What are tectonic plates?

The Earth's crust is made up of about 30 huge plates that float on the semi-liquid mantle surrounding the earth's core. There are currents and movements in the mantle, so these plates move very slowly in relation to each other - about 10 cm per year on average. The floor of the Pacific Ocean is one enormous plate. Some of the plates carrying continents also carry parts of the ocean floor. The thickness of the plates varies from 8 km to 200 km.

Earthquakes are evidence of movement of the tectonic plates that carry the continents. Most of the areas where earthquakes take place are along the edges of the plates. Friction holds the plate edges together for a while, but continuing movement means that huge stresses built up. The tension is suddenly released when the plates shift sharply and this causes earthquakes. There are other causes of earthquakes, but they all involve sudden movement of the crust along a fault or crack. The energy released in an earthquake is enormous - as much as 10,000 times the power released by the world's first atomic bomb.

Earth's Map

The earliest known maps were made in Babylon on clay tablets, as long ago as 2300 BC. Some Egyptian drawings and paintings found in tombs are almost as old. The Greek mathematician Ptolemy (AD 90-168) produced an enormous eight-volume guide to geography and map-making called The Guide to Geography. It was followed carefully by map-makers for almost 1,000 years.

It is not possible to draw the curved surface of the globe accurately on a flat sheet of paper. This problem puzzled mapmakers for many years. In 1569, Gerardus Mercator showed how to convert the rounded shape of the world into a cylindrical shape, which could be unrolled to make a flat map. However, this can distort the size of countries in the far north and south. Dividing the earth into "orange peel" segments gives a truer image of the size of countries.

Why did the world's climate change?

The appearance of living things changed the Earth's atmosphere, providing the conditions for climate change. The first living things were plant-like creatures. They used the sun's energy to change carbon dioxide and water into sugars, which they used for food, and oxygen, which they released into the atmosphere. Some of this oxygen turned into ozone, forming the layer that now protects us from the sun's ultraviolet radiation.

In more recent times, geologists and paleontologists have been able to make more accurate measurements of the age of ancient rocks. Trees produce a new 'ring' every year, and these can be measured and counted. The width of the ring shows how well the tree grew in any year, reflecting climatic conditions at that time. The rings of even partly fossilized trees buried in bogs can be measured, and these measurements provide accurate records of climate changes over the past 750,000 years. By measuring the radioactivity of once living material, scientists can make accurate measurements up to 40,000 years ago.

How can forecasters predict the weather?

Meteorologists use a wide range of instruments and techniques to help them track changes in the weather and to predict future weather conditions. Weather forecasters have used simple instruments such as thermometers, rain gauges, barometers and wind gauges for many years, but the advent of satelite photography has transformed weather forecasting. Weather satelites can track the movements of clouds, and show the positions of high and low pressure areas and weather fronts. Radar measures the size, speed and direction of storms, so accurate warnings of severe weather can be given. With the use of computers, increasingly accurate forecasting is now possible.

What are tornadoes and hurricanes?

Tornadoes are violent destructive whirlwinds whose force is concentrated into a much smaller area than that of a hurricanes. They are very common in the central United States, were they cause emourmous damage. Tornadoes travel across the land at high speed, and the roaring noise they make can be heard 40 km away. A tornado happens when huge masses of clouds moving in different directions meet. The air begins to spin ina spiral, and a funnel of twisting air reaches out towards the ground. The low pressure in the funnel sucks up soil, dust and anything else it touches, even cattle!

Hurricanes are powerful tropical storms that can cause serious damage. They from near the Equator, where warm,moist air rises, drawing in coller air below. The air moves in a spiral, eventually forming a whirling mass. Rising warm air continous to cool and drop down in the center of the spiral to repeat the process. The winds inside the hurricane travel at speeds up to 200km/h. Hurricanes form in the Atlantic Ocean, passing over the West Indies and the southern United States. Similar storms in the Pacific ocean are called typhoons, whereas in the Indian Ocean they are called cyclones.

What is the San Andreas Fault?

The San Andreas Fault is a long crack in the earth's crust that extends 970 kilometers through California, in the USA. It runs along the edges of the tectonic plates cerrying the Pacific Ocean and North America. These plates have shifted about 300 kilometers over the past 15 million years. They continue to move about 6 cm each year, building up emormous stresses in the crust. Along the fault line, the ground bulges, cracks open up and roads and railways develop a noticeable kink. The San Andreas Fault is carefully monitored because it is ewxpected to slip at any time.

What is a tsunami?

Tsunamis are tidal waves that are often caused by an undersea earthquake.Usually, there is some warning of a tsunami because scientists can detect the seismic waves caused by the earthquake. The tsunami rushes along at up to 970 km/h, building into a wall of water up to 30 meters high as it approaches land. Tsunamis are most common in earthquake zones, particularly around Japan.

Another form of tsunami is called storm surge, in which giant wave are whipped up by a storm. In 1970, a storm surge and cyclone hit Bangladesh, killing 266,000 people. It returned again in 1985, killing another 10,000 people.

What is smog?

Smog is a serious form of air pollution that can cause breathing problems. The tem smog was frist used to describe a type of air pollution in London, when smoke from coal fires combined with fog to produce dense yellow clouds that sometimes persisted for days. The term now describes the hazy fog that is caused mainly by vehicle exhausts and smoke from factories. It builds up when there is little air movement, and can happen when a layer of warm air traps cooler air from the ground, preventing the smog from being blown away. Cities in bowls or sheltered valleys are particularly liable to this condition.