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18.1 Magma. VOLCANIC ACTIVITY. Magma. Molten rock, mineral grains, and dissolved gasses deep inside Earth Rocks begin to melt at 800 o -1200 o C Depth, pressure and melting point have a direct relationship However, wet rock melts at a lower temperature (p.472). Types of Magma. - PowerPoint PPT Presentation
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VOLCANIC ACTIVITY18.1 Magma
Magma Molten rock, mineral
grains, and dissolved gasses deep inside Earth
Rocks begin to melt at 800o-1200oC
Depth, pressure and melting point have a direct relationship
However, wet rock melts at a lower temperature (p.472)
Types of Magma Basaltic- Hawaiian Islands Andesitic- Mount St. Helens Rhyolitic- Yellowstone National Park
Magma Composition Viscosity- resistance to flow Basaltic- low viscosity, low
gas, low silica, quiet eruptions Andesitic- found near
subduction zones, intermediate viscosity, 60% silica, intermediate eruptions
Rhyolitic- high silica, high gas, very explosive!
Viscosity Hotter = less viscous Basaltic lava
temperatures = 1000o- 1250oC
Rhyolitic lava temperatures = 700o- 900oC
More silica = higher viscosity
VOLCANIC ACTIVITY18.2 Intrusive Activity
Plutons Bodies of
intrusive igneous rock
Exposed on the surface by erosion
Batholiths- large formations that spread over at least 100 km2
Stocks- similar to batholiths but cover less than 100 km2 at the surface.
Laccolith- mushroom-shaped; form when magma flows between rock layers and pushes up the overlying rock layers.
Sill- parallel to surrounding rock layers.
Dike- cuts across layers of rock.
Sills and dikes vary in thickness from a few centimeters to hundreds of meters.
Plutons and Tectonics Plutons are caused by mountain-
building processes along convergent plate boundaries.
VOLCANIC ACTIVITY18.3 Volcanoes
Anatomy of a Volcano Lava erupts through openings in the
crust called vents. Bowl shaped areas atop vents are called craters.
Volcanoes can collapse and form larger depressions called calderas.
Types of Volcanoes Shield- broad, gentle
slope; non-explosive means of formation
Cinder-cone- small, steep sides, explosive; formed by piled debris
Composite- violent; composed of fragments and solidified lava
Volcanic Material Tephra- rock fragments thrown into the
air by an eruption Classified by size: dust, ash, lapilli,
volcanic blocks and volcanic bombs Blocks are angular; bombs are rounded
Pyroclastic Flow Rapidly moving
cloud of volcanic materialSpeeds can
exceed 100 mph - 450 mph
Temperatures can reach 1800oF
Where Do Volcanoes Occur? Convergent boundaries- subduction zones
where magma is forced upward Divergent boundaries- magma is forced
upward at ridges, rifts, fractures and faults
Where Do Volcanoes Occur? Hotspots- hot stationary plumes of magma far
from plate boundaries
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