In our Solar System the Earth is the third planet from the Sun and is located between Venus and Mars. Approximately 70% of the Earth is covered with water and the remaining 30% is land. As far as we know the Earth is the only planet that can sustain life.
The Earth is composed of several layers. It has an inner core of solid iron and an outer core mainly composed of molten iron which helps create the magnetic field that makes compasses work. Scientists estimate that the temperature at the centre of the Earth is about as hot as the surface of the Sun. Around the core is a very thick layer of semi-melted rock called the mantle. On top of the mantle is a thin solid layer called the crust. This layer is where we live and it varies quite a bit in thickness. It is thinner under the oceans (5-10 km) and thicker under the continents (20-70 km).
Plate Tectonics and Continental Drift
The surface layer of the Earth is strong and rigid, but is cracked into about 12 different sized pieces or tectonic plates. These plates ‘float’ on the mantle layer of semi melted rock called magma and the movement of plates was originally called ‘continental drift’.
Tectonic plates move at the speed of a growing fingernail, so over millions of years, there is great change in the position of the plates and the continents they carry. Plates move in different directions and at different speeds, so they may pull apart, slide past one another and even ride up over each other, continually shaping the surface of our planet.
Alfred Wegner, a German scientist, first suggested the theory of continental drift. In 1912 he noticed that rocks and fossils from different continents were often the same, indicating that the continents must have been joined at some stage. Eventually, this led to the theory of plate tectonics in the 1960s.
Volcanoes and Hot Spots
Volcanoes can occur where two tectonic plates move against each other or over special regions called ‘hot spots’.
Eruptions occur when magma rises from the mantle and bursts through the surface of the Earth to become lava. A main pipe or vent connects the mantle to the crater at the Earth’s surface and other side pipes can also form. How a volcano erupts and the shape of a volcano depends on the magma. For example some magma forms thick lava which flows slowly and doesn’t travel very far before it cools and sets, so it forms steep sided volcanoes. Very thick lava can also ‘clog’ the vent so that pressure builds up and an eruption can be explosive with rocks and clouds of ash being thrown out.
Hot spots are sources of high heat below the mantle that seem to ‘burn through’ the Earth’s crust at a weak spot. These volcanoes mainly erupt lava with very little ash. This type of volcano is often found on the bottom of the ocean where it continually releases lava that is cooled by the sea. Eventually enough lava is built up to form a small island. The entire chain of Hawaii’s islands was created as a tectonic plate moved slowly over a hot spot.
Tsunami is a Japanese word meaning ‘harbour wave’. Tsunami waves are a series of ocean waves that are caused by a large disturbance on or near the seafloor. Earthquakes, landslides, volcanic eruptions and meteorites can create tsunami waves that travel outward like ripples in a pond.
When tsunami waves first form they are difficult to detect as they slope gradually and may not be very tall. However, they travel at amazing speeds until they reach shallow water when they are slowed down by friction and then the waves can increase to 30 metres in height. Damage occurs as the waves inundate low lying coastal areas causing flooding and damage to coastal cities and towns.
Tectonic plates that form the surface of the Earth slowly move around, but sometimes the jagged edges of plates lock together. Stress builds until something gives and a large amount of energy is suddenly released at a point called the focus. The surface of the Earth immediately above the focus is called the epicentre. The energy travels in all directions through the Earth as seismic waves. Some seismic waves travel quickly and squeeze and stretch the rocks, some waves make the rock move up and down. Other waves cause a circular motion near the surface of the Earth and cause most damage.
The Richter scale is the best-known scale for measuring the magnitude of an earthquake. It is based on calculation of the strength of the earthquake at its source. An earthquake will feel different depending on its magnitude and how far you are from the epicentre. The scale is logarithmic which means one point on the scale represents a ten fold increase. For example, a recording of 6 indicates an earthquake with a ground motion 10 times larger than a recording of a 5. Earthquakes measuring 2 on the Richter scale are normally not felt by people, but an earthquakes with a Richter value of 6 or more can be considered a major event if it is close to a populated area. Smaller quakes are often felt after a major earthquake. These are called aftershocks and are usually one unit less on the Richter scale.
Today scientists are gaining more knowledge in predicting the likelihood and location of future earthquakes. Dangerous areas where earthquakes commonly occur are being identified and progress is being made in designing buildings that will withstand even the most severe earthquake.
A meteor is a small piece of matter entering the Earth’s atmosphere. As it enters the atmosphere the friction created by its speed causes the surface of the meteor to heat up and it appears as a bright streak of light in the night sky. These meteors are also called shooting stars.
Most meteors are so small that they burn up completely in the Earth’s atmosphere and never hit the ground. If a meteor survives its fiery descent and hits the ground it then becomes a meteorite. A large meteorite will leave a pit in the ground and may shatter into many fragments and spread over a large distance from the site of the impact. Very large meteorites may leave craters hundreds of metres across but these are very rare and only occur about once every 100 million years.