Impacts of ExtrusiveIgneous ActivityA2 Geographical Research
Learning Objectives• Look at the various aspects of extrusive igneous activity• formation of volcanic cones and fissures• The formation of Lava plateaux
What is extrusive igneousactivity?• In simple terms extrusive igneous activity refers to when magma reaches the surface of the earth and becomes known as lava.• The lava flows once cooled form landforms such as volcanoes
3 Forms of activity• Major extrusive activity can come as a gas, liquid or solid:• Gas - sulphur, hydrogen, carbon dioxide and hot steam(geysers).: When uprising magma decreases in pressure suddenly, gases within magma explode to the earths surface and cause destruction.• Solid - pyroclast: Mainly composed of: fragments of country- rock, solidified lava and fine materials of volcanic ash and dust. It can be classified in term of size into volcanic bombs, volcanic blocks, lapilli, volcanic ash and volcanic dust.• Liquid – lava, hot springs.
Extrusive Landforms• There are several types of extrusive landforms whose nature depends on how gaseous or viscous the lava is when it reaches the earth’s surface.• Lava produced by the upward movement of material from the mantle is Basaltic and tends to be located along mid-ocean ridges, over hot spots and along rift valleys.
• Lava that results from the process of subduction is described as andesitic (after the Andes) and occurs as island arcs or at destructive plate boundaries where oceanic crust is destroyed.
Fissure Eruptions• When two plates move apart lava may be ejected through fissures rather than a central vent.• The Heimaey eruption of 1973 in Iceland began with a fissure of over 3km.• This is small compared to Laki also in Iceland in 1783 where a fissure exceeding 30km in length opened up.• The basalt may form a large plateaux, filling hollows rather than building up into a typical cone- shaped volcanic peak.• The columnar jointing produced by the slow cooling of the lava provides tourist attractions e.g. Giant’s Causeway
Basic or shield volcanoes such as Mauna Loain Hawaii have lava flowing out of a centralvent and can spread over a wide area beforesolidifying. The result is a cone with longgentle sides made up of many layers of lavafrom repeated flows.Acid or dome volcanoes – acid lava quicklysolidifies in the air and this produces a steepsided, convex cone as most lava builds upnear to the vent. In the case of Mt Pelee thelava actually solidified as it came up the ventand produced a spine rather than flowingdown the sides.
Ash and cinder cones (e.g. Paricutin) form whenash and cinders building up into a symmetricalcone with a larger crater.Composite cones – many of the larger, classicallyshaped volcanoes result from alternating typesof eruption in which first ash and then lava(usually acidic) is ejected. Mt Etna is a result of aseries of both violent and more gentle eruptions.
Calderas - when the build up of gasesbecomes extreme, huge explosions mayclear the magma chamber beneath thevolcano and remove the summit of thecone. This causes the sides of the craterto subside, thus widening the opening toseveral kilometres in diameter. In the caseof both Thira and Krakatoa, the enlargedcrater or caldera has been flooded by thesea and within the resultant lagoons, latereruptions have formed smaller cones.
Minor Extrusive LandformsMinor extrusive landforms are often associated with, but notexclusive to, areas of declining volcanic activity. They includesoltfatara, fumaroles, geysers and mud volcanoes.
The Nature of Eruptions• Icelandic - lava flows gently from the fissure, usually on flat slopes• there is no central crater.• Giant cracks open in the ground and expel vast quantities of lava that spread far and wide to form huge pools that can cover almost everything around.• When these pools of lava cool and solidify, the surface remains mostly flat.• Since the source cracks are usually buried, there is often nothing "volcano-like" to see--only a flat plain.•
Hawaiian type (VEI 0-1)• there is a small vent through which small amount• fluid basaltic lava is thrown into the air in jets from a vent or line of vents (a fissure) at the summit or on the flank of a volcano.• The jets can last for hours or even days, a phenomenon known as fire fountaining.• The spatter created by bits of hot lava falling out of the fountain can melt together and form lava flows, or build hills called spatter cones.• Lava flows may also come from vents at the same time as fountaining occurs, or during periods where fountaining has paused.
Hawaiian type• Because these flows are very fluid, they can travel miles from their source before they cool and harden.• Hawaiian eruptions get their names from the Kilauea volcano on the Big Island of Hawaii, which is famous for producing spectacular fire fountains.• Two excellent examples of these are the 1969-1974 Mauna Ulu eruption on the volcano’s flank, and the 1959 eruption of the Kilauea Iki Crater at the summit of Kilauea.• In both of these eruptions, lava fountains reached heights of well over a thousand feet
Strombolian type (VEI 1-2)• These are distinct bursts of fluid lava (usually basalt or basaltic andesite) from the mouth of a magma-filled summit conduit.• The explosions usually occur every few minutes at regular or irregular intervals.• The explosions of lava, which can reach heights of hundreds of meters, are caused by the bursting of large bubbles of gas, which travel upward in the magma-filled conduit until they reach the open air.
Strombolian type • This kind of eruption can create a variety of forms of eruptive products: • Spatter, or hardened globs of glassy lava. • Scoria, which are hardened chunks of bubbly lava; lava bombs, or chunks of lava a few cm to a few m in size; ash • Small lava flows (which form when hot spatter melts together and flows downslope). • Products of an explosive eruption are often collectively called tephra.
