Midterm Study Guide – GLG115L

1. Identifying Rocks and Minerals

• Identification Tables: Start with the first (left-most) column and move right.

• Igneous Rocks: Begin with texture (aphanitic, phaneritic, etc.), then check composition (felsic, intermediate, mafic).

• No duplicate samples in rock boxes.

2. Texture & Rock Formation

• Texture: Not how a rock feels, but grain size, shape, and arrangement.

• Igneous Rock Texture:

  • Extrusive: Aphanitic, glassy, vesicular, pyroclastic.

  • Intrusive: Phaneritic, pegmatitic.

  • Porphyritic: Combination of coarse and fine grains.

• Sedimentary Rock Texture:

  • Fine ⇔ Coarse grained (low ⇔ high energy environment).

  • Well sorted ⇔ Poorly sorted.

  • Rounded ⇔ Angular (far ⇔ close to source).

• Metamorphic Rock Texture:

  • Foliated: Slaty, phyllitic, schistosity, gneissic (regional metamorphism).

  • Non-foliated: Microcrystalline to coarse-grained (contact metamorphism).

3. Minerals & Rocks

• Minerals = Elements: Example: Quartz (SiO2).

• Rocks = Minerals: Example: Granite (Quartz + Feldspar + Hornblende + Mica).

• Silicon & Oxygen: Most common elements in Earth’s crust → Silicate minerals dominate.

• Mafic vs. Felsic:

  • Mafic: More Fe & Mg, darker, denser.

  • Felsic: More Si, lighter, less dense.

4. Crust & Plate Tectonics

• Continental Crust: Older, thicker, less dense, felsic.

• Oceanic Crust: Younger, thinner, more dense, mafic.

• Plate Boundaries:

  • Convergent: Mountains, volcanoes, trenches, subduction zones.

  • Divergent: Rifts, ridges.

  • Transform: Earthquakes.

• Not all earthquakes occur at plate boundaries: Hotspots (e.g., Hawaii), anthropogenic causes (e.g., fracking).

5. Volcanism & Rock Formation

• Eruption Types:

  • Effusive (Mafic, low Si): Low viscosity, flows easily.

  • Explosive (Felsic, high Si): High viscosity, traps gases.

• Igneous Rock Formation:

  • Intrusive: Slow cooling, large crystals, phaneritic texture.

  • Extrusive: Fast cooling, small crystals, aphanitic, glassy, vesicular.

  • Porphyritic: Mixed cooling rates → Large & small crystals.

6. Sedimentary Rocks

• Types:

  1. Detrital (Clastic).

  2. Biochemical.

  3. Chemical.

• Formation Process:

  1. Weathering & Erosion.

  2. Transportation.

  3. Deposition.

  4. Lithification.

7. Metamorphic Rocks & Their Grades

• Grade Indicators:

  • Foliation presence & type.

  • Mineral arrangement.

  • Index minerals (e.g., micas, garnets).