1.7 Our Changing PLanet

The Earth consists of four concentric layers: inner core, outer core, mantle and crust.

The Earth is made up of four distinct layers:

1.    The inner core is in the centre and is the hottest part of the Earth. It is solid and made up of iron and nickel with temperatures of up to 5,500°C. With its immense heat energy, the inner core is like the engine room of the Earth.

2.    The outer core is the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel. It is still extremely hot, with temperatures similar to the inner core.

3.    The mantle is the widest section of the Earth. It has a thickness of approximately 2,900 km. The mantle is made up of semi-molten rock called magma. In the upper parts of the mantle the rock is hard, but lower down the rock is soft and beginning to melt.

4.    The crust is the outer layer of the earth. It is a thin layer between 0-60 km thick. The crust is the solid rock layer upon which we live.

There are two different types of crust: continental crust, which carries land, and oceanic crust, which carries water.

The Earth's crust and upper part of the mantle are broken into large pieces called tectonic plates. These are constantly moving at a few centimetres each year. Although this doesn't sound like very much, over millions of years the movement allows whole continents to shift thousands of kilometres apart. This process is called continental drift.

The plates move because of convection currents in the Earth’s mantle. These are driven by the heat produced by the natural decay of radioactive elements in the Earth.

Where tectonic plates meet, the Earth's crust becomes unstable as the plates push against each other, or ride under or over each other. Earthquakes and volcanic eruptions happen at the boundaries between plates, and the crust may ‘crumple’ to form mountain ranges.

Before Wegener

The theory of plate tectonics and continental drift was proposed at the beginning of the last century by a German scientist, Alfred Wegener. Before Wegener developed his theory, it was thought that mountains formed because the Earth was cooling down, and in doing so contracted. This was believed to form wrinkles, or mountains, in the Earth’s crust. If the idea was correct, however, mountains would be spread evenly over the Earth's surface. We know this is not the case.

Wegener’s theory

Alfred Wegener (1880 - 1930)

Wegener suggested that mountains were formed when the edge of a drifting continent collided with another, causing it to crumple and fold. For example, the Himalayas were formed when India came into contact with Asia. It took more than 50 years for Wegener’s theory to be accepted. One of the reasons was that it was difficult to work out how whole continents could move: it was not until the 1960s that enough evidence was discovered to support the theory fully.

The two main gases are both elements and account for about 99 percent of the gases in the atmosphere. They are:

about ⁴/₅ or 80 percent nitrogen (a relatively unreactive gas)

about ¹/₅ or 20 percent oxygen (the gas that allows animals and plants to respire and for fuels to burn)

The remaining gases, such as carbon dioxide, water vapour and noble gases such as argon, are found in much smaller proportions.

Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere was probably formed from the gases given out by volcanoes. It is believed that there was intense volcanic activity for the first billion years of the Earth's existence.

The early atmosphere was probably mostly carbon dioxide with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans.

Mars and Venus today

It is thought that the atmospheres of Mars and Venus today, which contain mostly carbon dioxide, are similar to the early atmosphere of the Earth.

There is evidence that the first living things appeared on Earth billions of years ago. There are many scientific theories to explain how life began. One theory involves the interaction between hydrocarbons, ammonia and lightning.

The Miller-Urey experiment - Higher tier

Stanley Miller and Harold Urey carried out some experiments in 1952 and published their results in 1953. The aim was to see if substances now made by living things could be formed in the conditions thought to have existed on the early Earth.

The two scientists sealed a mixture of water, ammonia, methane and hydrogen in a sterile flask. The mixture was heated to evaporate water to produce water vapour. Electric sparks were passed through the mixture of water vapour and gases, simulating lightning. After a week, contents were analysed. Amino acids, the building blocks for proteins, were found.

You will recall that about 78 percent of the air is nitrogen and 21 percent is oxygen. These two gases can be separated by fractional distillation of liquid air.

Liquefying the air

Air is filtered to remove dust, and then cooled in stages until it reaches –200°C. At this temperature it is a liquid. We say that the air has been liquefied.

Here's what happens as the air liquefies

·         water vapour condenses, and is removed using absorbent filters

·         carbon dioxide freezes at –79ºC, and is removed

·         oxygen liquefies at –183ºC

·         nitrogen liquefies at –196ºC.

    The liquid nitrogen and oxygen are then separated by fractional distillation.

Fractional distillation 

The liquefied air is passed into the bottom of a fractionating column. Just as in the columns used to separate oil fractions, the column is warmer at the bottom than it is at the top. The liquid nitrogen boils at the bottom of the column. Gaseous nitrogen rises to the top, where it is piped off and stored. Liquid oxygen collects at the bottom of the column. The boiling point of argon - the noble gas that forms 0.9 percent of the air - is close to the boiling point of oxygen, so a second fractionating column is often used to separate the argon from the oxygen.

Uses of nitrogen and oxygen

   liquid nitrogen is used to freeze food

   food is packaged in gaseous nitrogen to increase its shelf life

   oil tankers are flushed with gaseous nitrogen to reduce the chance of explosion

   oxygen is used in the manufacture of steel and in medicine.