Themes of the Day:

• Rock Types (Continued)
• Geologic Processes and the Rock Cycle
• Geologic Time Overview, Geologic Time Scale
• Principles of Relative Age Dating
• Interpreting Relative Ages in Geologic Cross-Sections

• Rocks are generally made up of one or more minerals.
• Igneous Rocks
  • Form by cooling and crystallization of magmas
  • Usually tight, interlocking crystals, generally silicate minerals
  • Crystal size (grain size) varies based on cooling rate - fast cooling = fine-grained, slow cooling = coarse-grained
  • Composition (chemical, mineralogical) varies from mafic to felsic
• Sedimentary Rocks
  • Form at or near the surface of Earth by reconstitution (lithification) of fragments or chemical constituents that result from the weathering process
  • Usually formed in stratified layers that obey Steno's Laws (Original Horizontality, Lateral Continuity, Superposition)
  • Clastic, Chemical, and Organic varieties
  • Often record important information about depositional environments
• Metamorphic Rocks
  • Rocks that have undergone changes (textural, mineralogical) in the solid state (no melting involved)
  • Often preserve information about their protolith (pre-metamorphic rock type) and the pressure and temperature conditions under which they equilibrated

Geologic Processes and the Rock Cycle

• Geologic Materials
  • Magma - molten rocks
  • Igneous Rocks - crystallized from a magma
  • Sediment - rock and mineral fragments produced by weathering
  • Sedimentary Rocks - reconstituted from weathered rock fragments or chemicals - usually deposited by wind, water, or ice
  • Metamorphic Rocks - pre-existing rock that has undergone changes in the solid state as a result of elevated pressure, temperature, or fluids
• Transitions within the Rock Cycle
  • Magma cools and crystallizes (solidifies) to form igneous rocks - ex. Hawaiian lava
  • Weathering is the physical or chemical breakdown of a rock (any type) and results in the formation of a sediment
  • Physical sediments can be lithified by compaction and cementation into sedimentary rocks - chemical sediments can precipitate from solutions to form sedimentary rocks
  • Igneous or sedimentary rocks can undergo metamorphism (solid state change) at high T or P or in the presence of fluids
  • Metamorphic rocks may melt to form magmas under conditions of increasing T, decreasing P, or increased fluids
• Rock Origins in Relation to Driving Forces: Internal and External

Geologic Time Overview: Geologic Time Scale

• James Hutton - The Father of Modern Geology
  • First to develop the notion of Deep Geologic Time, Uniformitarianism on a Geologic Time Scale
  • Angular Unconformity at Siccar Point, Scotland - GE
  • "... no vestige of a beginning, no prospect of an end ..."
• Relative vs. Absolute Methods of Age Dating
• Geologic Time Scale

Principles of Relative Age Dating

• Principles of Superposition, Lateral Continuity, and Original Horizontality
• Principle of Crosscutting Relations
• Unconformities
  • disconformity
  • angular unconformity
  • nonconformity

Interpreting Relative Ages in Geologic Cross-Sections

• Block Diagrams: Worked Example: Figure 18.8
• Cross Section #1: Figure 18.4
• Cross Section #2: Home on the Range
• Cross Section #3: Grand Canyon - GE