Where fresh rock is made
Deep underground, rock melts into MAGMA, which is lighter than the solid rock around it and slowly rises. Where it finds a way to the surface through a VENT, it erupts as LAVA, and when that cools it becomes brand-new rock, often basalt. Most volcanoes sit along the edges of the Earth's plates. About three quarters of the world's active volcanoes circle the Pacific Ocean in the Ring of Fire, where ocean floor slides down into the MANTLE and melts. Others, such as those of Hawaii, sit over hot spots in the middle of a plate.
In November 1963, fishermen off the south coast of Iceland saw smoke rising from the sea. An underwater eruption was building a brand-new island, named Surtsey, which had grown to more than a kilometre across by the time it stopped in 1967. Scientists have kept people off it ever since, to study how plants, insects and birds arrive on bare new land. Our rocks and minerals page covers the rocks that form.
A town frozen in time
In AD 79, VESUVIUS, near modern Naples, erupted and buried the Roman towns of POMPEII and Herculaneum under metres of hot ASH and rock. A young Roman writer, Pliny the Younger, watched from across the bay and later described the huge cloud rising like a pine tree, the first detailed eyewitness account of an eruption. His uncle, Pliny the Elder, died trying to rescue people by ship.
The ash preserved Pompeii remarkably well: streets, shops, paintings and even loaves of bread in ovens. In the 1860s an archaeologist, Giuseppe Fiorelli, noticed hollow spaces in the hardened ash where bodies had decayed, and filled them with plaster, making casts of the people and animals in their final moments. Vesuvius last erupted in 1944 and is still watched closely, because around three million people live near it.
Ash clouds and flying
In April 2010 an eruption under the Eyjafjallajökull glacier in Iceland threw a cloud of fine ash high into the sky. Volcanic ash can melt inside a jet engine and clog it, so air traffic across most of Europe was stopped for days, and around 100,000 flights were cancelled. The eruption of KRAKATOA in Indonesia in 1883 was far bigger: its ash spread round the world, and for months afterwards sunsets glowed unusually red.
A LAHAR is a fast, deadly mudflow of ash and water, often when an eruption melts snow on the SUMMIT. PUMICE is lava so full of gas bubbles that it floats. SULPHUR gives many volcanic places their rotten-egg smell.
Sleeping, waking and very big
An ACTIVE volcano has erupted recently or is showing signs of life, such as a TREMOR in the ground. A DORMANT one is quiet but could erupt again; FUJI, Japan's highest mountain, last erupted in 1707. ETNA, on Sicily, is one of the most active volcanoes in the world and erupts every few years. A CALDERA is a huge hollow left when a volcano collapses into its own emptied magma chamber, far bigger than an ordinary CRATER. The word VOLCANO itself comes from Vulcan, the Roman god of fire and metalworking, whose forge was said to be under a small island off Sicily called Vulcano. Every ERUPTION starts with MOLTEN rock.