planet earth volcanism and plutonism_powerpoint_presentation
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- 1. Volcanism and Plutonism
2. Extrusive Igneous Activity Volcanism 3. Summary of Important Concepts
- Volcanism.Refers to the rise of magma which makes its way to the earths surface as LAVA cooling above the earths surface.
- Magma - a mixture of liquid rock, crystals, and dissolved gases beneath the earths surface (within the crust).
- Volcanoes are conical or dome shaped landforms built by the emission of magma and its contained gasses from a constricted vent onto the earths surface.Magma rises in a narrow, pipe like conduit from a magma reservoir beneath and flows at the earths surface as LAVA.
- Volcanoes form from sources ofmagma insidethe earth. The main sources of magma are:
- -Subductionat convergent plate boundaries.
- -Sea floorspreading at divergent plate boundaries.
- -Hot spots .
4. Locations of Volcanic Activity and Volcanic Hazards Volcanic activity is controlled byplate tectonics , because plate movements relate to where sources ofmagmaoriginate inside the earth. Nearly all active volcanoes are located in one of three plate tectonic settings:
- Subduction zones at convergent plate boundaries
- Example: Volcanoes lining the trenches of the Pacific Ocean, forming the Pacific Ring of Fire.
- Rifting and sea floor spreading at divergent plate boundaries ,
- Example: Volcanic eruptions at mid-ocean ridges, and in some rift zones on the continents, like the East African Rift Valley.
- Hot spots
- Example: The Hawaiian Islands and the Galapagos Islands.
5. Map of the worlds active volcanoes, showing that the majority of active volcanoes (about 66%) occur in the PacificRing of Fire . 6. Because the western U.S.A. borders the Pacific Ring of Fire volcanic hazards exist primarily in the western states. On the map, regions with the greatest risk of volcanic hazards are shown in red, and those with the lowest risk are shown in blue. Areas that are uncolored are considered risk free. 7. Zooming in for a closer look at the western U.S., we see in this figure that the volcanoes of theCascade Range , which extends from northern California north into Oregon and Washington, are related to the presence of theCascadia Subduction Zoneoffshore.The most recent major eruption in the Cascade Range was that ofMount Saint Helensin 1980. The Cascade Range is part of the Pacific Ring of Fire. 8. Magma and the Driving Force Behind Eruptions
- Magma may be ejected onto the earths surface as:
- Pyroclastics or tephra- flour sized to boulder sized particles which are thrown in the air due to the built up pressure of gasses.
- The violence of a volcanic eruption depends on the magmasviscosityandgas content . The moreviscous(thick) and moregaseousthe magma, the more explosive the eruption.
Above: Gigantic eruption cloud Above:aa lava Right:pahoehoe lava 9.
- Viscosityis a measure of afluids resistance to flow . The main factor that determines the viscosity of magma is itssilica (SiO2) content . The more silica in the magma, the more viscous it is.The more viscous (silica-rich) the magma, the more violent the eruption .
- We are concerned, then, with three types determined by the chemical composition of the magma.
- Mafic or Basaltic magma low silicon and oxygen , high iron and magnesium. Simple silicate minerals. Dark magmas.Low Viscosity.
- Intermediate or andesitic magma - intermediate silicon and oxygen,intermediate iron and magnesium.High Viscosity.
- Felsic or rhyolitic magma - high silicon and oxygen , low iron and magnesium. Complex silicate minerals. Pale magmas. High Viscosity.
Left:The basic building block of all silicate minerals - the silica tetrahedron.Four oxygen atoms surrounding a single atom of silicon. 10.
- Thegas contentof a magma also relates to its behavior. A magma with low gas content will tend to flow out of a volcano as relatively quiet lava. A magma with high gas content will tend to blow apart violently upon erupting.The higher the gas content, the more violent the eruption .
- The composition of the gases in magma are:
- Water vapor
- Carbon Dioxide
- Minor amounts of sulfur dioxide, hydrogen sulfide, chlorine, and flourine gases
- But it is the amount of dissolved water that typically inspires a volcano to violence
11. Steps to a Volcanic Eruption
- Magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rock.
- As they rise they may encounter a depth (or pressure) where the dissolved gas no longer can be held in solution in the magma, and the gas begins to form a separate phase (makes bubbles).
