Tomothy Budd, of Ohoka School, asks :-
What replaces the lava inside the Earth when there is a volcanic eruption?
Margaret Bradshaw, a consultant geologist, responded.
The Earth is not hard all the way through. The layer below the crust (called the Mantle) acts like a warm, stiff plastic that is mobile. In fact, the Earth is rather like a toffee-filled chocolate; it looks hard enough from the outside, but when you bite into it, the inside is soft and can flow. It helps to think of the Earth as a football with a postage stamp stuck on it. The postage stamp represents the thinness of the hard crust compared to the size of the whole planet. The mantle layer is kept under a lot of pressure because of the Earth's rigid "skin".
When conditions are right for a volcanic eruption, parts of the upper mantle and crust become very hot, forming molten magma which rises to the surface (hot fluids always rise) to become erupted from a volcano as lava. Once the lava has escaped out onto the earth's surface, the surrounding parts of the mantle flow very slowly into the space left underground. Sometimes this replacement is so slow that the centre of a volcano may collapse into the partially filled space below to form what is known as a caldera.
Imagine our stamp covered football now filled with water. Push a pin through the stamp and water will start to ooze out. But there is no "hole" left inside the ball where it has leaked because the water inside has flowed sideways to replace the water that has escaped. The inside of the Earth is a little like that, only things happen much more slowly and we are dealing with material that is very, very stiff and treacly.