Transcript Volcanoes
Volcanoes Let’s Talk Magma… • Felsic Magma – – – – High silica content Thick Light colored Slow moving • Mafic Magma – – – – Low silica content Thinner Dark colored Easy flow Magma’s Journey • Magma rises from the asthenosphere to cracks in the lithosphere • On its way to the surface, decreasing pressure lets dissolved gases expand • Once on the surface, magma becomes LAVA (because previously dissolved gases have escaped to the atmosphere and new materials have been added, the chemical composition has changed) Lava Fragments • Tephra: solid fragments of lava • Ash: smallest pieces of tephra • Blocks: large pieces of tephra erupted as solids • Bombs: large pieces of tephra erupted as liquid (they harden as they fall) • Tephra + Gases = Superheated cloud Types of Volcanoes • • • • Cinder Cones Shield Volcanoes Composite Volcanoes Volcanic Domes Shield Volcanoes The common product of hotspots, but they can also be found along subduction-related volcanic arcs or all by themselves Almost exclusively BASALT (which is very fluid when erupted) Slow, steady, non-explosive flow gives it a shield-like shape Magma mixes with only oceanic crust on the way to the surface Lava has a LOW silica content (50%), so it has a LOW viscosity (resistance to flow) Composite Volcanoes Occur from the build-up of multiple eruptions that can be hundreds of years apart Almost exclusively ANDESITE Volcanologists call these "strato-" volcanoes because they consist of layers of solid lava flows mixed with layers of sand- or gravel-like volcanic rock called cinders or volcanic ash. Magma mixes with oceanic (basalt) and continental (rhyolite) crust on the way to the surface Lava has an intermediate silica content (60%), so…it moves at a medium pace Volcanic Domes Highly explosive! Mainly RHYOLITE Magma is typically too viscous to move far from the vent before cooling and crystallizing. So…they are very steep sided Magma mixes only with continental crust on way to surface Lava has a high silica content (70%) and moves slowly!