A Volcano is An opening in Earth s crust through

Transcription

1 Volcanoes

2 A Volcano is An opening in Earth s crust through which molten rock, gases, and ash erupt. Also, the landform that develops around this opening.

3 Kinds of Eruptions Geologists classify volcanic eruptions as quiet or explosive. How a volcano erupts depends on the magma. A quiet eruption moves slowly because it is low in silica and flows very easily. An explosive eruption has magma that is high in silica and does not flow easily. Magma builds up until it explodes out of the vent.

4 Magma Vs. Lava Magma molten rock, and solids found beneath the surface of Earth Lava molten rock expelled by a volcano during an eruption, found on the surface of Earth.

5 Magma and Erupted Materials Silica Content Gas Content Viscosity Least (~50%) Least Basaltic Magma Least viscous AndesiticMagma Intermediate (~60%) Intermediate Intermediate Rhyolitic Magma Most (~70%) Most Most viscous Type of Eruption Rarely explosive Sometimes explosive Usually explosive Melting Temp. Highest Intermediate Lowest Location Rifts, oceanic hot spots Subduction boundaries Continental hot spots

6 Stages of Volcanic Activity Geologists often use the term active, dormant, or, extinct to describe a volcano s stage of activity. Active volcanoes are erupting or showing signs that it will erupt soon. Dormant volcanoes are no longer erupting, but can again in the future. Extinct volcanoes are not likely to erupt any longer.

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8 Volcano Terms Ash Fine particles of pulverized rock blown from an explosion vent. Vent - The opening at the earth's surface through which volcanic materials issue forth. Magma chamber - The cavity containing the gas-rich liquid magma which feeds a volcano.

9 Volcano Terms Crater- A steep-sided, usually circular depression formed by either explosion or collapse at a volcanic vent. Throat-Entrance of a volcano. The part of the conduit that ejects lava and volcanic ash. Conduit (pipe)-a passage followed by magma in a volcano. Caldera -a large, circular depression that forms when the magma chamber below a volcano partially empties and causes the ground above to sink

10 Volcano Terms Base -the lowest part, bottom, or supporting layer of the volcano. Sill - intrusive igneous rock, parallel to the layering of the rocks into which it intrudes. Dike - intrusive body of magma that pushes its way across layers of sediments.

11 When a Volcano erupts the force of the expanding gases pushes magma from the magma chamber through the pipe until it flows or explodes out of the vent. The volcano has a pocket of magma below the surface called a magma chamber. A long tube, called a pipe, connects the magma chamber to the surface. The opening at the top of the pipe is called the vent. When it leaves the volcano magma becomes lava.

12 Viscosity Viscosity measure of how thick (viscous) and sticky a liquid is Lava Flow - A 'stream' of molten rock 'aa' lava flow - jagged, rubbly, broken surface; high viscosity (pronounced 'ah-ah') 'pahoehoe' lava flow - smooth, ropy surface; low viscosity (pronounced 'paw-hoeyhoey")

13 Where do Most volcanoes Occur? Latitude gives the position on Earth north or south of the equator Positive north of the equator, negative south of the equator Longitude gives the position on Earth east or west of the Prime Meridian Positive east of the Prime Meridian, negative west of the Prime Meridian

14 Why Volcanoes form Volcanic Eruptions occur when magma rises to the surface. This will happen when the asthenosphere melts enough to flow. There are three things that can cause this: 1- A decrease in pressure (like at a midocean ridge or rift valley) 2- An increase in temperature (like at a hot spot) 3- An increase in the amount of water in the asthenosphere (like at subduction)

15 Where Volcanoes Form Most volcanoes occur at: 1. DIVERGENT boundaries, and at 2. CONVERGENT boundaries that have SUBDUCTION. 3.Hot Spots

16 Volcanoes at Divergent Boundaries Decrease in pressure as plates pull apart lets magma rise. These fissure volcanoes are located at Mid-ocean ridges like Mid-Atlantic Ridge and the Great African Rift Valley

