THEME: Volcanism is a key process in Earth's dynamics

Key Questions:
Where do magmas come from?
How do magmas evolve?
How do the continents "grow" at the expense of the mantle?

OUTLINE:
Magma(lava on Earth's surface)
- naturally occurring mobile rock matter (silicate melt + dissolved gases + crystals).
Most of the crust consists of igneous rocks and their metamor equivalents.
Igneous rocks are classified on the basis ofSi0₂ content and texture (intrusive rocks are generally phaneritic whereas extrusive rocks are generally aphanitic)
Geothermal Gradient: = 30^o C/km (highly variable); - 1000 atm/3 km.
Pressure at Earth's center is = 52 million PSI; Temperature is = 5000^oC.
Magnatic Evolution - experimental data on the melting of granite suggest that granitic liquids can be generated at a depth of =20km.
- experimental data on the melting of basalt suggest that basaltic liquids are generated in the upper mantle (asthenosphere).
- magmas may evolve (by processes such as mixing, assimitiation or fractional crystallization) as they ascend.
- experimental data on the melting of peridotite accounts for partial melting in the asthenosphere and provides an explanation for the origin of basaltic magmas.
- near surface processes also affect magmas.
Global Pattern of volcanism:
- is not random.
- most volcanism occurs along plate boundaries and coincides with active seismic regions.
- type of volcanism depends on the type of plate boundary
- divergent zone (tension)
- quiescent basaltic volcanism in rift zone (produces flood basalts) or along mid-ocean ridges (creates new sea floor).
- convergent zone (compression)
- andesite or rhyolite magmas generated by partial melting of of oceanic crust and associated sediments erupt violently and produce ash flows (nuee ardente); mud flows (lahars) and ash falls.
Ex: Mt. St. Helens - shape of volcanic edifice related to magma viscosity (a function of Si0₂ content) and volatile (H₂0, C0₂, S0₂, etc.) content of magma
- shield volcano erupts basalt relatively benignly
- strato volcano (= composite) erupts andesite or rhyolite explosively
- basaltic volcanism also occurs over hot-spots within plates (Ex: Hawaii in middle of South Pacific)

The process of partial melting of peridolite in the asthenosphere yields basalt which intrudes along divergent zones and produces new oceanic crust.
The process of partial melting of oceanic crust and associated sediments along subduction zones produces andesite and some rhyolite and enables the continental crust to grow, i.e., the continental crust increases in volume at the expense of the mantle.

Review Questions Igneous Rocks

1. What is the difference between an extrusive and an intrusive igneous rock? Briefly explain how texture can be used to determine whether an igneous rock is extrusive or intrusive.
2. Identify and briefly describe two major variables controlling the generation of magmas. Why do magmas generally ascend towards the Earth's surface?
3. Briefly explain how a magma's volatile content (H₂0, C0₂, SO₂, etc.) and weight percent Si0₂ influence the eruptive style of a volcano.
4. Draw cross-sections of a shield volcano, a stratovolcano, and a cinder cone and explain why the external shapes of these volcanoes differ. Indicate the relative size of each volcano type on the cross-section.
5. What is the major chemical constituent of magma? Briefly describe the effect of composition on the fluidity of magma.
6. Compare and contrast volcanism (composition of lava, nature of eruption, type of volcanic edifice, etc.) at convergent plate boundaries with volcanism at divergent plate boundaries.
7. Use a completely labeled diagram to explain how the continental crust grows (i.e., increases in volume) at the expense of the mantle. Your answer should include information about the process of partial melting.
8. What is a "hot spot" and how does it help to explain the origin and evolution of the Hawaiian Islands?
9. Compare and contrast (size, shape, composition, etc.) a dike, a sill and a batholith.