Chapter 9 - Volcanoes Volcano – An opening in the Earth’s surface through which magma and volcanic gases pass. Magma – Molten rock, gases and water vapor inside the Earth. Lava – Molten rock that flows out onto the surface of the Earth. Eruption Types: Nonexplosive – The most common eruption where huge amounts of lava are released. Explosive – A much rarer but destructive type of eruption with hot debris, ash and gases rapidly shooting out from the volcano. Ash – Dust-sized particles of rock that get thrown into the atmosphere.

Magma Chamber – The underground body of magma that feeds the volcano. Vent – The opening in the Earth’s surface through which magma is released. The explosive potential of the volcano depends on the silica, water and gas content of the magma. The more water, the more explosive due to the underground pressure build up of the steam. Silica rich magma is thicker and can build up more pressure. What erupts from a volcano: Lava – Liquid magma that flows out through the vent. Blocky – Flows slowly; oozes and doesn’t travel very far. Pahoehoe – Flows slowly; has a glassy surface and resembles dripping wax. aa –Flows quickly; forms a brittle crust. Pillow lava – Flows quickly; forms underwater. Pyroclastic Material – Magma that has been blasted into the air and hardens before landing. ia

Types of Pyroclastic Material Bombs – Blobs of magma that cool as they are thrown high in the air. Lapilli – Pebble-sized bits of magma that harden before hitting the ground. Ash – Most of the material ejected during a volcanic eruption. Blocks – House-sized pieces of rock that are sheared from the vent throat and thrown into the air. Pyroclastic Flows – Dangerous eruptions in which huge amounts of hot ash, dust and gases blast from the volcano with temperatures of 1300°F and speeds of 130 m.p.h.

Types of Volcanoes The effects of a volcanic eruption are not only dangerous immediately but the enormous amounts of ash and gases ejected into the upper atmosphere can change the climate worldwide resulting in famine and disease. Shield Cones - can be enormous but are usually built on layers of lava in non-explosive eruptions. Cinder Cones – smaller and formed from moderately explosive eruptions that last a short time and are composed of pyroclastic materials such as ash. Composite Cones – one of the most common types formed from alternating explosive/non-explosive eruptions and lava flows.

Volcanic Landforms Crater – A funnel shaped pit at the top of many volcanoes. Caldera – A large depression formed after the eruption and much larger than the crater. A crater with collapsed walls. Lava Plateaus – Formed by repeated eruptions with massive outpourings of lava spreading out over a large area. These are usually formed by rift zones (huge cracks in the surface of the Earth). Causes of Volcanic Eruptions: The rock of the mantle is hot enough to melt but the pressure prevents the molecules from liquefying. However, when the pressure is decreased, magma forms and begins to rise because it is less dense than the surrounding rock. Where Volcanoes form: 80% of the worlds volcanoes form at convergent plate boundaries and 15% form at divergent boundaries. The Ring of Fire – The area of the Pacific plate perimeter where many volcanoes are found.

Tectonic Plate Boundaries Divergent Boundary – Areas where tectonic plates separate forming rift zones (deep cracks). The pressure rapidly decreases allowing the rock to melt and form magma which rapidly rises to fill in the rift zones. Mid-Oceanic Ridges – Undersea rift zones that produce volcanoes and mountains by producing new crust from erupted lava. Convergent Boundary – An area where tectonic plates collide. Subduction Zones – Areas where one tectonic plate is pulled underneath (subducted) under another. Oceanic crust will subduct under continental crust because it is denser than continental crust. This process generates tremendous heat and pressure that helps in the formation of magma and new volcanoes.

Hot Spots – Volcanically active places that are far from plate boundaries. These usually will form volcanic island chains. A. Mantle Plumes – Areas with rising magma that form hot spots. B. Cracks in the Earth’s crust that permit magma to rise upward. Volcanic Activity: A. Extinct – Volcano has not erupted within recorded history and most likely never will. B. Dormant – Volcano is currently not erupting but may again. C. Active – Volcano is currently erupting or showing signs of erupting in the near future. Eruption Prediction 1. Watch for an increase in earthquakes. 2. Observe the ratio of certain gases. 3. Tiltmeters and GPS detect changes in the volcano's slope. 4. Infrared satellite images watch for an increase in temperature.