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Geology: Volcanoes: 

Geology: Volcanoes


Volcano A mountain that forms when molten rock is forced to the Earth’s surface.

Non-explosive Eruptions: 

Non-explosive Eruptions Quiet eruption in which magma flows easily Usually gas dissolved in magma bubbles out gently Thin, runny lava oozes quietly from vent A’a pahoehoe

Non-explosive Eruptions: 

Non-explosive Eruptions Lava flows: river of red-hot lava

Non-explosive Eruptions: 

Non-explosive Eruptions Lava fountains: sprays of lava, rarely exceeding a few hundred meters in the air

Non-explosive Eruptions: 

Non-explosive Eruptions Relatively calm outpourings of lava can release a huge amount of molten rock. Some of the world’s largest mountains grew from repeated lava flows over hundreds of thousands of years. (EX: Hawaiian Islands)

Explosive Eruptions: 

Explosive Eruptions Thick and sticky magma will not flow out of volcano. Instead, it plugs the vent like a cork in wine bottle. Dissolved gases cannot escape, build up pressure until they explode.

Explosive Eruptions: 

Explosive Eruptions Clouds of hot debris and gases shoot out from the volcano, often at supersonic speeds. Molten rock is blown into millions of pieces that harden in the air. Dust-sized particles can circle globe for years in the upper atmosphere. Larger pieces fall closer to the volcano.

Explosive Eruptions: 

Explosive Eruptions Can blast millions of tons of solid rock and, in just a few minutes, can demolish rock formations that took hundreds or thousands of years to accumulate. Volcano can actually shrink due to repeated eruptions. See next slide: ‘shrinkage’ of Mount St. Helens, WA

Mount St. Helens, WA: 

Mount St. Helens, WA


Magma The composition of magma determines whether a volcanic eruption is non-explosive, explosive, or somewhere between. Water Silica

Magma: water: 

Magma: water A volcano is more likely to erupt explosively if its magma has a high water content. WHY????

Magma: water: 

Magma: water The effect water has on magma is similar to effect carbon dioxide gas has on a soft drink. When can is shaken, CO2 dissolved in soda is released. Because gases need more room than liquids, pressure builds up. When can is opened, soda shoots out.

Magma: silica: 

Magma: silica Explosive eruptions also are caused by magma with large percentage of silica. Silica-rich magma is thick and sticky. Flows slowly & tends to harden in volcano’s vent, plugging it, causing pressure build-up. Thick magma also prevents water vapour and other gases from easily escaping. (Think of popcorn!) Magma with less silica is thinner & runnier. Gases escape more easily, so less pressure builds.

What erupts from volcano?: 

What erupts from volcano? Lava Pyroclastic material

Lava types: 

Lava types Block Pahoehoe A’a Pillow

Lava: block : 

Lava: block Cooler and stiff Oozes from volcano Forms jumbled heaps of sharp-edged chunks. Mexico’s Colima volcano

Lava: pahoehoe: 

Lava: pahoehoe Flows slowly, like wax dripping from a candle Forms glassy surface with rounded wrinkles, ‘ropey’ appearance

Lava: a’a: 

Lava: a’a A`a (pronounced "ah-ah") is slightly stiffer Pours out quickly and forms a brittle crust which is torn into jagged pieces as molten lava underneath continues to move.

Lava: pillow: 

Lava: pillow Forms when lava erupts underwater Forms rounded lumps the size and shape of pillows.

Pyroclastic Material: 

Pyroclastic Material Produced: when magma explodes from volcano and solidifies in mid-air when existing rock is shattered by powerful eruptions

Pyroclastic Material: Size: 

Pyroclastic Material: Size Volcanic blocks: solid rock blasted out of volcano Volcanic bombs: large blobs of magma that harden in air Lapilli: “little stones” (Italian)- pebble-like bits of magma that become solid before hitting the ground. Ash: forms when gases in stiff magma expand rapidly & walls of gas bubbles explode into tiny glasslike slivers.

