Lava is melted rock that has reached the Earth's surface through a volcano's main vent or through side vents and fissures.

Some volcanoes produce little or no lava. Some volcanoes eject pyroclasts, which are fragmented or broken rock. The word pyroclastic comes from a Greek word that means "Rock broken by fire".

When volcanoes do produce lava flows they are classified as either Pahoehoe or Aa. The lava is identical in both pahoehoe and aa lava flows, the difference comes from the amount of lava erupted and the speed of cooling. Pahoehoe lava flows are produced from a small amount of lava that moves slowly, while aa flows usually are associated with a large volume of lava that moves swiftly. Aa flows are generally 6-15 feet thick and pahoehoe flows are usually 1-3 feet thick.

Click on the "Next" button to see the two lava flows side by side!