Science

Earthquakes and global warming

Earthquakes produce shock waves that cause damage. There are two types of seismic wave - p-waves and s-waves. Seismometers can detect these waves and provide evidence of the Earth’s structure.

The ozone layer reduces the amount of ultraviolet light from the Sun that reaches the Earth’s surface. Exposure to ultraviolet radiation can lead to sunburn and skin cancer, but sunblocks can reduce this damage.

Increased energy use and deforestation raise the amount of carbon dioxide in the atmosphere. This leads to global warming.

Earthquakes

The Earth’s crust and upper mantle are broken up into huge tectonic plates. Where these meet, the Earth’s crust becomes unstable and earthquakes and volcanic eruptions occur.

Earthquakes cause damage to buildings, and often lead to loss of life. It is difficult to predict exactly when an earthquake might happen and how bad it will be, even in places that are known for having earthquakes.

Seismic waves

Earthquakes produce shock waves. These travel through the Earth and can be detected using a device called a seismometer.

There are two types of seismic wave.

Properties of seismic waves

  p-waves s-waves
Type of wavelongitudinal transverse
Relative speedfaster slower
What can they travel through? solids and liquidssolids only

P-waves are longitudinal waves like sound waves, and s-waves are transverse waves like light waves

Ultraviolet radiation

Ultraviolet radiation is found naturally in sunlight. Exposure to ultraviolet radiation can cause:

  • our skin to tan
  • sunburn
  • skin cancer

We cannot see or feel ultraviolet radiation, but our skin responds to it by turning darker. This happens in an attempt to reduce the amount of ultraviolet radiation that reaches deeper skin tissues.

Darker skins absorb more ultraviolet light, so less ultraviolet radiation reaches the deeper tissues. This is important because ultraviolet radiation can cause normal cells to become cancerous.

Sunblocks

Sunblocks can reduce the damage caused by ultraviolet radiation. They contain chemicals that absorb a lot of the radiation and prevent it from reaching our skin. They may also contain chemicals that reflect some of the radiation away from the skin.

Manufacturers of sunblocks make products with different sun protection factors:

  • the higher the factor, the longer you can stay out in the sun without burning
  • high factor sunblocks reduce the risks from ultraviolet radiation more than low factor sunblocks

If, for example, you would get sunburnt after ten minutes in the sun, with Factor 5 applied you could stay in the sun for 50 minutes - or for 1500 minutes with Factor 150 applied. But the real time is usually lower, because some of the sunblock gets absorbed by the skin, and some gets rubbed off.

Global warming

Dust from volcanoes reflects heat radiation from the Sun, leading to cooling. But dust from factories reflects heat radiation from cities, stopping it from escaping. This leads to warming in urban areas.

Carbon dioxide is a greenhouse gas. It absorbs heat energy that is radiated from the Earth’s surface and prevents it from escaping into space. This keeps the Earth warmer than it would otherwise be. But human activities are increasing the amount of carbon dioxide in the atmosphere. This increases the greenhouse effect and leads to global warming.

The percentage of carbon dioxide in the atmosphere has risen from 0.028 in 1700 to 0.035 in 1990.

Carbon dioxide in the atmosphere has risen at a higher rate since the 19th century

The earth's global average temperature has risen from  13.5º C in 1860 to 14.4º C in 1995 (temperatures over a 5 year average).

The temperature of the earth has risen over the years

Human activities leading to global warming include:

  • increased use of energy
  • deforestation - cutting down trees

Increased use of energy

Most of the energy we use comes from burning fossil fuels. These release carbon dioxide when they burn. So the increasing use of energy is leading to increasing amounts of carbon dioxide in the atmosphere.

Deforestation

When land is cleared for timber and farms, there are fewer trees to remove carbon dioxide from the atmosphere by photosynthesis. Also, if the fallen trees are burned or left to rot, additional carbon dioxide is released into the atmosphere.

Stable Earth - higher

Seismic waves

A knowledge of how seismic waves travel through the Earth provides us with evidence of the Earth’s structure. Remember that:

  • p-waves travel through solids and liquids, so they can travel through all of the Earth’s layers
  • s-waves cannot travel through liquid rock, so they cannot travel through the outer core

The speed of p-waves and s-waves increases as they travel deeper into the mantle. They travel through the Earth in curved paths, but they change direction suddenly when they pass through the boundary between substances in different states. The diagrams show what happens when p-waves and s-waves pass through the Earth:

S-waves

the S waves are deflected
  • transverse
  • slow moving
  • travel through solids only

P-waves

P waves travel through
  • longitudinal
  • fast moving
  • travel through liquids and solids only

S-waves cannot pass through the liquid outer core, but p-waves can. When p-waves pass from solid to liquid, then from liquid to solid, there are sudden changes in direction.

The ozone layer

Ozone (O3) absorbs ultraviolet radiation. The ozone layer is the part of the upper atmosphere where ozone is found in the highest concentrations. The ozone there absorbs ultraviolet radiation, preventing most of it from reaching the ground. This is important because ultraviolet radiation can lead to skin cancer.

Chlorofluorocarbons, or CFCs, are chemicals that were once used widely in insulating foam and aerosol spray-cans. They release reactive chlorine atoms in the upper atmosphere. These break down ozone and damage the ozone layer. CFCs have now been almost completely replaced by chemicals that do not destroy ozone.

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