Mar 16, 2016
Volcano:
an openingopening in the earth’s surface through which lava, hot gases, and rock fragments erupterupt
Magma 50-100 miles below the earth’s surface slowly begins to rise to the surface
As the magma rises it melts gaps in the surrounding rock
As more magma rises a large reservoir forms as close as 2 miles below the surface (magma chamber)
How do they form?
Pressure from the surrounding rock causes the magma to blast or melt a conduit (channel) to the surface where magma erupts onto the surface through a vent (opening)
How do they form?
The magma, now called lava, builds up at the vent forming a volcano
How do they form?
Often the volcano sides will be higher than the vent forming a depression called a crater
How do they Form?
Crater:
Caldera: an unusually large crater or the remains when the cone collapses into its own magma chamber
The Birth of a Volcano
Anatomy of a VolcanoCone:
the above ground structure built from lava and/or tephra
Conduit: the path that magma takes from the magma chamber to the vent
Magma Chamber: the reservoir located under the volcano where magma collects and becomes the supply of magma/lava to build the volcano
Lava: molten, liquidliquid rock on the surface of the earth
Parasitic Cone: a smaller secondary volcano
built on the side of or near the main volcano, but sharing the same conduit to the magma chamber
Fumarole: a secondary vent that emits only gases
Fissure: a long fissure (crackcrack) from which lava flows
Vent: opening of the volcano, through which lava, ash and gases flow
Take a minute to label the parts on the diagram (not all parts are shown)
Magma chamber
conduitmantle
Parasitic Cone
Ash Cloud/Gases Vent
Lava Flow
Crater
Two factors determine the type of eruption:
Amount of water vapor & other gases in the magma
The chemical composition of the magma
Types of Volcanic Eruptions
Trapped gases under high pressure will violently explode when the magma reaches the lower pressure of the surface.
Explosive EruptionsExplosive Eruptions
Has granitic magma is very thick and plugs the vent causing the pressure to build until it blows violently out the vent
Explosive EruptionsExplosive Eruptions
Mt. St. Helens
The high water content of the magma produces more water vapor which when mixed in granitic magma produces explosive eruptions
Explosive EruptionsExplosive Eruptions
Mont serrat
Explosive Eruptions-Examples
Mt. Pinatubo
Mt. St. Helens
Mt. Fuji
Eruption of Mt. St. Helens
Low pressure gas
Quiet EruptionsQuiet Eruptions
Has basaltic magma (is more fluid and will flow instead of explode)
Quiet EruptionsQuiet Eruptions
…and has low water content
Examples: Hawaii
Quiet EruptionsQuiet Eruptions
Typical Hawaiian Eruption
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Cinder conesCinder conesoSmall base, steep-sided, loosely consolidatedoUp to 1000 feet talloLife span of a few yearsoCommonly built from gravel size lava rock fragments called cindersoViolent eruptions, dangerous when close---High pressure gas bubbles causes thick lava to explode into the air, lava begins to cool as it rises and falls becoming very stickyoWhen lava hits the ground it sticks rather than flowsoThis builds a steep cone with a small base
Cinder Cones:
Shield volcanoesShield volcanoesoLarge baseLarge base, , gentle slopegentle slope, lava rock layers, lava rock layersoA few A few milesmiles high highoLife span of a Life span of a millionmillion years or more years or moreoThe lava is hot, thin, very The lava is hot, thin, very fluidfluid often often basalticbasaltic
oExample: Example: Hawaiian Islands, IcelandHawaiian Islands, Iceland
Shield volcano on Mars; Taken from space
Take a look at these examples:http://www.volcano.si.edu/world/tpgallery.cfm?category=Shield%20Volcanoes
The Mauna Loa volcano in Hawaii—the largest volcano on Earth—has the broad expanse characteristic of shield volcanoes. It spreads across half the island of Hawaii.
