Rock melts when the temperature within the earth (geotherm) exceeds the melting point (solidus) of rock. This happens for different reasons at (1) subduction zone volcanoes, (2) mid-ocean ridge volcanoes, and (3) hotspot volcanoes. Normally the geotherm does not cross the solidus, so there is no melting.
(THE MANTLE IS SOLID!!!!) BUT, it is very close at about km in depth ( Asthenosphere). What are the 4 main forms of volcanoes?
1. Seafloor Subduction Subduction Zones: wet melting Mt. Fujiyama Cotopaxi Volcano, Equador Cotopaxi, by Frederic Church, 1862 Stratovolcano (Composite Cone) Cerro Negro, Nicaragua Cerro Negro, Nicaragua Paricutin, Mexico (1946) Mayon, Philippines (1984) Pacaya, Guatemala Pacaya, Guatemala (Agua volcano in background) Pacaya, Guatemala (2004) Pyroclastic flow sweeps down the side of Mayon Volcano, Philippines, 1984. Mayon, Philippines (1984) Mayon, Philippines (1984) Mayon, Philippines (2000) Mt. Pinatubo, Philippines, 1991. Pinatubo, Philippines (1991) Pinatubo, Philippines (1991) Pinatubo, Philippines (1991) A small lahar triggered by rainfall in Guatemala, 1989. Pinatubo, Philippines (1991) Pinatubo, Philippines (1991) Mt. Pinatubo: So much ash into the atmosphere that Earths temperature dropped, and sunsets were redder. What are the 4 main forms of volcanoes?
2. Mid-Ocean Seafloor Spreading Ridges: pressure release melting Figure 4-15b Ocean Crust Layers What are the 4 main forms of volcanoes?
3. Continental Rifting Continental Rifting leaves a complex structure beneath passive margins like the east coast of North America What are the 4 main forms of volcanoes?
4. Hotspot Mantle Plumes Hawaii rises more than 5 miles above the seafloor. Hawaii Kilauea, Hawaii Mauna Loa, Hawaii Figure 4-7b Kilauea, Hawaii Figure 3-1 Pahoehoe lava, Hawaii Aa lava flow, Kilauea, Hawaii Figure 4-10b Figure 4-10c Mt. St. Helens: Giant Eruption May 18, 1980 Mt. Adams Mt. St. Helens Mt. St. Helens: Before May, 1980 After Phase 1: Small earthquakes and puffs of steam indicate that magma is rising.Bulge develops in North face. Phase 2: A magnitude 5.1 earthquake shakes mountain, dislodging bulge which slides down mountain. Decreased pressure on magma starts lateral blast. Phase 3: Eruption causes a second block to break free, exposing more magma and initiating an eruption column. Lateral blast goes at 300 mph, covers 230 square miles. Phase 4: The Eruption Column reaches 80,000 feet in less than 15 minutes. Mt. St. Helens Earthquakes: 1995-2005 The Dome is Growing Again The center of the Yellowstone Caldera is rising up at 7 cm/year!
Magma chamber structure and uplift in Yellowstone. Recent GPS and InSAR studies show that the Yellowstone caldera is uplifting at a rate of 7 cm/yr, which is apparently related to a magma recharge (Chang et al., 2007). In receiver functions recorded by EarthScope station H17A from 100 teleseismic earthquakes in 2008, two P-to-SV converted phases exist that are consistent with the top and bottom of a low velocity layer (LVL ) at about 5-km depth beneath the Yellowstone caldera. P- and S-wave velocities suggest at least 32% melt saturated with about 8% water plus CO2 by volume. (from Chu et al., 2010) Chang, W.L., R.B. Smith, C. Wicks, J.M. Farrell, and C.M. Puskas, Accelerated uplift and magmatic intrusion of the Yellowstone Caldera, 2004 to 2006, Science, 318, 952 956, 2007. Chu, R., D. V. Helmberger, D. Sun, J. M. Jackson, and L. Zhu, Mushy magma beneath Yellowstone, Geophys. Res. Lett., 37, L01306, doi: /2009GL041656, 2010. Yellowstone Plume Shear-wave velocity anomalies in a cross section aligned parallel to the track of the Yellowstone hotspot and the absolute velocity of the North American plate (Obrebski et al., in review). The section shows the strong low-velocity anomaly in the upper 300 km beneath the eastern Snake River Plain and a low-velocity conduit extending as deep as can be resolved (1000-km depth) beneath Yellowstone. (left) Thermal model for a plume rising beneath Yellowstone showing the effects of the moving lithosphere (Lowry et al., 2000). The imaged low-velocity conduit is more complex in shape than in the simple thermal model, which is likely due to its interaction with other objects in the mantle (not shown). Lowry, A.R., N.M. Ribe, and R.B. Smith, Dynamic elevation of the Cordillera, western United States, J. Geophys. Res., 105, 23,37123,390, Obrebski, M., R.M. Allen, M. Xue, S.-H. Hung, Plume-Slab interaction beneath the Pacific Northwest, in review. Finished 15 minutes early again. Not many questions
Finished 15 minutes early again. Not many questions. Got to Chemical weathering in the next lecture. Shear-wave velocity anomalies in a cross section aligned parallel to the track of the Yellowstone hotspot and the absolute velocity of the North American plate (Obrebski et al., in review). The section shows the strong low-velocity anomaly in the upper 300 km beneath the eastern Snake River Plain and a low-velocity conduit extending as deep as can be resolved (1000-km depth) beneath Yellowstone. (left) Thermal model for a plume rising beneath Yellowstone showing the effects of the moving lithosphere (Lowry et al., 2000). The imaged low-velocity conduit is more complex in shape than in the simple thermal model, which is likely due to its interaction with other objects in the mantle (not shown). Lowry, A.R., N.M. Ribe, and R.B. Smith, Dynamic elevation of the Cordillera, western United States, J. Geophys. Res., 105, 23,37123,390, Obrebski, M., R.M. Allen, M. Xue, S.-H. Hung, Plume-Slab interaction beneath the Pacific Northwest, in review.