- Shield Volcanoes - Low, rounded profiles; slope angles 2-10 ; composed of numerous flows of mafic composition and little explosive activity - Largest of all volcanoes! Shield volcanoes consist of numerous thin basalt lava flows that build up mountains with slopes rarely exceeding 10 degrees.! Profile of Mauna Loa in Hawaii. Mauna Loa is one of five huge shield volcanoes that make up the island of Hawaii.!1
Shield volcanoes are made up mostly of basalt lava flows, but some pyroclastic materials are erupted at lava fountains.!2
- Cinder Cones: made of pyroclastic material - Composed of pyroclastic materials that accumulate around the vent; steep slopes (33 ) - Usually short-lived and may represent final eruptive stages of activity of larger volcanoes - Many form on the sides or within the caldera of larger basaltic volcanoes - small = up to 400m - basaltic to intermediate (andesite) This 400-m-high cinder cone named Paricutín formed in a short time in Mexico in 1943, when pyroclastic materials began to erupt in a farmer s field. Lava flows from the volcano covered two nearby villages, but all activity ceased by 1952.!3
Eldfell, a cinder cone in Iceland. Eldfell began erupting in 1973 and in two days grew to 100 m high. The steam visible on the left side of the image resulted from aa lava flowing into the sea. Another cinder cone, known as Helgafel, is also visible.! - Composite Volcanoes (stratovolcanoes) - made of alternating layers of lava flows and pyroclastic fragments - mafic to felsic in composition - may be gentle or violent! Composite volcanoes, also called stratovolcanoes, are made up mostly of lava flows and pyroclastic materials of intermediate composition, although mudflows (lahars) are also common.!4
- Lava Domes - High-viscosity, felsic magmas move slowly upward to form steep-sided lava domes Lava Domes!! Mount St. Helens in Washington State as it appeared before and after it erupted.!5
! - Lahar: volcanic mudflow made of unconsolidated pyroclastic materials that flow downhill.!6
- Sudden collapse or explosive eruption may cause a nuée ardent to move rapidly downslope, incinerating everything in its path. Also called an ash flow. May travel 100 s of km/hr and flow outward for 100 s of km - Ash flow tuff: any rock formed from an explosive eruption. Pumice - Welded ash flow tuff: rock form from tuff that was put under a lot of heat and pressure!7
St. Pierre, Martinique, after it was destroyed by a nuée ardente from Mount Pelée in 1902. Only two of the city s 28,000 inhabitants survived.! Body casts of some of the volcano s victims in Pompeii. About 2,000 bodies in this city of 20,000 have been found, but certainly far more were killed.!8
! - Supervolcano - Explosive eruption of hundreds of cubic kilometers of pyroclastic material - magma type: felsic - example: Yellowstone National Park!9
- Volcanic neck - Pipe - vent that has filled with magma that has solidified - usually harder than surrounding rock. - as a result, the neck gets exposed - a tube that connects the asthenosphere and the volcano - Fissure Eruptions and Basalt Plateaus - Columbia River basalts flowed from fissures to cover large areas in WA and OR - Low viscosity, mafic lavas spread out and built up a basalt plateau!10
- Is it Possible to Predict Eruptions?! 11
- Distribution of Volcanoes - Most are at or near plate boundaries - Circum-Pacific, Mediterranean and mid-oceanic linear trends are recognized!12
- Plate Tectonics and Volcanoes - Divergent Plate Boundaries - Rift zones: Place where two lithospheric plates are moving apart. Magma replaces the gap. This magma comes from the asthenosphere that has risen and melted due to pressure relief melting - most form in the oceanic basins - Ocean crust is primarily basalt and gabbro - Emplaced as vertical dikes, gabbro plutons, and pillow lava - Mid-Atlantic Ridge, East Pacific Rise, Indian Ridge are examples - Convergent Plate Boundaries - Subduction Zones: Place where two plates are colliding. One plate is driven downwards underneath another plate. The plate that is subducted is oceanic crust, the other is continental crust - What happens - As the oceanic crust is pushed down, hot rock melts in the asthenosphere and rise due to: 1) friction 2) partial melting 3) high water content - Composite volcanoes found in the circum-pacific and Mediterranean belts where partial melting produces in termediate/felsic magmas, lava domes, and violent eruptions!13
! - Intraplate Volcanism - Occurs as a plate moves over a stationary hot spot in the upper mantle - Mantle plume: mass of magma rising out of the mantle!14
(800 o C) Low Temperature (1400 o C) High Temperature High Silica Degasses poorly (explosive) High viscosity (thick) Lighter color rock Lighter weight rock (less dense) Felsic (high SiO2) Low Silica Degasses easily (not explosive) Low viscosity (thin) Darker color rock Heavy weight rock (more dense) Mafic (high Fe & Mg)!15
- Volcano monitoring - Physical and chemical changes - Tiltmeters, seismic activity, past history - Changes in magnetic and electrical fields - Gas emissions, groundwater level and temp - While timely warnings have been issued in the past, volcanoes remain unpredictable and only a few are regularly monitored - If over oceanic crust: gentle - If over continental crust: more explosive - Currently 1000 active volcanoes!16