- When a gas bubble forms, it will also continue to grow in size as pressure is reduced and more of the gas comes out of solution.In other words, the gas bubbles begin to expand.
12. Steps to a Volcanic Eruption: Two Possibilities
- If the magma has a low viscosity, the gas will easily expand to atmospheric pressure at the earths surface and simply burst, and a non-violent eruption will occur, usually as a lava flow.
- If the magma has a high viscosity the gas will not be able to expand very easily creating ahigh pressure inside which will cause them to burst explosively on reaching atmospheric pressure.This will cause and explosive volcanic eruption.
13. It turns out that viscosity and gas content of magma both relate to plate tectonic setting: Divergent boundaries(mid-ocean ridges) andhot spotsboth draw their magmas from theupper mantle . This magma is calledMAFIC MAGMA , and is characterized byrelatively low silica and low gas content .In contrast, the magma atconvergent boundariescomes from melting ofsubducted oceanic plates . This magma is calledFELSIC MAGMA , and is characterized byrelatively high silica and high gas content . 14. Bottom line: The worlds most dangerous volcanoes are those at convergent plate boundaries! 15. Hazard Summary
- The mainHAZARDSassociated with volcanic eruptions are:
- Lava flowsthat burn and destroy what they overrun.
- Ash fallsthat cover vast areas of landscape, creating respiratory problems, messy conditions, and potential lahars.
- Pyroclastic flows : hot, fluid mixtures of rock particles and gas that travel at great speed down the flanks of a volcano; have caused thousands of fatalities.
- Lahars : fast-moving mud flows caused by mixing volcanic ash with water (from rain or from eruptions melting snow and ice on the volcano); responsible for more death and destruction than any other volcanic hazard.
- Gasesemitted during eruptions that may be toxic and/or corrosive; the most common is CO2 gas - when present in large enough quantities it causes suffocation.
16. Lava Flows
- Lava is molten magma that flows out and onto the Earths surface.
- Lava flows are typically formed from low viscosity mafic magma that erupts at divergent boundaries and hot spots .
Fluid basalt lava flows are called Hawaiian type lava and can come in two varieties.Smooth, runny pahoehoe (top) and chunky aa (bottom). 17. The lower silica content (and therefore low viscosity) of mafic magma allows the lava to run down slopes easily. Lava flow eruptions are fairly gentle and quiet. They may cause property damage, but rarely fatalities. 18. Ash Falls
- Ash fallsform when an eruption column of tephra and gas is blown into the air by an explosive eruption. The eruption column can rise up more than 20 km into the atmosphere.
- Tephra is a general term for any size of fragmental material blown out of a volcano.
19. Shown right:Volcanic Bombs form from the rapid cooling of lava thrown in the air.When the lava cools in mid-air, it forms the characteristic ellipsoidal shape. Shown left:Pele is the goddess of the volcano.Teeny tiny volcanic bombs are known as Peles tears. Large-sized tephra typically falls back to the ground on or close to the volcano. 20. Volcanic ash , the smallest tephra fragments, can travel hundreds to thousands of kilometers downwind from a volcano. 1980 Eruption of Mt. St. Helen and map of ash fall distribution and thickness. 21. Ash spreads in upper atmosphere around the globe.The suspended ash can decrease the insolation from the sun and lower global temperatures! 22. Pyroclastic Flows
- Pyroclastic flows (Ash Flows ) are avalanches of a very hot (1300-1800F) mixture of hot rock particles and hot gas that are blown out of the vent of the volcano as an eruption column which subsequently collapses and moves very rapidly down the flanks of the volcano at speeds from 50 to over 200 km per hour and can travel for 10s of kms burning, burying and suffocating everything in their path.
Above:1997 eruption on island of Montserrat in the West Indies.Right:1986 eruption of Augustine Volcano, Alaska. 23. ...destroy by direct impact. Pyroclastic flows.. ...bury sites with hot rock debris. ...burn forests, crops, and buildings. 24. One of the greatest volcanic disasters in history was the destruction of the city of Saint Pierre, Martinique, by a pyroclastic flow from Mount Pelee in 1902.Before the eruption rig