Geology Chapter 5

In what type of setting (plate boundary) do we typically find mafic magma/lava?, felsic magma/lava; a combination of the two?

Subduction Zones - more things going on Initial melt - mafic, derived from asthenosphere Can also have melting of sediments, and slab itself Results in a range of mafic, intermediate (andesitic) and felsic magma Overriding plate Continental lithosphere - melting adds more silica - more felsic lavas/magma Island arcs - Oceanic lithosphere -melting results in andesitic - basaltic lavas, magmas

What is the difference between an active, dormant, and extinct volcano?

Active Volcanoes - erupted in recent history Dormant Volcanoes - no historical eruptions, but is not eroded, worn down Can become active again Extinct Volcanoes - very unlikely to erupt again Active, dormant, & extinct Not set in stone, different things to different volcanoes Active - eruption in past 10,000 years Dormant - no eruptions in past 10,000 years (but expected to erupt again) Extinct - not expected to erupt again

How does addition of fluids cause magma to melt?

Addition of fluids? Subduction zones! Water - present in rocks of seafloor (not liquid) Also present in sediments Plate and sediments are subducted - temperature rises Fluids are driven off - rise into overlying asthenosphere Like using salt on ice (adding something that reduces the melting temperature

How do magmas originate? What causes melting?

Asthenosphere - very close to its melting temp Two ways to cause melting Reduce pressure Addition of fluids (like water)

What are some of the major areas prone to volcanic hazards in the United States? What is a Volcano Observatory?

Cascade Range - subduction zone beneath the Pacific Northwest - 9 volcanoes currently with thermal activity Aleutian Islands - Alaska Long Valley & Yellowstone Calderas Mammoth Lakes In California 1980 - sudden earthquakes 4 magnitude 6 quakes in 48 hours in May 1980 Many smaller quakes Mammoth Lakes - lies WITHIN Long Valley Caldera 13 km long oval depression Violent pyroclastic eruption 700,000 years ago 50,000 years ago - smaller eruptions Caldera - enlarged volcanic crater Formed by explosion or collapse due to emptying magma chamber No"volcano" visible on surface Example of Crater Lake

What are some of the secondary effects of volcanic eruptions? How can volcanoes impact climate or the atmosphere?

Climate and the Atmosphere Single eruption can change global climate - ash high in the atmosphere - causes cooling Dust - can take years to "settle" Krakatoa - temps dropped by 0.5°C - effects felt for 10 years Tambora - 1815 - in 1816 - the "year without summer" Snowstorms in the Eastern U.S. in June Gases also have an impact - sulfur rich gases 1982 El Chichon in Mexico - produced a lot of sulfur gases Created sulfuric acid & acid rain Mt. Pinatubo 1991 in the Phillipines Huge amounts of dust 20 million tons of SO2 created aerosols that blocked sunlight Dust and SO2 - led to declines in temperature

pyroclastic flows or nuee ardentes

Hot glowing cloud of ash and gas Denser than air - travels along surface of ground - clip Name means glowing cloud Temperatures over 1000°C, moves down slopes up to 60 mph Burns everything - extremely dangerous Movie

Are there precursors to volcanic eruptions? If yes, what are they?

Increase is seismic activity Movement of magma in lithosphere results in stress Small (and sometimes large) earthquakes Monitor seismicity around active volcanoes very closely Again, USGS Volcano Observatory Example Harmonic Tremors - continuous, rhythmic tremors - very different from seismic waves Indicate possibility of impending eruption Changes in the ground surface Bulging, swelling, uplift Rising magma and/or buildup of gas pressure Kilauea - inflation of volcano precedes eruption Yellowstone - seismicity and ground deformation Changes in composition of gases emitted Increase in SO2? Changes in ground surface temperature Really an inexact science Mt. Pinatubo - able to evacuate 80,000 people Mt. St Helens in 1980 - exact time of eruption only known seconds before hand Can't predict volume of erupted material Recent evidence - strong earthquakes may occur just prior to volcanic activity Chaitén - eruption occurred after 5.2 magniutde earthquake

Lava

LAVA - not usually life threatening - usually easy to get out of the way Will burn structures, destroy homes ( up 2000°F) Can also engulf large areas - then solidify Movie cinder cones and shield volcanoes

Lahars

Lahars - mixture of volcanic ash and water Creates a fast moving mudflow = lahar Choke stream channels, cause flooding 1991 - Mt Pinatubo in the Phillipines Eruption was followed by rain Triggered devastating lahars Snow capped volcanoes - heat melts snow - produces a lahar as well Hazard for Mt Rainier - near Tacoma Long lasting effects - ash and mud just don't disappear Rivers and streams - clogged - causes additional flooding Cleanup is very expensive composite volcanoes

What is the volcanic explosivity index? What does it measure?

