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Volcanoes!. What is a Volcano?. Volcano : a mountain that forms when layers of lava & volcanic ash erupt & build up. Active Volcanoes spew smoke, steam, ash, cinders, & lava (Tephra: rock fragments thrown into air during a volcanic eruption)
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What is a Volcano? • Volcano: a mountain that forms when layers of lava & volcanic ash erupt & build up. • Active Volcanoes spew smoke, steam, ash, cinders, & lava • (Tephra: rock fragments thrown into air during a volcanic eruption) • Currently about 550 active volcanoes; most are dormant on Earth
How do volcanoes form? • Deep inside Earth, between the molten iron core and the thin crust at the surface, there is a solid body of rock called the mantle. When rock from the mantle melts, moves to the surface through the crust, and releases pent-up gases, volcanoes erupt. But why does this solid rock melt and come to the surface?
From Magma to Lava • Magma being less dense than the surrounding rock causes it to rise & sometimes escape though areas in Earth’s plates called vents(the opening where magma reaches earth’s surface) • Magma that has reached the surface is called lava.
What is Magma? • Magma: a mixture of molten rocks, minerals, & gases. • Factors that affect magma formation: 1. Rock composition: different minerals melt at different temps 2. Pressure: affects temp needed to melt rocks 3. Water Content: affects melting temp 4. Gas Content: more gas, more explosive eruptions 5. Silica Content: more silica, higher viscosity 6. Viscosity: the internal resistance of a substance to flow High silica content: high viscosity Low silica content: low viscosity
Lava Types • Magma comes in different "flavors," or compositions. Each of these will produce a different lava, from fluid, fast-moving basalt to slower, more viscous andesite. Because rocks are made up of collections of minerals that melt at different temperatures, the makeup of the rock being melted affects the magma that results. http://www.learner.org/exhibits/volcanoes/movies/movies3.html
Lava Types:Magma that reaches Earth's surface comes in three essential compositions that produce the three common volcanic rock types: basalt, andesite, and rhyolite. • Basaltic magma, the most common form of magma, results from the partial melting of rock in the zone of Earth's mantle called the asthenosphere. Basalt has the lowest silica content of the three common volcanic rocks, which makes it the least viscous. It also has the lowest gas content. • Fastest flowing lava (up to 16 km/hr) • Quiet eruptions • Usually formed from melted oceanic plate • Ex: Hawaiian Islands
Types of Basaltic Lava Ropy pahoehoe Pahoehoe flow A'a flow
Lava Types:Magma that reaches Earth's surface comes in three essential compositions that produce the three common volcanic rock types: basalt, andesite, and rhyolite. • Andesite can arise in a variety of ways, but most is produced by the partial melting of wet basalt. This often occurs at colliding plate margins where oceanic crust formed of basaltic magma is subducted beneath another plate. Andesitic magma can also be generated by the melting of continental crust. Compared to basaltic magmas, andesitic magmas are normally more viscous and contain more dissolved gases. • Usually formed from interaction of oceanic plate + continental plate • Ex: Mt. Saint Helens
Types of Andesitic Lava Stubby Andesite Flow -- Thick, short andesite flow on the flanks of the Colima Volcano, Mexico. Courtesy of J.C. Gavilanes, Universidad de Colima. Andesite flows on Lascar Volcano -- Located in Chile, the Lascar volcano is the most active stratovolcano in the central Andes. Displaying here is a large blocky lava flow.
Lava Types:Magma that reaches Earth's surface comes in three essential compositions that produce the three common volcanic rock types: basalt, andesite, and rhyolite. • Rhyolitic lava is rich in silica and therefore quite viscous. It can arise from the partial melting of continental crust. Because of its viscosity, which results in the magma solidifying before it can reach the surface, rhyolitic lava is not common. • Involved in explosive eruptions due to high viscosity & gas content (think of shaking up a soda can & opening it) • Ex: Yellowstone National Park
Types of Rhyolitic Lava Obsidian flow -- The Long Valley Caldera, California, was created by crustal collapse associated with an explosive eruption about 650,000 years ago. Since that time, felsic eruptions of de-gassed magma have generated viscous rhyolitic domes and short felsic flows. The example shown in the foreground is Obsidian flow, which erupted about 600 years ago. The flow is about 1.5 km wide across the photo. Wilson Butte, left of Obsidian flow, is a felsic lava dome that erupted about 1400 years ago. Courtesy of S.R. Bradley, USGS.
California's Mt. Shasta Why do volcanoes erupt in different ways? • Most volcanoes occur on plate boundaries. Plate boundaries are areas where Earth's shifting plates meet or split apart, usually with violent results. • Volcanoes can occur on convergent or divergent plate margins or over a hotspot, a spot inside the mantle that heats areas of the plate above it.; hotter than normal.
Volcanoes at Convergent Boundaries • Along convergent margins, two plates meet and one descends beneath the other, a process called subduction. As the descending plate is forced deeper into the mantle, parts of it begin to melt and form magma that rises to the surface, often in explosive eruptions. Convergent margins tend to create large, classic, cone-shaped volcanoes called stratovolcanoes, such as Mt. St. Helens in Washington State. • 80% of all volcanoes • Ex: Ring of Fire Mt. St. Helens before the 1980 eruption -- Mt. St. Helens was considered by many to be one of the most picturesque of the Cascade volcanoes. This view of the northeastern flank of the volcano is taken from Spirit Lake. Courtesy of Jim Nieland, U.S. Forest Service
The most volcanically active belt on Earth is known as the Ring of Fire, a region of subduction zone volcanism surrounding the Pacific Ocean.
