Lava flows, lava domes and lahars.
Lava flows
There are two common types of lava flows, known in Hawaiian terms as:
- Aa - clinkery, rough-textured, higher-velocity flows.
- Pahoehoe - less viscous, smooth-textured, lower-velocity flows.
Silica-rich magmas produce block-lava flows (whose surfaces consist of dense solidified blocks the size of televisions to small houses) and obsidian flows (which contain large masses of glassy, crystal-poor rocks.)
Most lava flows travel less than 10 km, but fluid pahoehoe flows can reach lengths of 50 km or more. In 2002 for example, fluid lavas from Nyiragongo volcano in the Democratic Republic of Congo flowed through the business district of the city of Goma.
The Global Volcanism Project has some amazing pictures of lava flows
Lava flows obviously cover whatever gets in their path. The methane gas, produced as lava buries vegetation, can migrate in subsurface voids and explode when heated.
Lava Domes
These are formed when viscous magma (silica-rich dacitic-to-rhyolitic compositions) piles up around a volcanic vent. Silicon promotes rigidity in magmas because it has a +4 charge and forms multiple bonds with other elements.
Domes are steep-sided and can reach a few hundred meters high. They can form at the summit of a volcano, on its flanks, or as independent volcanic centres.
Domes can form during single eruptive episodes or by periodic lava extrusion- which happened during the 1980-1986 eruption of Mount St. Helens.
A dome collapse can produce pyroclastic flows and debris avalanches.
Lahar
Lahar is an Indonesian term for a volcanic mudflow. These lethal mixtures of water and tephra have the consistency of wet concrete, yet flow at speeds similar to fast-moving streams of water.
Lahars carry debris ranging in size from ash to lapilli, to boulders more than 10 meters in diameter and can reach temperatures of up to 100 degrees Centigrade, the boiling temperature of water.
The Global Volcanism Project has some fasinating photos of lahars (volacanic mudflows).
When lava erupts beneath a glacier or flows over snow and ice, meltwater from the ice and snow can result in far-reaching lahars. If lava enters a body of water or water enters a lava tube, the water may boil violently and cause an explosive shower of molten spatter over a wide area.
