methane in the atmosphere reacted with the oxygen.

Nitrogen gas was released, partly from the reaction between ammonia and oxygen, but mainly from living organisms such as denitrifying bacteria. (remember that nitrogen is a very unreactive gas and it has built up slowly).

THE LAST 2½ BILLION YEARS OR SO

As soon as the oxygen was produced by photosynthesis it was taken out again by reacting with other elements (such as iron). This continued until about 2.1 billion years ago when the concentration of oxygen increased markedly. As oxygen levels built up and then . . .

VIP-The ozone layer was formed which started to filter out harmful ultraviolet rays. This allowed the evolution of new living organisms in the shallow seas.

The structure of the Earth

Imagine a Scotch egg......

  1. The outer shell of the Earth is called the CRUST      (breadcrumbs)
  2. The next layer is called the MANTLE     (sausagemeat)
  3. The next layer is the liquid OUTER CORE     (egg white)
  4. The middle bit is called the solid INNER CORE      (egg yolk)

DEAD EASY!

The deepest anyone has drilled into the earth is around 12 kilometers, we've only scratched the surface. How do we know what's going on deep underground?

There are lots of clues:

  • The overall density of the Earth is much higher than the density of the rocks we find in the crust. This tells us that the inside must be made of something much denser than rock.
  • Meteorites (created at the same time as the Earth, 4.6 billion years ago) have been analyzed. The commonest type is called a chondrite and they contain iron, silicon, magnesium and oxygen (Others contain iron and nickel). A meteorite has roughly the same density as the whole earth. A meteorite minus its iron has a density roughly the same as Mantle rock (e.g. the mineral called olivine).
  • Iron and Nickel are both dense and magnetic.
  • Scientists can follow the path of seismic waves from earthquakes as they travel through the Earth. The inner core of the Earth appears to be solid whilst the outer core is liquid (s waves do not travel through liquids). The mantle is mainly solid as it is under extreme pressure (see below). We know that the mantle rocks are under extreme pressure, diamond is made from carbon deposits and is created in rocks that come from depths of 150-300 kilometers that have been squeezed under massive pressures.

VIP- The Earth is sphere (as is the scotch egg!) with a diameter of about 12,700Kilometres. As we go deeper and deeper into the earth the temperature and pressure rises. The core temperature is believed to be an incredible 5000-6000°c.

VIP- The crust is very thin (average 20Km). This does not sound very thin but if you were to imagine the Earth as a football, the crust would be about ½millimetre thick. The thinnest parts are under the oceans (oceanic crust) and go to a depth of roughly 10 kilometers. The thickest parts are the continents, (continental crust) which extend down to 35 kilometers on average. The continental crust in the Himalayas is some 75 kilometers deep.

VIP- The mantle is the layer beneath the crust, which extends about half way to the center. It's made of solid rock and behaves like an extremely viscous liquid. The convection of heat from the center of the Earth is what ultimately drives the movement of the tectonic plates and cause mountains to rise.

VIP- The outer core is the layer beneath the mantle. It is made of liquid iron and nickel. Complex convection currents give rise to a dynamo effect, which is responsible for the Earth's magnetic field.

VIP- The inner core is the bit in the middle!. It is made of solid iron and nickel. Temperatures in the core are thought to be in the region of 5000-6000°c and it's solid due to the massive pressure.

This diagram shows a detailed picture of the Earth's interior. Crust is being created at the mid ocean ridges and being eaten at the subduction zones. The movement processes are driven by the convection currents created by the heat produced by natural radioactive processes deep within the Earth.

Outer core:  depth of 2,890-5,150 kilometers
The outer core is a hot, electrically conducting liquid (mainly Iron and Nickel). This conductive layer combines with Earth's rotation to create a dynamo effect that maintains a system of electrical currents creating the Earth's magnetic field. It is also responsible for the subtle jerking of Earth's rotation. This layer is not as dense as pure molten iron, which indicates

Inner core:  depth of 5,150-6,370 kilometers
The inner core is made of solid iron and nickel and is unattached to the mantle, suspended in the molten outer core. It is believed to have solidified as a result of pressure-freezing, which occurs to most liquids under extreme pressure.

D" layer:  depth of 2,700-2,890 kilometers
This layer is 200 to 300 kilometers thick. Although it is often identified as part of the lower mantle, seismic evidence suggests the D" layer might differ chemically from the lower mantle lying above it. Scientists think that the material either dissolved in the core, or was able to sink through the mantle but not into the core because of its density.

Lower mantle:  depth of 650-2,890 kilometers
The lower mantle is probably composed mainly of silicon, magnesium, and oxygen. It probably also contains some iron, calcium, and aluminium. Scientists make these deductions by assuming the Earth has a similar abundance and proportion of cosmic elements as found in the Sun and primitive meteorites.

Transition region:  depth of 400-650 kilometers
The transition region or mesosphere (for middle mantle), sometimes called the fertile layer and is the source of basaltic magmas.  It also contains calcium, aluminium, and garnet, which is a complex aluminium-bearing silicate mineral. This layer is dense when cold because of the garnet. It is buoyant when hot because these minerals melt easily to form basalt, which can then rise through the upper layers as magma.

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Upper mantle:  depth of 10-400 kilometers
Solid fragments of the upper mantle have been found in eroded mountain belts and volcanic eruptions. Olivine (Mg, F e)
2SiO4 and pyroxene (Mg,Fe)SiO3 have been found. These and other minerals are crystalline at high temperatures. Part of the upper mantle called the asthenosphere might be partially molten.

Oceanic crust:  depth of 0-10 kilometers
The majority of the Earth's crust was made through volcanic activity. The oceanic ridge system, a 40,000 kilometer network of volcanoes, generates new oceanic crust at the rate of 17 km
3 per year, covering the ocean floor with an igneous rock called basalt. Hawaii and Iceland ...

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