Volcano

As new rock forms around a volcanic vent, the resulting volcano takes shape according to the kind of erupted material, which is in turn related to lava composition. The most common types of volcanoes, from largest to smallest, are the shield volcano, composite volcano, and cinder cone.

A shield volcano is a very large, broad, and low profile volcano consisting of layers of basaltic rocks. Shield volcanoes most commonly form in the middle of oceanic plates or in continental rifts, which are areas in which the continental crust is being pulled apart. The shape of shield volcanoes resembles the round shields used by warriors of ancient times to protect themselves in battle. The tallest individual mountain on Earth—the island of Hawaii—is a shield volcano. This volcanic island slopes gently down from the 13,796 ft (4205 m) summit of Mauna Kea to the ocean abyss more than 30,000 ft (9 km) below. Kenya's Mt. Kilimanjaro, the tallest mountain in Africa, is a shield volcano, as is Olympus Mons, the tallest mountain on Mars. Mountains very much like shield volcanoes have been mapped on Venus by the Magellan spacecraft radar mapping expedition. Shield volcanoes can produce large amounts of lava and bury large areas, but are not known for for violent explosive eruptions.

A composite volcano, or stratovolcano, is a large, steep-sided andesitic volcano made of alternating sequences of lava and pyroclastic debris. Composite volcanoes are most commonly located along plate boundaries. Japan's Mt. Fuji is a composite volcano, as are Mount St. Helens in Washington, Mt. Ararat in the Caucasus, and Popocatepetl, near Mexico City. Composite volcanoes can grow over millions of years and then collapse in a cataclysmic event, forming a large volcanic crater known as a caldera.

A cinder cone is a small, steep-sided volcano made of pyroclastic material, with lava composition ranging from basaltic to rhyolitic. Cinder cone eruptions occur as an incandescent liquid solidifies in midair and falls into a heap. Landslides sculpt the sides of the still-hot rock A volcanic cinder cone and lava flow in northern Arizona. Photograph by Tom Bean. Stock Market. Reproduced by permission.
pile, forming such cinder cones as Mexico's Paricutin, New Mexico's Mt. Capulin, and Arizona's Sunset Crater.

Some mafic eruptions occur through cracks known as fissures. Instead of building a mountain in one place, fissure eruptions cover broad areas with basaltic lava flows known as flood basalts. A fissure eruption occurred in Iceland in 1783, and the resulting environmental catastrophe wiped out one-fifth of its population. Fissure eruptions have:

• Filled the Rio Grande Rift with hundreds of feet (100 m) of volcanic rock at Taos, New Mexico.
• Constructed the Columbia River Plateau in Washington and Oregon.
• Covered southern India with approximately 240,000 cubic mi (1 million cubic km) of basalt 65 million years ago, forming the Deccan Traps.
• Formed the Siberian Flood Basalt plateau in northern Russia 250 million years ago, the largest flood basalt in the world.

Some of these basaltic eruptions happened at the same time great extinctions occurred. No conclusive evidence has been found for a connection between fissure eruptions and global extinction, but there is no doubt that the gas and heat released could affect environmental conditions for the worse all over the planet's surface.