Strombolian type• Strombolian eruptions are often associated with small lava lakes, which can build up in the conduits of volcanoes.• They are one of the least violent of the explosive eruptions, although they can still be very dangerous if bombs or lava flows reach inhabited areas.• Strombolian eruptions are named for the volcano that makes up the Italian island of Stromboli, which has several erupting summit vents.• These eruptions are particularly spectacular at night, when the lava glows brightly
Vulcanian type (VEI 2-3)• A Vulcanian eruption is a short, violent, relatively small explosion of viscous magma (usually andesite, dacite, or rhyolite).• This type of eruption results from the fragmentation and explosion of a plug of lava in a volcanic conduit, or from the rupture of a lava dome (viscous lava that piles up over a vent).• Vulcanian eruptions create powerful explosions in which material can travel faster than 350 meters per second (800 mph) and rise several kilometres into the air.• They produce tephra, ash clouds, and pyroclastic density currents (clouds of hot ash, gas and rock that flow almost like fluids).
Vulcanian type• Vulcanian eruptions may be repetitive and go on for days, months, or years, or they may precede even larger explosive eruptions.• They are named for the Italian island of Vulcano, where a small volcano that experienced this type of explosive eruption was thought to be the vent above the forge of the Roman smith god Vulcan.
Vesuvian type• Typified by the eruption of Mount Vesuvius in Italy in A.D. 79, great quantities of ash- laden gas are violently discharged to form cauliflower-shaped cloud high above the volcano• It has a long period of inactivity but right after it erupts with enormous power
Peléan eruption (VEI 3-4)• They can occur when viscous magma, typically of rhyolitic or andesitic type, is involved, and share some similarities with Vulcanian eruptions.• The most important characteristics of a Peléan eruption is the presence of a glowing avalanche of hot volcanic ash, a pyroclastic flow.• Formation of lava domes is another characteristic feature. Short flows of ash or creation of pumice cones may be observed as well.• The initial phases of eruption are characterized by pyroclastic flows.
Peléan eruption • The tephra deposits have lower volume and range than the corresponding Plinian and Vulcanian eruptions. • The viscous magma then forms a steep- sided dome or volcanic spine in the volcanos vent. • The dome may later collapse, resulting in flows of ash and hot blocks. The eruption cycle is usually completed in few years, but in some cases may continue for decades. • The 1902 explosion of Mount Pelée is the first described case of a Peléan eruption, and gave it its name.
Krakatoan or Plinian (VEI 4-8)• They are caused by the fragmentation of gassy magma, and are usually associated with very viscous magmas (dacite and rhyolite).• They release enormous amounts of energy and create eruption columns of gas and ash that can rise up to 50 km (35 miles) high at speeds of hundreds of meters per second.• Ash from an eruption column can drift or be blown hundreds or thousands of miles away from the volcano.• The eruption columns are usually shaped like a mushroom (similar to a nuclear explosion) or an Italian pine tree; Pliny the Younger, a Roman historian, made the comparison while viewing the 79 AD eruption of Mount Vesuvius, and Plinian eruptions are named for him.
Point of Note• Vesuvian and Plinian are often to referred to as one and the same but differentiations are made within certain textbooks• Plinian eruptions are extremely destructive, and can even obliterate the entire top of a mountain, as occurred at Mount St. Helens in 1980.• They can produce falls of ash, scoria and lava bombs miles from the volcano, and pyroclastic density currents that raze forests, strip soil from bedrock and obliterate anything in their paths.• These eruptions are often climactic, and a volcano with a magma chamber emptied by a large Plinian eruption may subsequently enter a period of inactivity.
Eruptions and the VEI• VEI was proposed in 1982 as a way to describe the relative size or magnitude of explosive volcanic eruptions.• It is a 0-to-8 index of increasing explosivity. Each increase in number represents an increase around a factor of ten.• The VEI uses several factors to assign a number, including volume of erupted pyroclastic material (for example, ashfall, pyroclastic flows, and other ejecta), height of eruption column, duration in hours, and qualitative descriptive terms
Eruptions and the VEIThe classification of the eruption shows some similarity to the Volcano Explosivity Index(VEI) developed in the USA.
•In the figure, the volumes of severalpast explosive eruptions and thecorresponding VEI are shown.• Numbers in parentheses representtotal volume of erupted pyroclasticmaterial (tephra, volcanic ash, andpyroclastic flows) for selectederuptions; the volumes are foruncompacted deposits.• Each step increase represents a tenfold increase in the volume of eruptedpyroclastic material.
What Determines Eruption Type?• The crystal and gas content and temperature of a magma help determine a volcano’s eruption style.• Crystals in magma make it more viscous, so magma with a high crystal content is more likely to explode than flow.• Gases create explosions if they cannot easily escape from viscous magma, but they can also be released without explosions (or with only minor ones) from fluid magma.• High-temperature magmas usually erupt effusively, while low-temperature magmas cannot flow easily and are more likely to erupt explosively.
Lava Domes• Are mounds that form when viscous lava is erupted slowly and piles up over the vent, rather than moving away as a lava flow.• The sides of most domes are very steep and typically are mantled with unstable rock debris formed during or shortly after dome emplacement.• Most domes are composed of silica-rich lava which may contain enough pressurized gas to cause explosions during dome extrusion.