P1 Revision Notes

The Earth

when it first formed, the Earth was completely molten (hot liquid). Scientists estimate the Earth is 4500 million years old as it  has to be older than its oldest rocks. The oldest rocks found on Earth are about 4000 million years old. Studying rocks tells us more about the Earth's structure and how it has changed as a result of the following processes:

• erosion - the Earth's surface is made of rock layers, on top of another. The oldest is usually at the bottom. The layers are compacted sediment produced by weathering and erosion. Erosion changes the surface over time.
• Craters - the moon's surface is covered with impact craters from meteors. Meteors also hit the Earth but craters have been erased by erosion.
• Mountain formation - if new mountains weren't being formed, the Earth's surface would have eroded down to sea level.
• Folding - some rocks look as if they've been folded in half. This required huge force over a long time.

Further evidence of the Earth's age can be found by studying:

• fossils of plants and animal in sedimentary rock layers which show how life has changed.
• the radioactivity of rocks. A rock's radioactivity decreases over time and radiactive dating measures radiation levels to find out a rock's age.

The structure of the Earth

Thin Rocky Crust

- thickness varies between 10km and 100km

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