Pyroclastic Materials: 

Pyroclastic Materials Volcanic block Volcanic bomb Lapilli Volcanic ash

Pyroclastic Flow: 

Pyroclastic Flow a ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano at 100 km/hour or more. temperature may be greater than 500° C, sufficient to burn and carbonize wood. Pyroclastic flow, Philipines

Where do volcanoes occur?: 

Where do volcanoes occur? In three kinds of places: Divergent boundaries Convergent boundaries Hot spots


Divergent boundaries: magma forced upward to Earth’s surface, erupts as lava (EX: Iceland) Grimsvotn volcano, below

Volcano: divergent boundary: 

Volcano: divergent boundary Mantle material rises to fill space opened by separating tectonic plates. As the pressure decreases, mantle begins to melt. (Decreased pressure lowers melting point of mantle.) Because magma is less dense than surrounding rock, it rises to surface, where it forms new rock on ocean floor.


Convergent boundaries: melting of subducted plate creates magma, which is forced upward to surface where it erupts as lava (EX: Japan and Philipines) Toya caldera, Japan, below

Volcanoes: convergent boundary: 

Volcanoes: convergent boundary As oceanic plate moves downward beneath continental plate at subduction zone (because oceanic crust is denser and thinner), some of rock melts and forms magma. When magma is less dense than surrounding rock, it rises toward surface.


Hot spots: magma is forced up through cracks in lithosphere and spills out as lava (EX: Hawaiian islands) Mauna Kea, right

Volcanoes: hot spots: 

Volcanoes: hot spots Hot spots are places on Earth’s surface that are directly above columns of rising magma, called mantle plumes. A hot spot often produces a long chain of volcanoes because the mantle plume stays in the same spot, while the tectonic plate moves over it. EX: Hawaiian islands

Volcanoes: Ring of Fire: 

Volcanoes: Ring of Fire

Ring of Fire: 

Ring of Fire Surrounds Pacific plate. Known for numerous earthquakes and volcanoes. Caused by Pacific plate subducting beneath the surrounding plates. Pacific Ocean is slowly shrinking.


Volcanoes Types: 1) shield 2) cinder cones 3) composite (stratovolcanoes)

Shield volcanoes: 

Shield volcanoes Look like a warrior’s shield. Broad, slightly dome-shaped volcanoes are the world’s largest volcanoes. May erupt many times over a period of more than a million years. Generally have lava that flows easily and can spread over great distances, building up in layers to form the low, wide mountain.

Shield volcanoes: 

Shield volcanoes Shield volcanoes are almost exclusively basalt, a type of lava that is very fluid when erupted. For this reason these volcanoes are not steep. (You can't pile up a fluid that easily runs downhill.)

Shield volcanoes: 

Shield volcanoes

Shield volcanoes: 

Shield volcanoes Mauna Loa is the world’s largest active volcano. It is one of the five shield volcanoes that make up the island of Hawaii. From the floor of the Pacific Ocean to its summit, Mauna Loa is about 6 miles high (taller than Mount Everest), and it is 60 miles long and 30 miles wide.

Cinder Cones: 

Cinder Cones A cinder cone volcano is built up from ashes, cinders, and rocks that burst from Earth during a violent eruption. The rocks fall back to Earth near the opening, to form a cone. Meanwhile, hot ash covers a larger surrounding area. Erupt for a short period of time, so most are not taller than 300 m (about 1000 feet). Occur in clusters or on sides of shield and composite volcanoes. Erode quickly due to pyroclastic material not being cemented together by lava.

Cinder cone: 

Cinder cone

Cinder cone: Mount Etna, Italy: 

Cinder cone: Mount Etna, Italy

Cinder Cone: Pu`u ka Pele, Mauna Kea, Hawaii: 

Cinder Cone: Pu`u ka Pele, Mauna Kea, Hawaii

Composite Volcanoes: 

Composite Volcanoes A composite volcano switches between quiet eruptions of flowing lava and violent eruptions of thick, gas-rich lava (pyroclastic material). This type of volcano has the most powerful eruptions of all. Composite volcanoes, also called stratovolcanoes, form tall conical mountains.

