The tallest mountain, Ruapehu, is an andesite cone that was once twice as high but has been eroded. All of the cones are composite stratovolcanoes made up of layers of lava and tephra. They are in the intermediate category, meaning that they have a moderate amount of silica in the magma. The percentage of silica is the deciding factor in the thickness or viscosity of the lava. This contrasts with ignimbrite which is a gas-rich material ejected from the ground at high speed forming a giant white hot fountain 55 kilometres high. When the ignimbrite falls it flows outwards at high speed (600-900 km/h) and can travel many kilometres, even going up and over hills and mountains before it cools, trapping air inside. Especially around Lake Taupo the eroded ignimbrite includes pieces of pumice, a lightweight rock that floats on water. The top, unwelded layer of the ignimbrite sheet is relatively soft and easily eroded by streams and rivers, which transport the pumice to the lake. The lower, welded layers of the ignimbrite sheets are more dense, though still porous. These lower compressed layers often show the glint of heated silicon specks. This stone, called Hinuera stone (after the quarry of origin), is used for building cladding.