EPS 101 Spring 2003– Adrian Brearley - Lectures 10-11

The Oldoinyo Volcano in Africa - Giraffe for Scale

Volcanic Igneous Rocks

Keywords and concepts

General Terminology

Volcanism – processes by which magma, formed by melting processes in the interior of the Earth, rises to surface and is erupted as lava.

Volcano – Hill or mountain formed by the accumulation of volcanic material from eruptions.

Lava – magma erupted onto the surface of the Earth that cools and crystallizes into an extrusive igneous rock.

Lava Types

Basaltic lavas – dark, mafic (poor in SiO₂) magma, erupts at temperatures of 1000-1200ºC. Very fluid, can move fast and long distances.

Flood basalts – Highly fluid basaltic lava – erupts on flat terrain. Spreads out to form thin sheets as a flood of lava.

Pahoehoe – basaltic lava with a ropy surface texture produced by the formation of a thin, flexible crust which is dragged and twisted into coils that resemble ropes.

Aa – basaltic lava that has lost its gases and is more viscous. Moves slowly allowing thick skin to form that gets broken into rough, jagged, angular fragments or blocks of rock.

Pillow lavas – basaltic lavas that are erupted underwater. Tongues of molten lava develop plastic skin on contact with water and form masses of ellipsoidal, sacklike blocks that pile up on one another.

Rhyolitic lavas – very silica-rich (~65-70 wt% SiO₂), light colored lava – very viscous, erupts at temperatures of 800-1000ºC. Moves 10 times more slowly than basaltic magma.

Andesitic lava – lava with intermediate silica content and properties between those of basalt and rhyolite.

Textures of lavas

Volcanic ejecta (tephra) – fragmentary material ejected into the air during a volcanic eruption (pyroclasts).

Volcanic ash – finest grained fraction of pyroclastic material with a grain-size less than 2 mm.

Volcanic tuff – volcanic rock formed by the accumulation and lithification (welding) of volcanic ash.

Volcanic breccia –volcanic rock formed by the accumulation and lithification of larger, angular fragments of pyroclastic debris.

Pyroclastic flow – Glowing cloud of hot ash, dust and gas ejected from a volcano and moving at speeds of up to 200 km per hour, e.g. Mt Pelee, Martinique.

Eruptive Styles and Landforms

Central eruptions – volcanic eruptions where discharge occurs from a central vent, resulting in the formation of classic volcano shaped hill or mountain.

Shield volcanoes – Lava cone built up of successive lava flows erupted from a central vent. Lava spreads easily and widely. Forms broad, shield shaped volcanoes with gentle slopes, i.e. Mauna Loa on Hawai’i.

Cinder cone volcanoes – Formed by the eruption of pyroclastic materials from a central vent. Large fragments form stable, steep upper slopes, but fine fragments travel further from vent and form gentle slopes at volcano base. Classic concave-shaped volcano cone.

Composite volcano – volcano formed by the alternating eruption of pyroclastic material and lava flows.

Crater – Central bowl-shaped depression at the summit of most volcanoes surrounding the central vent.

Calderas – Large, steep walled, basin shaped depression formed by the collapse of a volcano after the magma chamber below the volcano has emptied due to extensive eruptions, e.g. Valles Caldera, NM.

Phreatic explosions – Violent volcanic explosions produced when large quantities of ground water or seawater come in contact with hot, gas-charged magma, e.g. Krakatoa, 1883.

Diatremes – form when hot volcanic material from deep interior of the Earth erupts with explosive force. Vent and channel below went are filled with highly brecciated volcanic materials and rocks and minerals from great depths.

Fissure eruptions – volcanic eruptions that occur along long (10s of kilometers) fractures in the Earth’s surface, e.g. mid ocean ridges.

Flood basalts – large volumes of basaltic magma produced from fissure eruptions. Lavas flow long distances and cover huge areas of land, e.g. Columbia River basalts (200,000 km²).

Ash-flow deposits – Formed from fissure eruptions of pyroclastic materials. Produces extensive sheets of volcanic tuffs, e.g. Yellowstone National Park.

Other volcanic phenomena

Lahars – torrential mudflows of wet volcanic debris. Formed when pyroclastic flow meets a river of snowbank, e.g. Nevado del Ruiz in Columbia.

Edifice collapse – Catastrophic breakup of a volcano, when summit breaks off, due to an earthquake or similar event. Produces massive destructive landslide. e.g. Mt St Helens in 1980.

Volcanic gases. – Gases released from volcanoes consisting of mainly water (70-95%), carbon dioxide and sulfur dioxide.

Fumaroles – vents of gas fumes and steam emanating from volcanos and hot lava flows for many years after eruptions as the volcano and lava cools.

Hot springs and geysers – All phenomena associated with circulating groundwater that reaches buried cooling magma. Water is heated and returns to surface as hot springs and geysers. Geyser is a hot water fountain that spouts intermittently with great force.