Shield Volcanoes
Mauna Kea
Composite or stratoComposite or stratoLarge mountain volcano often Large mountain volcano often snow snow cappedcapped, a few miles high, a few miles high
Life span of million years or moreLife span of million years or more
Composite or stratoComposite or stratooHave alternating eruptions of Have alternating eruptions of tephratephra (air- (air-borne) and borne) and lavalava. The tephra adds . The tephra adds heightheight to the volcano and the lava to the volcano and the lava cementscements the the tephra together and adds to the tephra together and adds to the basebase. .
oFound mostly in Found mostly in subductionsubduction zones and zones and have violent eruptions.have violent eruptions.
STrato/CompositeSTrato/CompositeLlaima Volcano, Chile.
Composite (strato) Volcanoes Composite (strato) Volcanoes examples: examples:
Mt. RainierMt. Fuji
Mt. Kilimanjaro
Types of Volcanoes Review
Where do Volcanoes Occur?
Where do Volcanoes Occur?location Examples
Divergent Boundaries: where plates move APARTAPART
IcelandIceland
Where do Volcanoes Occur?location Examples
convergent Boundaries: where plates come come togethertogether
Cascade range Cascade range volcanoes, Andesvolcanoes, Andes
Cascade Volcanoes
Where do Volcanoes Occur?location Examples
Hot Spots: plates riding over plates riding over an especially hot an especially hot place in the place in the mantlemantle
Hawaii, Yellowstone, Hawaii, Yellowstone, IcelandIceland
Hot SpotsHot Spots: Formation of the Hawaiian Islands
What comes out of volcanoes?What comes out of volcanoes?
•Lava•Tephra•Gases
Lava—3 kinds:
Pahoehoe lava:
Cooler, thicker, slow moving
harden with a relatively smooth surface
Often has a ropy or wrinkled appearance
Pahoehoe lava:
Aa lava: • Hot, thin, fast
flowing• Hardens with
a rough, jagged, sharp edge surface
Pillow Lava: Lava suddenly cooled
by water shows sack-like
segments (stuffed pillows)
Can you identify the kinds of lavalava from the pictures? Circle
your choice.
Tephra Basically, rockrock fragments• Also known as pyroclasticpyroclastic rock
fragments.• There are many different possible
sizes, from very small (volcanic ashash or dustdust to much larger rocks (called volcanic bombsbombs)
Ash & dust
Volcanic Bombs
This dacite breadcrust bomb (about 15 cm in diameter) was erupted from the lava dome at Mount St. Helens, Washington.
Lahar (mudflow): mixture of ash, eroded land,
and water flowing down river valleys
Lahar (mudflow):
GASESwater vapor, carbon dioxide, nitrogen, sulfur dioxide, hydrogen sulfide, chlorine
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Pg. 3-5Pg. 3-5Textbook pg. 441 Textbook pg. 441
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Active (awake): Has erupted within recent time
and can erupt again at any time. Pre-eruption activities:
Increase in earthquake activity under the cone
increase in temperature of cone, melting of ice/snow in the crater swelling of the cone steam eruptions minor ash eruptions
Volcano Activity Levels (Stages)
Mt St. Helens
Dormant (sleeping):
No eruption within recent times, but there is record of past eruptions
Can become active and erupt again after a “wake up” period
ExampleExample: Mt. Rainier
Extinct: No eruption within recorded
history Not expected to ever erupt
againExample: Mount Mazama (Crater
Lake)
Crater Lake
Mount RainierMount Rainier• The most dangerousdangerous volcano in the US• The danger is mostly from laharslahars traveling
down river valleys at a speed of 25mph and destroying everything in its path
• 100,000 people live on the solidified mudflows of previous eruptions
Mount Rainier• The mountain is dangerously unstableunstable, a
tall, steep heap of loose rock held together by the force of gravity and a cubic mile of glacier ice that could be melted or shaken loose
• Lahar flows average every 500 years500 years and have gone as far as the Puget Sound lowlands
• Mount Rainier has erupted 4 times4 times in the last 4000 years with the last eruption 200 years ago
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Pg. 6-9Pg. 6-9Textbook pg. 448 Textbook pg. 448
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