|Terms: extrusive, intrusive, aphanitic, phaneritic, volcano, stratovolcano, shield volcanoe, caldera, pyroclastic, lahar, harmonic tremors|
Significance of volcanism
Volcanic eruptions can be deadly (The Deadliest Volcanic Eruptions). Eruptions cause local devastation, emit poisonous gases and can generate tsunami. Colossal eruptions cause short- or long-term climatic changes leading to crop failures, famine, and possibly extinction. The eruption of Mount Toba 70,000 years ago nearly wipe out Humankind. (See Ambrose, 2005, Bradshaw Foundation). The eruption of Krakatoa in 535 brought on a sustained period of cold lasting nearly 10 years which brought on the famine, The Plague, civil unrest and the Dark Ages.
Towering volcanoes, broad plains and plateaus, and deep basins are just some of the landforms created by volcanism. The type of landform and the ejecta that blankets it reflected the eruptive style of the volcano. Yellowstone National Park lies within a large basin within a plateau built of volcanic ash. Both these features indicate that this sleeping giant with its geyser basins has the eruptive potential to change civilzation as we know it.
Extrusive (volcanic) Landforms
Extrusive landforms are created by igneous activity at the surface of the earth. There is wide range of volcanic landforms that includes large depressions (calderas and Maars), conical mountains (volcanoes), and broad lava plains and plateaus. The size and shape of these landforms is controlled by a number of factors most of which are listed below.
Figure 1. Common volcanic landforms from least explosive (top) to most explosive (bottom). The increasing violence of a volcanic feature is largely related to magma viscosity which is a function of magmas composition. (Mondified from Topinka, USGSICVO, 1997, obtained from the U.S.G.S. Cascade Volcanic Observatory)
1. Type of eruption (eruptive style):
Volcanoes may produce voluminous outpourings of fluid lava, like the Hawaiian volcanoes, or explode violently emitting huge quantities of tephra (ash and the like) and gas. The deadly nature of an eruption, related to the exposiveness and shear volume of material produced, is quantified using the Volcanic explosivity Index (VEI). Fortunately supervolcanos (VEI >7) don't erupt often. The factors that most influence the eruptive style are silica composition and gas content (fig. 2).
- Effusive eruptions are typically nonviolent outpourings of lava. Usually they can be outrun if notified in time. Basaltic lava flows are the most fluid (USGS photo glossary). In contrast, silica-rich felsic lavas are very thick and pasty and produce mounds of glassy rubble. (See Vic Camps Lava Flow Types / mafic flow: Video eruption of Nyragongo / felsic flow: Video of Mayon flow ).
- Pyroclastic eruptions are moderately to extremely explosive eruptions of ash and coarser material (tephra). A pyroclastic flow is a hot cloud of ash and gas that hugs the ground and flows over a hundred kilometers/hour. They are nearly impossible to outrun and deadly. (See USGS photo glossary/Vic Camp pyroclastic flows / Video eruption of Mt. St. Helens/ Video of Pyroclastic flow in Japan, Video and explanation of pyroclastic flow generated by collapse of World Trade Center)
Effects of Temperature and Gas Content: T
- Basaltic and andesitic flows become less fluid as lava cools and gas content decreases. (Note these videos of the 2006 eruption of Mount Etna, the flows are fluid near the vent and pasty farther on after the magma has cold and lost much of its gas.)
- Felsic (silica-rich magmas) are explosive when charged with gas. Later gas-poor eruptions produce pasty flows of glassy rubble. (Observe this video of the 1944 eruption of Mt. Vesuvius in Italy. Note the initial pyroclastic eruption is followed by thick, slow, but unstoppable flows.)
A continuum exists between non-violent effusive eruptions and violent explosive eruptions of pyroclastic flows. This is because there is a continuum between the composition of magmas (fig. 2).
- Eruptive style governed by:
- Silica composition of the magma (controls viscosity) learn more from VolcanoWorld
- gas content (propels lava into the air)
- Water Vapor (50=80%)
- Other gasses include SO2, CO2, H2S, Cl2
Figure 2. Relationship between silica content, eruptive style, and volcanic landforms.
Silica-rich, felsic (rhyolite-dacitic) lavas produce pyroclastic flow, and less commonly lava flows. Some eruptions may be so violent as to create a giant explosion crater, or explosion caldera. Lava flows are thick and pasty, and don't travel far from the vent. Silica-poor, mafic (basaltic) lavas predominantly produce flows. Flow are initially fluid (pahoehoe) but can become rubbly(aa) when cool and gas poor. Rapid release of gas from a basaltic lava will produce non-violent fountaining an cinder eruptions. Fluid basalt flows can travel several kilometers and rapidly flood large areas. Intermediate (andesitic) magmas may exhibit both styles of extrusion depending on range of silica compsition and gas content. Because of the nature of the eject produced different landforms are created.
(Image after Smith and Punn, 2005)
2. Vent characteristics, such as the size, shape (i.e. pipes and fissures), and number can control whether a the landform develops into a volcano (conical mountain) or a broad plateau. (vent: USGS photo glossary)
3. Volume of material erupted over time governs the size and extent of th feature.
1. Cylindrical landforms
- Volcanoes (material accumulates around a central vent or radiating fissures)
- Shield volcano: built from numerous is mafic lava flows (USGS photo glossary). Shield volcanoes are large and broad having a profile similar to a warriors shield.
- Examples (Mauna Loa and Kilauea in Hawaii) Mauna Loa rises 17,179meters (56,000 feet) above the ocean floor making it the largest mountain on earth.
- Composite cone (stratovolcano): composed of andesitic-to-silicic pyroclastic material and thick, but small lava flows (USGS photo glossary). Composite cones are tall and steep.
- Examples: Pinatubo, Mt Fuji (Japan), Krakatoa (Indonesia), Mt. Pelee (Martinique), volcanoes in the Cascade Range (e.g. Mts. St. Helens, Hood, Shasta, etc)
- cinder cone: small satellite scoria cones associated with local fountaining of fluid basaltic or andesitic magma (USGS photo glossary)
- Crater: General term for a depression formed over a volcanic vent
- Caldera: Large crater or basin formed by extremely violent eruptions of felsic pyroclastic material (USGS photo glossary)
- Maars:USGS photo glossary
- Examples: Lake Nyos, Camaroon
- Lava dome: Viscous, felsic plug of degassed magma (obsidian flow) that forms a dome over the crater or caldera vent. Typically formed during the last eruptive stage of a stratovolcano or caldera eruption.(USGS photo glossary)
Figure 2. Pictogram of most common volcanic landforms. (Topinka, USGSICVO, 1998, obtained from the U.S.G.S. Cascade Volcanic Observatory)
2. Plains and Plateaus
- Basalt flows, plains and plateaus: Typically composed of fluid basalts (flood basalts) extruded from fissures (dikes); may be dotted with cinder cones
- Examples: Snake River Plain, Columbia River Plateau, Iceland
- Ignimbrite sheets and plateaus: Composed of welded pyroclastic flows from one or more violent caldera eruptions
- Examples :Katmai ignimbrite sheet; Yellowstone Plateau, Bishop Tuff from the Long Valley Caldera, CA
- 1973 eruption in south-central Iceland--1/3 died from slow poisoning
- 1986: 1700 died of suffocation from an eruption of CO2 from Lake Nayos (maar)
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