1. 18.1 Magma
VOLCANIC ACTIVITY

2. 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)

3. Types of Magma Basaltic- Hawaiian Islands Andesitic- Mount St. Helens
Rhyolitic- Yellowstone National Park

4. 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!

5. Viscosity Hotter = less viscous
Basaltic lava temperatures = 1000o- 1250oC
Rhyolitic lava temperatures = 700o- 900oC
More silica = higher viscosity

7. 18.2 Intrusive Activity
VOLCANIC ACTIVITY

8. Plutons Bodies of intrusive igneous rock
Exposed on the surface by erosion

9. Batholiths- large formations that spread over at least 100 km2
Stocks- similar to batholiths but cover less than 100 km2 at the surface.

10. Laccolith- mushroom-shaped; form when magma flows between rock layers and pushes up the overlying rock layers.

11. 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.

12. Plutons and Tectonics
Plutons are caused by mountain-building processes along convergent plate boundaries.

13. 18.3 Volcanoes
VOLCANIC ACTIVITY

14. Anatomy of a Volcano
Lava erupts through openings in the crust called vents.
Bowl shaped areas atop vents are called craters.

15. Volcanoes can collapse and form larger depressions called calderas.

16. 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

17. 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

18. Pyroclastic Flow Rapidly moving cloud of volcanic material
Speeds can exceed 100 mph mph
Temperatures can reach 1800oF

20. 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

21. Where Do Volcanoes Occur?
Hotspots- hot stationary plumes of magma far from plate boundaries