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Volcanoes!. What is a Volcano?. 1. A volcano is an opening, or rupture , in a planet's surface or crust , which allows hot magma , volcanic ash and gases to escape from below the surface. 2. Where do Volcanoes Occur?.
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What is a Volcano? 1. A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, volcanic ash and gases to escape from below the surface
2. Where do Volcanoes Occur? a. Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together.
2. Where are Volcanoes found? b. Volcanoes may also occur in the middle of plates, away from plate boundaries in locations called hotspots. Hotspots result from the slow movement of a tectonic plate across a hot region (plume) beneath the surface.
3. Parts of a volcano • Conduit – pathway from magma reservoir to the surface • Vent – opening at the surface • Crater – large vent
4. Volcanic Cones When lava erupts from a central vent hole
a. Shield Volcanoes Form from overlapping fluid (pahoehoe) lava flows. They are broad with gentle slopes (less than 6o ) Examples: Mauna Loa,
b. Cinder Cones Made of loosely piled cinders and ash. They have very steep slopes (over 350) but are usually less than 1000’ tall. Examples: Paricutin, Mexico
Paricutin, Mexico • Grew in the middle of a cornfield • Rose 1300’ in one year
c. Composite Cone (Stratovolcano) Made up of alternating layers of lava flows and cinder ash falls. Steeper than shield volcanoes (20 – 300 ) and taller than cinder cones (up to 12,000’). Examples: MSH
d. Plateau Basalts Plateau Basalts - cover 1000s of square miles; these huge seas of rock start as fissureeruptions: lava floods pour out of long cracks to form layers 100s of feet thick; Ex: Columbia/Snake River Basalt Plateaus (200,000 square miles!)
e. Calderas Calderas - circular depression that forms when a volcano collapsesor explodes Ex: Yellowstone Caldera, Crater Lake (Note: <1 mile diameter is called a crater)
f. Volcanic Necks Exposed remnant of the rootsof a volcanic pipe that fed lava to the surface; very hard rock that refuses to erode away; good source of diamonds and gold; Ex: Shiprock, N.M.
5. Types of Eruptions Related to chemicalcomposition of lava.
5. Types of eruptions a. Lavas like basalt are lowin silica (SiO2) and tend to be very fluid and runny and erupt very easily. Gases escape easily from these lavas, so they don’t build up much pressure. These smoothly flowing eruptions are called Quiet Eruptions or Hawaiian eruptions. They often feature lava fountains and rivers and produce pahoehoeand scoria. Hawaiian eruptions are typical of shieldvolcanoes
5. Types of Eruptions B. Lavas like rhyolite are highin silica (SIO2) and tend to be thick and pasty and do not erupt very easily. The thick, pasty lava forms a lava domewhich plugs the volcano’s vent. Trapped gases cause pressure to build, leading to an Explosiveeruption. Explosive eruptions often feature explosive, poisonous pyroclastic surges and slower rivers of hot gases, pumice and ash called pyroclasticflows, as well as huge clouds of ash and dust piercing the stratosphere, but rarely have much in the way of lava flows. Explosive eruptions are typical of stratovolcanoes.
