Volcano

Lava flows are streams of molten rock that flow onto Earth's surface from a vent or fissure, and most commonly have a basaltic composition. Two common types of basaltic lava have been distinguished in the Hawaiian language. The Hawaiian words adopted into English to describe these different lava rocks are aa (pronounced AH-ah), also called blocky lava, and pahoehoe (pronounced pa-HOY-hoy) or ropey lava. Aa forms from viscous and slow-moving, and aa flows are characterized by an irregular, jagged appearance. Pahoehoe flows, in contrast, are characterized by smooth, wavy surfaces. Submarine eruptions of basalt form large lobes known as pillows, and are commonly referred to as pillow basalts.

Pyroclastic ("fiery fragment") deposits are the result of explosive eruptions. Explosive eruptions occur when magma containing water or gases (or magma that has been in contact with ground water) rises near enough to the surface that the pressure exerted by the rock above it can no longer keep the magma from boiling. The result is an explosive eruption of pyroclastic debris. Volcanic dust, ash, cinders, and blocks are collectively known as tephra. Ash from pyroclastic eruptions can cover large areas, thinning with distance from the volcano. The rock produced by a volcanic ash fall is known as tuff.

An ash flow, or pyroclastic flow, is a dense body of ash, superheated gases, and rock that moves as a fluid from an erupting volcano, crossing the landscape and filling valleys with the fluid mixture. This material deflates as it cools and produces a rock known as ignimbrite, or welded tuff. Ignimbrites can cover hundreds of square kilometers of landscape, such as the Mitchell Mesa Tuff of West Texas. Pyroclastic flows from a prehistoric eruption of Taupo, a volcano in New Zealand, produced ignimbrite deposits that covered the tops of hills hundreds of meters tall.

A pyroclastic surge is a kind of pyroclastic flow that occurs when magma encounters groundwater close to Earth's surface. Also called a nuee ardente, a French phrase meaning "glowing cloud," this was the kind of eruption that destroyed the city of St. Pierre, on the Caribbean island of Martinique, in 1902. The volcano that is formed by this kind of eruption is called a maar or tuff ring.