17 Divergent- Great Rift Valley

18 Volcanoes at Convergent Boundaries (Subduction) Subduction increases the amount of water in the asthenosphere, which lowers the melting temp. As the denser oceanic crust is pushed lower, it melts into magma. When continental and oceanic plates converge, a volcano forms on land. When 2 oceanic plates converge together, a volcano forms an island. Mount Hood/ Portland

19 Volcano- Convergent Sibayak, Indonesia Mt St. Helens

20 How Volcanoes Form Volcanic activity takes place primarily at subduction boundaries, 1. Water in the subducted rock is released into the asthenosphere. 4. Some of the magma reaches Earth s surface, and volcanoes form on the overriding continental plate. Oceanic lithosphere Continental lithosphere Asthenosphere 2. The water lowers the melting temperatures of materials in the asthenosphere, leading to magma formation. 3. The magma is less dense than its surroundings, so it rises.

21 Hot Spots Sometimes, volcanoes occur at places that aren t plate boundaries. We call these HOT SPOTS. Hot spots are areas where hot magma rises from deep in Earth s mantle. Magma escapes where the crust is the thinnest or weakest. It starts out solid then it melts when it reaches areas of lower pressure.

22 Example of Hot Spot Volcanoes that make up Hawaii.

23 Types of Volcanoes There are 4 main types of Volcanoes: Shield Volcanoes Cinder Cone Volcanoes Composite Volcanoes Fissure Volcanoes

24 Volcano Categorization Type of volcano is based on: Eruption style Structure And primarily the composition of magma Tephra is fragmental material produced by a volcanic eruption regardless of composition, or fragment size. Volcanologists also refer to airborne fragments as pyroclasts. Once pyroclasts have reached the ground its tephra

25 Shield Volcanoes Shield Volcanoes form from runny lava (low viscosity) that tends to flow long distances before hardening. They generally have quiet eruptions. Volcano has a broad base and gently sloping sides. Looks like a warrior s shield hence the name Volcano has a less explosive eruption than other types because the lava flows more easily Type of magma- Mafic Basalt: black, runny lava Made of layers of lava Example: Mauna Loa, Hawaii

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27 Cinder Cones Form when molten lava is thrown into the air from a vent. As it falls, it breaks into fragments called ash or tephra that harden before hitting the ground. Most common type of volcano The ash and tephra make a cone-shaped mound. They are smaller than other types. Erupt explosively because magma is thick, which allows pressure to build up. (medium viscosisty) Magma composition: Felsic Rhyolite (light colored)

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29 Cinder Cones New Guinea Iceland

30 Composite or Stratovolcano Form from alternating eruptions of quiet lava and explosive ash. The layers build up and make a moderate-sized volcano. High viscosity Made of layers of ash/tephra and lava called strata. Magma composition- Andesite: medium color

31 Example of Composite/Stratovolocano Mt. Pinatubo-Philippines

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33 Fissure Volcanoes Form in long cracks where plates are pulled apart and near other volcanoes where the crust is weakened. Cinder Cone or Shield Volcanoes may also be nearby.

34 Supervolcano A volcano that ejects more than 1,000 cubic km (240 cubic miles) of material, When one explodes, it can cover an entire continent in ash. Form huge calderas after erupting

35 Supervolcano Effect Eruptions harm climate and living organisms by blocking sunlight Affects photosynthesis (decreases) and temperature (decreases) Thought to be behind several mass extinctions in Earth s history

36 Yellowstone Check out how large some of the lava flows were from the Yellowstone Volcano!!!

37 How does a Caldera Form? A) A Volcano s pressure starts to build up B) The Volcano releases large amounts of Lava C) As the Lava is released, the pressure decreases and the volcanic mountain begins to collapse, forming a concave shape in the center of the volcano. D) The center of the volcanic mountain may begin to fill with water and form a lake.

38 Crater Lake in Oregon State should actually be called Caldera Lake

39 Volcanic Explosivity Index This scale is designed to express the severity of volcanic eruptions. Based on the amount of erupted tephra

40 Label the parts of a volcano Word Bank: Main Vent/Crater Secondary Vent Magma Chamber Lava Ash & Dust Cloud Tephra Bombs Cone

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