L8: Volcanoes Flashcards
What are the different manifestations of volcanic matter?
- Central eruptions
- Pyroclastic flows
- Phreatic explosions
- Fissure eruptions
- Diatremes
List the types of central eruptions
- Shield volcanoes
- Volcanic domes
- Cinder cone volcanoes
- Composite volcanoes
- Craters/calderas
Shield volcanoes
- Basaltic lavas are relatively silica-poor (45-50% silica) therefore its less viscous compared to more silica-rich andesitic and rhyolitic lavas
- Build relatively broad, shield-shaped volcanoes (large circumference, low diameter)
- Individual lava flow = thin; build up and can make it thick
Volcanic dome
- Viscous, silica rich (70%) lava – usually rhyolite and or andesitic, otherwise generally cooler lava
- Flows less readily, these lavas pile up close to the vent and can contribute to the building of compact, high peaks
Cinder-cone volcano
- Formed entirely of pyroclastic materials that accumulate around the volcanic vent
- Silica magmas = more viscous
- Materials fall symmetrically to form a cone shape; steep sides w/ larger fragments falling towards the middle finer fragments carried further to form gentle slopes near the base of the cone
- Its short so that’s indicative of short duration of activity
Stratovolcano
- Aka composite volcanoes
- Comprise alternating pyroclastic layers and lava flows
- Grow larger than both dome and cinder cones – have slopes of intermediate steepness
- Characterized by intermittent eruptions over a long-time span, making them among the most dangerous
- Gas-laden and andesitic in composition, the magma can accumulate in significant reservoirs in subduction zones
Caldera
- “Craters”
- Form in depressions of volcanoes after an eruption by the recession of lava down the vent
- Erosion contributes significantly to the crater size
- The magma chamber below may not survive an eruption so the roof collapses to form a caldera
What are resurgent calderas?
- May see resumed activity, being one of the most destructive natural phenomena known
- Ex. under Yellowstone
What are the stages in a caldera?
1: Fresh magma fills a magma chamber and triggers a volcanic eruption
2: The eruption continues and the magma chamber becomes partly depleted
3: Mountain summit collapses into the empty chamber, forming a caldera. Large pyroclastic flows accompany the collapse, blanketing the caldera and a surrounding area of hundreds of square kms
4. A lake forms in the caldera. As the residual magma in the chamber cools, minor eruptive activity continues in the form of hot springs and gas emissions. A small volcanic cone forms in the caldera
What are pyroclastic flows?
- AKA Nuee Ardente
- Mixtures of hot springs, gas, and rocks (pyroclasts) can form a super-heated and dense current capable of moving 200 km/hr
What is pyroclastic flow?
Because solids are buoyed up on a layer of hot gas so that there’s little frictional resistance. These flows are generated during some violent eruptions, during which molten material often cools before falling to Earth
What occurs when water meets magma?
Turns to superheated steam
Phreatic Eruptions
- The result of excessive subsurface pressure due to steam build-up, akin to an exploding boiler
- Ex. 1883 island of Krakatoa
What are fissure eruptions?
- Highly fluid basaltic lava may flow out of fissures, rather than a single pipe or vent, constituting a “fissure eruption”
- Provides majority of volcanic rock
- Most important occur at the spreading centres of mid-ocean ridges
What are large igneous provinces?
Large fissure eruptions on land
Define what a mass extinction event is.
Event in which many types of organisms die out over a very short period
What are the stages in a diatreme?
- Gas charged magma from deep in the mantle forces its way upward, fracturing the lithosphere
- Rapidly ascending magma breaks off and carries crust and mantle fragments as it explodes at supersonic speed
- After the eruption, the feeder channel forms a distreme made up of solidified magma and these rock fragments, or breccia
- The softer sediments of the cone and surface of the crust erode, leaving the diatreme core and radiating dikes we see today
Kimberlites
- Another form of diatreme (a volcanic form of peridotite)
- May include many other mantle fragments, including diamonds
Global pattern of volcanism
- 80% - found at convergent boundaries
- 15% - found at divergent boundaries
- The rest - found at intraplates
How does lava composition vary?
Varies between different plate-tectonic settings
What are the volcanic and plate tectonic margins
Divergent margins, convergent margins, intraplate zones
Divergent margins
- Basaltic magma is drawn from a broad region of the asthenosphere and forced upwards through the relatively narrow vents
- The melt overflows and in this way forms the ocean ridges, volcanoes, and seafloor crust
- They occur at spreading zones
Convergent margins
- Magmas that arise from subduction are much more varied in composition than those basaltic magmas at spreading ridges
- Amount of silica present alters the amount of sediment that is subducted
_______ is easier to melt than ___________ rocks.
- Hydrous material
- Anhydrous
Intraplate zones
- Hotspots and how deep magmatic mantle plumes rise from the mantle and ascend to the lower pressures at earth’s surface where it induces the melting of the asthenosphere and the eruption of basaltic lava
- Hot spots leave a trail of extinct, progressively older volcanoes as plates move over the mantel; plumes are thought to be fixed in the mantle
Lahar
Special combination of volcanic ash and water forming mud
How do pyroclastic deposition cause damage?
- Primarily through large volumes of “ash fall”
- Volcanoes emit various gases
How can climate effects be felt globally from volcanic eruptions and explosions?
- Dust can block sunlight, lowering the global temperature in a manner akin to a “nuclear winter” hypothesis and take years to settle
- Sulfur-rich gases can induce surface acid rain and aggravate ozone depletion
- Ex. Mt. Helens producing detectable ash fall felt halfway across the US
Define deposition
Accumulation of sediment following transport