Like a Richter Scale for Volcanic Eruptions How much pyroclastic material, length of eruption, how high in atmosphere material rises

If the outer core is the only part of the earth that is molten, how does magma get generated in the upper mantle & crust?

Magma forms by partial melting of upper mantle and crust. Partial melt means that only a fraction of the available material forms a melt, and that the remainder stays solid. The partial melt rises because of its lower density and ascends through he crust.

Where does magma originate? What tectonic settings are magmas generated in?

Magma originates in the lower part of the Earth's crust and in the upper portion of the mantle Magma generation Upper mantle (50 - 250 km)

What is a composite volcano (or stratovolcano)? What kind of magma occurs in a stratovolcano?

Not all volcanoes erupt the same way, at the same time Pyroclastics, lava, more lava, more pyroclastics, etc. Form Composite Volcanoes (or stratovolcanoes) - made up of layers of different types of material Grow larger than other volcanoes Most of the potentially dangerous ones Lavas - intermediate in silica (SiO2)content - sometimes can trap a lot of gas, sometimes flow SiO2 rich lavas - can ooze out on surface & form a lava dome

How does reduction in pressure cause magma to melt?

Reduction in pressure Divergent boundaries (mid ocean ridges) Overlying plates split and part Melting in the asthenosphere Hot spots Rising plume of mantle material As plume rises, pressure drops, causes melting

What is a shield volcano? What type of magma do we generally find there?

Shield Volcanoes Result from mafic magma Flows freely and over long distances Builds flat, low volcanoes Build-up of many thin flows over time Hawaii - Mauna Loa - about 2.5 miles above sea level (6 miles above sea floor!)

What is meant by the terms felsic and mafic?

SiO2 (silica) - most important Magma (or rock) "poor" in silica = Mafic Silica poor (45 - 50% silica) Magma rich in silica = Felsic Up to 75% silica Continuum between Felsic & Mafic

How are felsic and mafic magmas and lavas different from each other in terms of physical properties? Which type of magma/lava usually has more violent eruptions? Why?

Viscosity of magma - how easily it flows.... Mafic lavas - low in viscosity ("thin") flow easily Mafic Magma Mauna Loa in Hawaii Recent Activity in Hawaii Felsic lavas - higher viscosity "Thick" flow very slowly - stiffer Mafic -gases escape easily Felsic - trap gases, builds up pressure and energy - more explosive eruptions Felsic magma has more silica and alumina content, which makes magma more viscous. Mafic magma has less silica content and more iron and magnesium in it.

What is a fissure eruption? What type of magma do we generally find there?

is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is often a few meters wide and may be many kilometers long. ... Fissure vents are often part of the structure of shield volcanoes. Malfic(basalt) magma

What are the volcanic hazards we discussed in class

lava, ash, pyroclastics, lahars, pyroclastic flows or nuee ardentes, toxic gases, phreatic eruptions

phreatic eruptions

think of a volcanic island surrounded by the ocean Seawater can seep into rock, get heated by magma and turn to steam Huge change in volume as water turns to steam and expands Blows up the whole volcano - called a phreatic eruption

What is a cinder cone?

usually basaltic magma Build up of chunky volcanic cinders Erupted from vent, and fall close Form a symmetrical cone shaped heap

Pyroclastics

violently erupted volcanic material Range in size from ash to house sized blocks! Volcanic Bombs - blobs of liquid lava thrown from a volcano Pyroclastics - fragments of hot rock and lava - more dangerous than lava More sudden, more explosive, spread faster, spread farther Large blocks, bombs - more destructive, but don't travel as far Ash - carried further from volcano, volume can be huge Composite Volcanoes

toxic gases

volcanoes emit carbon dioxide, carbon monoxide, hydrochloric acid Carbon Monoxide, hydrochloric acid - poisonous Carbon Dioxide - not poisonous, but can cause suffocation Cameroon, Africa - 1986 - CO2 emitted from lake with near surface magma below Suffocated 1700 people Mammoth Mountain California Movement of CO2 along fault lines Tree kills, warnings to residents