EduR14 Pumice

3D Model

77×60×31mm

Quaternary (<2.6Ma)

Italy

Pumice is an igneous rock with the same silica-rich composition as granite, evidenced by its light colour as many silica-rich minerals are lighter coloured. Such rocks are typically generated from melting of the crust. It has tiny crystals that are not visible to the naked eye as it often forms when magma is erupted explosively at the surface, meaning it cools quickly and crystals do not have time to grow. As the magma rises, the pressure decreases, the gas bubbles within expand, causing the magma to erupt in an explosion, throwing bits of bubble-filled frothy lava into the air. Since the silica-rich magma is very thick and viscous, like honey, and the lava cools and solidifies so quickly, the air bubbles are trapped inside the resultant rock, making it extremely porous and very light.

Being so porous and light, after the eruption of underwater or coastal volcanoes, large floating patches of pumice, known as rafts, can be seen floating on the ocean surface.