How the Earth is changing

Evidence from rocks
1. Rocks provide evidence for changes in the Earth. In 1785 James Hutton presented his idea of a rock cycle to the Royal Society. He detailed ideas of erosion and sedimentation taking place over long periods of time, making massive changes to the Earth’s surface.
  1. Geologists can use other evidence from the rocks themselves such as:
    1. looking at cross-cutting features (rock that cuts across another is younger)
    2. using fossils (species existed/ became extinct during certain time periods)
    3. deepness of the rock (younger rocks are usually on top of older ones).
Wegener’s theory
1. Alfred Wegener proposed the theory of continental drift at the beginning of the 20th century. His idea was that the Earth's continents were once joined together, but gradually moved apart over millions of years. It offered an explanation of the existence of similar fossils and rocks on continents that are far apart from each other. But it took a long time for the idea to become accepted by other scientists.
2. Before Wegener
  1. 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. The heating effect of radioactive materials inside the Earth prevents it from cooling.
  2. 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.
3. Wegener’s evidence for continental drift was that:
  1. the same types of fossilised animals and plants are found in South America and Africa
  2. the shape of the east coast of South America fits the west coast of Africa, like pieces in a jigsaw puzzle
  3. matching rock formations and mountain chains are found in South America and Africa.
Seafloor spreading
1. In the centres of many oceans, there are mid-ocean ridges. At these places, the tectonic plates are moving apart. Molten material, known as magma from inside the Earth oozes out and solidifies. This movement of the mantle is referred to as convection due to heating by the core of the Earth. This process is called seafloor spreading. It results in seafloors spreading by a few centimetres each year.
Inside the Earth
1. All our evidence for changes in the Earth comes from looking at rocks. Folds and fossils in sedimentary rocks, radioactive dating and the weathering of ancient craters show that the oldest rocks are about 4000 million years old. That means the Earth must be at least as old as this.
2. The only thing that we have been able to observe directly is the Earth’s crust, which is the very thin outer rocky layer.
3. Evidence from earthquakes shows that the Earth has a very dense core surrounded by a solid mantle.
4. The Earth is almost a sphere. These are its main layers, starting with the outermost:
  1. The crust, which is relatively thin and rocky
    1. The mantle, shown here as dark red, which has the properties of a solid, but can flow very slowly
      1. The outer core, shown as orange, which is made from liquid nickel and iron
        The inner core, shown as yellow, which is made from solid nickel and iron
The Earth's magnetic field - Higher tier
1. The typical speed of seafloor spreading is slow: about 10 cm per year. When the magma oozing out of mid-ocean ridges solidifies into rock, the rock records the direction of the Earth’s magnetic field. The Earth’s magnetic field changes with time, and sometimes even reverses its direction. These changes are recorded in the rocks. The same magnetic patterns are seen on both sides of the mid-ocean ridges.
Plate tectonics - Higher tier
1. Movement of tectonic plates - Higher tier
  1. Volcanoes, mountains and earthquakes occur at the edges of tectonic plates - their creation depends on the direction the plates are moving.
2. Volcanoes
  1. If the plates are moving apart, as at mid-ocean ridges, volcanoes are produced as molten magma is allowed to escape. This happens in Iceland.
3. Mountains
  1. If the plates are moving towards each other, the edges of the plates crumple, and one plate ‘dives’ under the other. This is called subduction. It produces mountains, like the Himalayas. The friction of the movement can also melt rocks and produce volcanoes. This is also part of the rock cycle, because the plate that dives under the other one becomes part of the mantle and emerges much later from volcanoes and in seafloor spreading.
    1. There are two other ways in which mountains can be formed. At destructive margins mountain chains can be formed as plates push against each other. If an ocean closes completely then continents can collide. This occurs slowly but the collision would still result in the formation of a mountain chain.
4. Earthquakes
  1. If the plates are moving sideways, stresses build up at the plate boundary. When the stress reaches some critical value, the plates slip suddenly, causing an earthquake. It is hard to predict when such an event may happen.
    1. In California on the western coast of the USA, the San Andreas fault at the edge of the North American tectonic plate marks the point at which two plates are moving sideways. Earthquakes are common in this region. They include the Great San Francisco Earthquake of 1906.
5. Detecting wave motions
  1. The vibrations of an earthquake are detected using a seismometer that records the results in the form of a seismogram. The vibrations that are detected from the site of an earthquake are known as seismic waves.

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How the Earth is changing

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