Lava is hot, liquid rock. It begins as a yellow or red liquid and cools to a brown or black solid. It tends to flow in tongues rather than sheets.

The source of lava is deep underground, usually below a volcano. There is a large chamber of molten rock below a volcano. This molten material while underground, is called magma, and the chamber is the magma chamber. When the magma rises up the volcano and spills out over the ground, it is called lava. If the magma is sent out of the volcano in an explosion, it is called ash. Ash and lava are the same basic material.

Lava can be very runny and move very quickly, or it can be sticky and move slowly. The runniest lava is known as basalt lava, and it cools to give a black lava. Because it is runny, any gases trapped in the lava as it runs free easily break out of it. Basalt lava is therefore relatively free of holes where the bubbles would have been. It is also very finely grained with no crystals visible without a hand lens. Basalt lava is too runny to produce steep-sided cones. The Hawaiian volcanoes are made from lava. They are immense, but the slopes from the vent are gentle.

Other volcanoes produce less runny lava. This lava is often brown or even pink (it is called rhyolite). It does not flow far from the cone and helps to build a cone. It can contain large holes and larger crystals. Pumice, which is light grey, and was once the surface of the sticky lava, is full of bubble holes, and that is why it floats on water. The explosive volcanoes all tend to produce lava after they have first exploded and produced ash. Fujiama volcano, Japan and Mt St Helens, USA, are both cones built up with sticky lava and ash.