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