Composite volcanoes: 

Composite volcanoes

Composite volcano: Mount St. Helens: 

Composite volcano: Mount St. Helens

Composite volcano: Mt. Fuji, Japan: 

Composite volcano: Mt. Fuji, Japan

Craters & Calderas: 

Craters & Calderas Crater: a funnel-shaped pit at the top of the central vent of most volcanoes Results from explosions of material out of the vent as well as the collapse of material from the crater’s rim back into the vent Caldera: a circular depression formed when a magma chamber empties & causes the ground above to collapse & sink Generally much bigger than craters


Craters Sunset Crater, Arizona, USA


Calderas Crater Lake NP, Oregon, USA Coatepeque caldera, El Salvador

Valles caldera, NM: 

Valles caldera, NM The remnants of the former Jemez Volcano, now known as Valles Caldera, can be seen in this photograph. The caldera is an almost circular, cliff-ringed pit 14 miles (23 kilometers) across. A composite volcano reached its maximum height 1 million years ago; shortly thereafter, it burst forth with two extremely violent eruptions, spewing out incredible volumes of volcanic gases, ash, pumice, and broken rock. These two massive explosions depleted the magma chamber far beneath the volcano. No longer supported from below, the volcano collapsed, forming a vast caldera.

Valles caldera, NM, from the inside: 

Valles caldera, NM, from the inside

Other Types of Volcanic Activity: 

Other Types of Volcanic Activity Hot spring: forms when groundwater heated by a nearby body of magma rises to the surface and collects in a natural pool. Low Geyser Basin, Yellowstone NP, WY, USA

Other Types of Volcanic Activity: 

Other Types of Volcanic Activity Geyser: a fountain of superheated water and steam that erupts from the ground. Old Faithful, Yellowstone NP, WY, USA

Landforms from Magma: 

Landforms from Magma Volcanic neck: forms when magma hardens in a volcano’s pipe; softer rock erodes away, exposing the hard rock of volcanic neck EX: Shiprock, NM

Landforms from Magma: 

Landforms from Magma Dike: forms when magma forces itself vertically across rock layers and hardens Sill: forms when magma squeezes itself between horizontal layers of rock.

Types of Volcanoes: 

Types of Volcanoes Active: currently erupting Dormant (sleeping): not actively erupting, but likely will in the future Extinct (dead): not likely to erupt in the future

Volcanic effects: immediate: 

Volcanic effects: immediate Clouds of ash can flow like an avalanche, chocking and searing everything in its path Large deposits of ash mix with snowmelt or rain, flowing downhill and picking up additional debris Volcanic ash falling: Buildings collapse under weight Dam rivers Smother animals and crops

Volcanic effects: climatic: 

Volcanic effects: climatic Ash & sulfur-rich gases can reach upper atmosphere Block sunlight >> drop temperatures Last for several years Wetter, milder summers Longer, harsher winters

Major Eruptions: Past 150 Years: 

Major Eruptions: Past 150 Years 1883 – Indonesia: Krakatau: threw 18 km3 ash into air, blast heard 5,000 km away 1902 – Martinique: Mount Pelee: 29,000 residents of nearby St. Pierre killed w/in 2 minutes of eruption (2 survivors) 1912 – Alaska, USA: Mount Katmai: as much as Krakatau 1980 – Washington, USA: Mount St. Helens: 60 people died, 600 km2 forest destroyed 1991 – Philippines: Mount Pinatubo: ash lowered global temp by .5o C, pyroclastic flow reached 250 km/hr 1995 – Montserrat: Soufriere Hills: 2-year eruption destroyed tourism economy of this Caribbean island

Predicting Volcanic Eruptions: 

Predicting Volcanic Eruptions Measure small quakes: produced as magma moves within them Measure slope: bulges may form as magma pushes against the inside of the volcano Measure volcanic gases: change in ratio of sulfur dioxide to carbon dioxide Measure temperature from orbit: satellite images record infrared radiation and track temperature changes

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