Gravity keeps us in place. Fortunately, it also keeps the atmosphere in place so that we have air to breathe.

Gravity is simply due to mass, the mass of anything. When you drop a pencil, it is attracted to the Earth's centre of gravity (centre of mass). But the Earth is attracted to the pencil and the Earth comes up to meet it - admittedly by a very tiny amount! When you jump off a diving board, the whole of the gravity from the whole of the Earth is needed to pull you downwards. It makes you accelerate at 9.81 m/s², the 'value' of gravity.

This figure seems quite accurate - but it can vary very slightly. Near the poles, the "acceleration due to gravity" is very slightly more than at the Equator; because the polar radius is slightly less, the North Pole is therefore nearer the centre of mass of the Earth. As you climb Everest, it should get easier as the pull of gravity gets smaller - very slightly!

When measuring gravity with this precision, the rocks below the Earth's surface can affect the value. Dense rocks, such as gabbro, have more mass per unit volume and the gravimeter reading is therefore higher than average. A positive gravity anomaly is recorded.

Rock salt, in great domes or diapirs, is less dense than average and shows up as a negative gravity anomaly. A gravity survey - another remote sensing method - is very useful way of suggesting that there may be a salt dome below the ground surface, and therefore a possible site for an oil trap.

On a global scale, the pull of gravity influences the balance of forces acting on the surface. There is an overall balance - called isostasy - despite the apparent differences between the deep ocean basins and the tall mountain ranges. The ocean crust is thin and dense and the continental crust is less dense and thick, sinking into the mantle until it is balanced (or, if you like, until it floats) and isostasy is achieved.

From time to time, the balance is upset, as for example when thick ice sheets develop on land (which they do relatively quickly) at the start of an ice age. The continents have extra weight on them and they sink further into the mantle. When the ice melts (which it does relatively quickly), the landmass rebounds slowly. Some land areas are still rising, 10 000 years after the ice melted. How do we know? What part do you think that raised beaches play in this discussion?

Raised beaches now above sea level, Arran. The former cliff is on the left.

• describes how gravity is measured with gravimeters;
• explains the variations in gravity;
• outlines how the Bouguer anomaly is determined;
• shows how gravity surveys are carried out and the importance of anomalies;
• explains isostasy and isostatic adjustments.

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