Volcanoes at Divergent Boundaries • At divergent margins, plates are coming apart and hot rock forces its way to the surface. Many divergent plate margins are under the oceans, creating long undersea rift zones that fill with lava. In some eruptions at divergent margins, the relatively calm, smooth flow of lava creates volcanoes with gently sloping sides, called shield volcanoes. • Usually balsaltic lava flows so quiet eruptions • Accounts for 15% of all volcanoes • Ex: Mid Atlantic Ridge, Iceland Black smoker from the mid-Atlantic Ridge -- These dark, billowing clouds of hot hydrothermal fluid are rich in dissolved metals. As they rise into the cool ocean water, they precipitate these metals along the sides of the vent, thus creating a "chimney" that builds over time. The fluids also provide nutrients to a variety of unusual plants and animals that congregate around the vents in an area of the ocean where life would generally not exist. Courtesy of Peter Rona, NOAA.
Volcanoes at Hot Spots • Hotspots can also cause shield volcanoes to form. As plates move over hotspots, volcanoes spring up and die down in turn, often creating an island chain. The Hawaiian Islands are the result of a hotspot. • 5% of all volcanoes • Provide important info about plate movement (plate moves while hot spot is stationary) http://www.learner.org/exhibits/volcanoes/movies/movies2.html
Types of Volcanoes • Although every volcano has a unique eruptive history, most can be grouped into three main types based largely on their eruptive patterns and their general forms.
Composite Volcanoes • Eruptions vary between quiet & explosive • Made from alternating layers (quiet & explosive) • Usually contain Andesitic Lava • Found mostly at convergent boundaries, above subduction zones • Ex: Mt Saint Helens
Cinder Cone Volcanoes • Formed from explosive eruptions that throw lava high into the air. This lava cools quickly & forms different sizes of volcanic material collectively called tephra (volcanic ash, cinders, “bomb” rocks,…) • Usually contain Rhyolitic Lava • Steep sides • Smallest overall size Ex: Mount Shasta, Mt. Pele
Shield Volcanoes • Formed from quiet eruptions • Gently sloping sides with nearly circular bases • Usually contains Basaltic lava • Generally the largest • Ex: Hawaiian Islands (Mauna Loa is the world’s largest active volcano) & Iceland
What are Pyroclastic Flows? • Pyroclastic flows are high-density mixtures of hot, dry rock fragments and hot gases that move away from the vent that erupted them at high speeds. They may result from the explosive eruption of molten or solid rock fragments, or both. They may also result from the nonexplosive eruption of lava when parts of dome or a thick lava flow collapses down a steep slope. Most pyroclastic flows consist of two parts: a basal flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow. Ash may fall from this cloud over a wide area downwind from the pyroclastic flow. http://volcanoes.usgs.gov/Movs/anim_pf.mov
What are Pyroclastic Flows? • Pyroclastic Flow: High-speed avalanches of hot ash, rock fragments, and gas move down the sides of a volcano during explosive eruptions or when the steep edge of a dome breaks apart and collapses. These pyroclastic flows, which can reach 1500 degrees F and move at 100-150 miles per hour, are capable of knocking down and burning everything in their paths. Remnant of a building in Francisco Leon that was destroyed by pyroclastic flows and surges during the eruption of El Chichon volcano in southeastern Mexico between March 29 and April 4, 1982. Francisco Leon was located about 5 km SSE of the volcano. The reinforcement rods in the concrete wall are bent in the direction of flow (right to left).
Structure of a Volcano • A volcano constitutes a vent, a pipe, a crater, and a cone. • The vent is an opening at the Earth's surface. • The pipe/conduit is the main passageway in the volcano in which the magma rises through to the surface during an eruption. • The crater is a bowl-shaped depression at the top of the volcano where volcanic materials like, ash, lava, and other pyroclastic materials are released. • A side vent is a vent on the side of the volcano connected to the main vent by a dike or fissure. • Solidified lava, ashes, and cinder form the cone. Layers of lava, alternate with layers of ash to build the steep sided cone higher and higher.
Nice Side Vent! • This photograph is of a volcanic cone. The crater is located at the top. The side vent is active and a lava flow is running down the side of the cone. A fissure is bringing the magma to the side vent.
Inside a Crater! • You are looking at the inside of a volcanic crater. The steep walls were produced be many eruptions ejecting very liquid lava. This lava then lands on the crater walls building them higher and higher. The lava in the main vent is extremely hot (probalbly about 1800 degrees F.) The lava on top cools and hardens because the air that it is in contact with is so much cooler than the lava. This hardened lava will then be dragged back down under the surface and remelted. You probably noticed the same process if you have ever heated soup on the stove. If you did not keep stirring the soup it formed a "scum" on top.
Holy Caldera! • A large volcano crater is called a caldera. Caldera’s are formed when the volcano “blows its top.”
Crater Lakes • Many craters eventually fill in with water and become crater lakes, like this one; Crater Lake, Oregon.
