Unit 3: Geologic Time, the Evolution of the Biosphere

& Human Impact

Part 1: Construction of the Geologic Time Scale and Reading the Rock Record

Scale of Geologic Time• Time is so vast compared to human

experience need metaphors to help comprehend– Ex. 4.6 billion years = 1 calendar year– Ozmandias

• How do we know???– Geologists can read the rock record for clues to

earth history and dates

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Reading the Rock Record• Type of rock tells about environment of formation

– sedimentary vs. igneous, limestone vs. conglomerate)

• Relative and absolute dating techniques can give date of rock layer – 543 mya vs. older than this

• Fossils within the rock or presence of certain minerals or structures can tell about the environment and biosphere at the time of the rock formation– Fern vs. coral, oxidized minerals vs. unoxidized

Sedimentary Rocks

• What does this Rock Tell you???

Psst…Listen up Bucko,Back in my day…

Sedimentary Rocks and their environments

• Limestone = ocean, shallow or deep depending on types of fossils

• Sandstone = coastline, stream or lake edge, desert• Shale = quiet deep ocean or lake bottom• Coal = swamp• Conglomerate = fast moving stream, active

coastline, glacial deposit

Igneous Rocks and their environments

• Coarse-grained igneous (ex. Granite)= cools deep underground

• Fine-grained igneous (ex. Rhyolite, Basalt)= cooled rapidly at surface

• Gassy/glassy igneous (ex. Obsidian, pumice)= cooled instantly in air

• Ash=volcanic event

Metamorphic & Other Rocks and their environments

• Localized metamorphic rocks= heat from volcanic activity or fluids

• Regional metamorphism= pressure from tectonic forces (direction can be determined from foliation)

• Shocked quartz, iridium spikes, metallic sediment= meteor impact

• Unsorted, angular sediment sedimentary rock=glacial deposit

Fossils and Structures• Fossils indicative of environment (ex. Fern

vs. coral)

• Fossils help with dating and correlations based on principle of fossil succession: fossils succeed one another in a definite order, and therefore any time period can be recognized by its fossils

• Fossil structures such as ripples, footprints, burrows, crossbedding give evidence of environment

Correlation and Dating using fossils

• Fossil succession

• Idex fossils vs. groups of fossils

Relative Dating: Key Terms

• Uniformitarianism vs. Catastrophism

• Law of Superposition

• Principle of Original Horizontality

• Principle of Cross-cutting Relationships

• Inclusions

• Unconformities