Changes in The Earth and it's atmosphere

The Earth is nearly sphereical, it has a layered structure that consists of...

• a thin crust (0-50km thick)
• a mantle (poroperties of solid but does flow)
• an outer core (liquid)
• an inner core (solid)

Rocks at the Earth's surface are always being broken up , reformed and changed in an ongoing cycle, known as the rock cycle. The changes are v..e..r..y  s...l...o..w.

Evidence showed scientists that the East Coast of Soth America and the west coast of Africa have...

• closely matching coastlines
• similar patterns of rocks, which contain fossils of the same plants and animals.

This evidence led Alfred Wegner to propose that South America and Africa at one point had been joined as part of a single land mass. He proposed that movement of the crust was responsible for the seperation,i.e continental drift. This is tectonic theory. It took 50 years for scientists to discovered how the crust moved.

The Earth's lithoshpere is cracked into tectonic plates.

Intenses heat by radioactive decay deep inside the Earth, creates convection currents in the mantle. These currents cause the tectonic plates to move apart very slowly by only a few centimeters a year.

In convection in a gas or liquid, the matter rises as it is heated. As it gets further away from the heat source, it cools and sink. This is what is happening in the Earth.

1. Hot new molten rock rises to the surface,creating new crust

2.The older, cooler crust, then sinks down where the convection current starts to fall.

3. The land masses on these plates move very slowly.

The movements are usually small and gradual, but sometime they can be sudden and disastrous. Earthquakes and volcanic eruption are common occurances at plate boundries. They are hard to predict.

There are many theories as to how life began on Earth beacue there is little direct evidence therefore a number of assumptions have been made.

Two scientists named Miller and Urey, tried to test a possible theory for how life on Earth began. They mixed together the chemicals thought to be present in the early atmosphere- water (vapour), methane and ammonia. The mixture was heated and sparks to represent ultraviolet radiation from the Sun was passed through it . The mixture was cooled and repeated.

After many tries the mixture contained simple organic molecules, like amino acids, that are essential for life. This is known as the primordial soup theory, one of the many theories to how life began.

Intense volcanic activity released...

• mainly carbon dioxide (like the atmospheres of Mars and Venus today)
• water vapour (condenses to form oceans)
• small amounts of ammonia and methane

Green plants and algae evolve and ...

• carbon dioxide is reduced as the plants take it in and give out oxygen
• microorganisms that can't tolerate oxygen are kill off
• carbon from carbon dioxide becomes locked up in sedimentary rocks formed from the shells and skeletons of marine organisms
• other gases react with oxygen to release nitrogen
• nitrogen is also produced by bacteria removing nitrates from decaying plant material.

There are now about 20% oxygen and 80% nitrgoen in the atmosphere The amount of carbon dioxide has decreased significantly.