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Geological Time

In order to understand landforms, it is useful if you know something about the rocks that lie under them - how they are formed, when they were formed and some of the forces that have shaped them. For your geography GCSE, you are required to know that geological time is on a different scale to human time scales, to know a simplified geological time scale and know the position of certain key rocks and the last ice age within the framework of this time scale.

Geological time is measured in millions of years and is almost impossible to imagine. If you assume that the average life span of a human being is about 75-80 years, it would take over 12,000 lifetimes to experience one million years. Think how long it has been since the end of the First World War - that is just over one average human lifetime ago.

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Now think back to 1066 when William the Conqueror invaded Britain, that's about 12 average lifetimes ago - both of these seem a long way in the past. To begin to appreciate geological time needs you to multiply that by hundreds and thousands of times. The oldest rocks that have been found at the surface of the Earth formed nearly 4,000 million years ago, getting on for 50,000 human lifetimes ago!

Life seems to have started on Earth about 3,500 million years ago and was in the form of single-celled microbes. Around 1,500 million years ago, simple multicelled organisms had evolved. These were soft-bodied and finding traces of them is difficult. All this happened during the precambrian which is by far and away the longest period of geological time, it accounts for 88% of the time that has passed since the planet Earth formed. All the time, weathering and erosion has been taking place and tectonic activity has gradually provided us with the rocks that form our landscapes today. If you look at old pictures of mountains, taken 100 years ago, apart from changes in vegetation, they look the same as they do today. They change on the geological time scale, not the human time scale.

About 540 million years ago, life really took off - this has been called the cambrian explosion. Since then, life has continued to evolve, with mass extinctions and other significant events marking the end of each period and era. The largest divisions of geological time are called eons. There are only four eons recognised since the Earth was formed. Eons are broken down into eras. The eras are broken down into periods and that is as far as you need to know for GCSE geography. For your general knowledge, periods are broken down into epochs and epochs are broken down into ages, stages and substages!

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  1. What is the estimated age of the Earth?
    Billion here is used in the American sense - one billion is one thousand million. We will stick to that for the rest of the quiz
  2. The oldest rocks that seem to contain fossils of microbes have been found in Australia. How long after the formation of the Earth were they thought to have been formed?
    The rocks have been dated as being about 3,500 million years old
  3. The South Downs are made from chalk formed during the ...
    The end of the cretaceous period is marked by the mass extinction that killed off the dinosaurs
  4. The Cotswolds are partly formed from a type of limestone that formed during the ...
    Limestone is formed in shallow warm seas
  5. Another type of limestone is found in Britain, for example, in the Pennines. When was this formed?
    The carboniferous period lasted 60 million years (359-299 million years ago)
  6. The last great ice age is said to have started about 120,000 years ago and ended about 10,000 years ago. This puts it in which era and period?
    When answering multiple choice questions, look at the alternatives closely. Two of these (the first and last) are completely wrong, they match up the era with a period from a different era, so that narrows down your choice to two, giving you a 50% chance if you have to take a guess
  7. Granite is a very hard rock that is found in several places in Britain. There are significant outcrops in Scotland (formed about 400 million years ago) and the SW peninsula (formed about 280 million years ago). Which of the following combinations of periods is correct (Scottish granite first)?
    Granite is an intrusive igneous rock that creates the landforms known as tors on the uplands of Devon and Cornwall
  8. If you were able to travel back in time, to which of the following periods would you go to see a dinosaur?
    Dinosaurs appeared in the triassic and became extinct at the end of the cretaceous
  9. Imagine that you are a farmer and your farm is in an area where the main rock is carboniferous limestone. What would you be most likely to be farming?
    Soils in carboniferous limestone areas are generally too poor to grow crops commercially
  10. One analogy to help you visualise the scale of geological time is to compare it to London's tallest building, the Shard. If the height of the Shard represents all of geological time, which one of the following would represent human history?
    We are newcomers to the planet but already have changed it more than any other species before us

Author: Kev Woodward

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