Chapter 13 Volcanoes 101 Videoclip
Section 1 Many volcanic eruptions are caused by moving plates- often on or around plate boundaries Most volcanoes are found near the Ring of Fire along the Pacific Plate (see map) Volcanoes are found: Subduction zones Mid Ocean Ridges Hot spots (visual concepts-online book)
Subduction Zones Site of many volcanoes and trench formation Plates slide underneath another plate
Reminds me of a song…….. Ring of Fire
Mid Ocean Ridges Divergent boundaries form new crust as lava cools Most eruptions are not noticed because they are under water
Hot Spots Not on plate boundaries Columns of hot material from the mantle-plumes When the lithosphere drifts, hot spot moves to a different location, creating a new volcano and new continental crust ex) Hawaiian Islands
Hot Spots
Formation of Magma Liquid rock Rock has a melting point at which it becomes liquid Magma can form under 3 conditions: Temperature exceeds melting point of minerals in rock Pressure from surrounding rock is removed Fluids such as water are added and lower the melting point
Magma Can be described as Mafic-high in iron and magnesium –dark in color-forms oceanic crust Felsic –high in silica-light colored-forms continental crust See the visual concept on magma formation Q: Why does magma rise to the surface? Magma is less dense than surrounding rock
Volcanism Any activity in which magma rises to the surface Magma is less dense than rock Volume of magma increases-magma melts rock as it rises to the surface Volcano-vent through which magma and gases travel to the surface Lava-magma on the surface Brain POP: Volcanoes
Intrusive Activity When magma does not reach the Earth’s surface, the magma may cool and solidify inside the Earth This formation is called a pluton Devil’s Tower, Wyoming
Viscosity: resistance to flow Eruption Types Quiet Explosive Viscosity: resistance to flow
Lava has low viscosity-gases can escape easily-usually mafic lava Quiet Eruptions Lava has low viscosity-gases can escape easily-usually mafic lava
Lava Flows aa Pahoehoe Blocky Pillow aa Pahoehoe
Explosive Eruptions Lava has high viscosity-high amount of trapped gases-usually felsic lava
Pyroclastic Materials Rocks that form from lava during an eruption Classified according to size Volcanic dust-less than .25 mm Volcanic ash- .26 mm to 2 mm Lapilli- 2 mm to 64 mm Volcanic Bombs-large clots of lava-rounded shape Volcanic Blocks-the largest rock particles-solid rock blasted from the vent
Composite or Stratovolcano Types of Volcanoes Shield Cinder Cones Composite or Stratovolcano
Shield Volcanoes Broad at the base with gently sloping sides Quiet eruptions, mafic lava, found at hot spots and diverging boundaries Ex) Hawaiian volcanoes
Cinder Cones Usually the smallest in size, steep slopes, small explosive eruptions, felsic lava, converging boundaries
Composite or Stratovolcanoes Large in size, quiet and explosive eruptions, steep slopes, both mafic and felsic lava, convergent boundaries
Composite Volcano in Washington Mount St. Helens Composite Volcano in Washington
Eruption May 18, 1980
Eruption May 18, 1980 USGS Video What information do scientists use to predict volcanic eruptions? Are eruptions a single event or a group of multiple eruptions? Before and After Photos
Calderas-How do they form? Caldera Formation Demo-USGS Crater Lake Oregon