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Origin of magma There is no permanently molten layer within the Earth’s mantle or crust. • Melting within the upper mantle/crust occurs due to local conditions • Increasing temperature • Decreasing pressure • Change in chemistry (addition of water) Factors that lead to melting: Pressure Increase pressure = increase melting temperature Decrease in pressure = decompression melting Occurs at mid-ocean ridges Mantle peridotite Mid-ocean ridge basalt MORB Origin of magma Factors that lead to melting • Role of volatiles • Primarily water • H2O -- 70-85% • CO2 -- 10-15% • Wet rock = lower melting temperature • Subduction zones Partial melting of basalt and mixing with subducted ocean sediments, produces andesitic/rhyolitic magma Origin of magma Factors that lead to melting • Heat • Normal geothermal gradient = 25°C/kilometer • Mantle plumes (hot spots) Hawaii Hotspot Track Origin of magma There is no permanently molten layer within the Earth’s mantle or crust. • Partial melting/Fractional crystallization • Igneous rocks are mixtures of minerals • Melting occurs over a range of temperatures • Produces a magma with a higher silica content than the original rock Peridotite Bulk composition of Earth’s mantle Basalt Andesite Rhyolite Formation of a batholith Granitic batholiths in N America Magma/country rock interactions A sill in the Salt River Canyon, Arizona Basalt dike, Grand Canyon AZ Quartz dike, Lone Pine CA Shiprock NM, a volcanic neck Volcanic eruptions Eruptive violence is due to: • Magma temperature • Magma composition • Dissolved gases in the magma All of these affect magma’s viscosity • Viscosity is a measure of resistance to flow Basaltic Lava Flows Pillow basalts Nicasio reservoir Marin County CA Basaltic Lava Flows • Pahoehoe (pa-hoy-hoy) is a Hawaiian word describing basalt that solidifies with a glassy, ropy texture. – Pahoehoe forms when extremely hot basalt forms a skin. – With flow, the skin is rolled into ropy ridges and furrows. Basaltic Lava Flows A’a flows Basaltic Lava Flows Columnar jointing at Devil’s Postpile Basaltic Lava Flows Columnar jointing at Giant’s Causeway Andesitic Lava Flows • • • • Higher silica content makes andesitic lavas very viscous. Andesitic lava flows slowly (1-5 m/d). The outer crust fractures during flow, creating rubble. Andesitic lava flows remain close to the vent. Rhyolitic Lava Flows • Most viscous lava because of very high silica content. • Rhyolitic lava rarely flows. • Rather, lava piles up as a lava dome, plugging the vent. Explosive Volcanic Eruptions • The products of volcanic eruption take three forms: – Lava flows – sheets of lava that move over the ground before solidifying. – Pyroclastic debris – fragments blown out of a volcano that fall and accumulate. – Volcanic gases – vapor and aerosols that exit a volcano. Pyroclastic Debris • Fragmental material ejected from a volcano. • Consists of glass shards, fragmented lava and rock. • Wide size range: – Ash – powdery glass shards. – Lapilli – pea to plum-sized material. – Blocks and bombs – apple to refrigerator-sized. • Blocks – Pre-existing rock torn from the volcano. • Bombs – Streamlined fragments of ejected lava. Pyroclastic Debris • Fragmental material ejected from a volcano. • Consists of glass shards, fragmented lava and rock. • Wide size range: – Ash – powdery glass shards. – Lapilli – pea to plum-sized material. – Blocks and bombs – apple to refrigerator-sized. • Blocks – Pre-existing rock torn from the volcano. • Bombs – Streamlined fragments of ejected lava. Pyroclastic Debris • Fragmental material ejected from a volcano. • Consists of glass shards, fragmented lava and rock. • Wide size range: – Ash – powdery glass shards. – Lapilli – pea to plum-sized material. – Blocks and bombs – apple to refrigerator-sized. • Blocks – Pre-existing rock torn from the volcano. • Bombs – Streamlined fragments of ejected lava. Pyroclastic Debris • Fragmental material ejected from a volcano. • Consists of glass shards, fragmented lava and rock. • Wide size range: – Ash – powdery glass shards. – Lapilli – pea to plum-sized material. – Blocks and bombs – apple to refrigerator-sized. • Blocks – Pre-existing rock torn from the volcano. • Bombs – Streamlined fragments of ejected lava. Pyroclastic Debris • Fragmental material ejected from a volcano. • Consists of glass shards, fragmented lava and rock. • Wide size range: – Ash – powdery glass shards. – Lapilli – pea to plum-sized material. – Blocks and bombs – apple to refrigerator-sized. • Blocks – Pre-existing rock torn from the volcano. • Bombs – Streamlined fragments of ejected lava. Volcanic glass Pumice Pumice Obsidian Obsidian Ash A size comparison of three types of volcanoes Shield Volcano: Mauna Loa Composite Volcano: Mt St Helens Cinder Cone: SP crater, N. Arizona Paracutin (central Mexico) in eruption Plinean eruption: Explosive eruption in which ejecta is lifted vertically by hot gases and distributed over a wide area A pyroclastic flow on Mt. St. Helens Mt Unzen, Japan (1991) Soufrier Hills volcano (West Indies (1996) The eruption of Mt. Vesuvius in 79 A.D. buried the Roman city of Pompeii beneath several meters of ash Archaeologists began excavating the city in the 18th century Crater Lake, Oregon (Mt Mazama; eruption ~6000 years ago) Crater Lake in Oregon Fissure flows occur when low viscosity lava extrudes along a linear fracture. Flows are often high volume The Columbia River basalts Columbia Plateau flood basalts -- flows cover 200,000 km2 in Oregon, Washington to an average thickness of 1000 meters Deccan Traps, India -- eruption ~ 65 million years ago, covers 5-10 million km2 to an average thickness of 2-4 km Long Valley eruption ~730,000 years ago Cross-section of Long Valley today Yellowstone Hotspot Track Stage 1: Massive Basalt National Parklands Stage 2: Rhyolite Blue #’s = Age of Volcanism (Million Years Ago) Phreatic (steam) eruptions occur when groundwater comes in proximity with magma