Geology Ch. 6 Notes + Flashcards

6.1-6.4 Wednesday

6.6-6.7 Friday

No class next friday

Read chapter 9

Igneous Rocks

What is a Rock?

1. Solid

2. Generally composed of minerals

3. Formed through specific processes

• Igneous rocks → “born of fire”, forms when molten minerals solidify

• Intergrown minerals in thin sections

• Creation of rock → 50-250km, >800 degrees C below the surface

• Heath, pressure, fluids, mineral

• Undergoes decompression melting underground

• Radioactive decay, friction, and residual are the 3 main types of heat

• Heat → Geothermal gradient (15-50 C/km)

• Felsic (continental) rocks: made from more silica

• Mafic (ocean.) rocks: made from less silica

• Sedimentary rock → lithified sediments, form when rock debris transported by the agents of erosion solidify

• Grains held together by cement in thin section

• Metamorphic rocks → “changed form”, form when pre-existing rock is exposed to excessive heat and pressure

• Grains with parallel alignment in thin sections

• Mafic Volcanoes

• Non Viscous lava; spreads out in a thin sheet

• Dark

• Forms at oceanic crust

• Felsic Volcanoes

• Viscous lava; forms thick, dome-like blob

• Magma: changes in composition

• Fractional Crystallization: Magma starts out as mafic cools and the magma that doesn’t cool yet have silica, making it felsic

• Assimilation:

  • Xenoliths: Wall rock embedded in igneous rock

• Rise of magma

• Density: Magma in the mantle is more dense than rock

• Pressure flows up

• Gas expands, propelling the magma up more

• Magma is likely to stop before it gets to the crust

• Cooling of Magma

• The deeper magma cools, the larger the crystals

• The closer magma is to the surface, the faster it cools, producing smaller crystals

• Intrusive (plutonic, below the surface)

• Cools quickly

• Large crystals

• Phaneritic (visible) crystals

• Most activity is intrusive

• Extrusive (volcanic, outside the surface)

• Cools slowly

• Small crystals

• Aphanitic (without) crystals

Major Types of Igneous Rocks

• Vesicular: rock is full of vesicles, air pockets

• Ex: Pumice, Scoria

• Glassy: Happens when lava flows into the water

• Ex: obsidian

• Pyroclastic: broken pieces of ash from a volcanic eruption

• Ex: Tuff

Igneous Rock Structures

• Sill

• Forms horizontally

• Dike

• Forms vertically

• Batholith

• An extremely large area of granite rock

• Ex: Sierra Nevada batholith

• Pillow Basalt

• Bulbus structures that cool from the outside instantly, creating a glass structure on the outside