While still underground this molten rock is called magma. Once it reaches the surface to erupt and flow down the side of a volcano it is called lava

Deep inside the Earth, between the molten iron core and the thin crust on the surface, there is a solid body of rock called the mantle. It is still hot from the formation of our planet about 4.6 billion years ago - since rocks are good insulators, the heat has been slow to dissipate.

When rock from the mantle melts, it becomes magma, moving to the surface through the Earth’s outer crust and releasing pent-up gases. When the pressure is too much, volcanoes erupt. Pressure builds up if the supply of magma to the volcano from the Earth's mantle is high. Alternatively, pressure can increase within the magma chamber inside the volcano. This is because as the magma in the chamber starts to cool, it releases gases which expand, thereby increasing the pressure.

When the pressure gets too high, the rocks that make up the volcano break and the magma escapes to the surface to erupt. In some volcanoes, the supply from within the Earth is relatively constant so eruptions are frequent; in others the magma rises in blobs every 100 or even 1000 years and eruptions are infrequent.

When it is still underground, this molten rock is called magma. Once it reaches the surface to erupt and flow down the side of a volcano, it is called lava. In general, the hotter the lava, the more fluid its consistency and the faster it flows. Hawaiian lavas tend to be at the hot end of the range. When they erupt they are about 1,175ºC (2,140ºF)