Earth's Geological Time Scale and Changes

• The geologic time scale is used to organize Earth's history.
• It is continually updated as new rock and fossil evidence is discovered.

Modeling Earth's History

• Earth's history can be represented by a 12-month calendar to conceptualize the vastness of geologic time.

Geologic Change

• Geologic processes like weathering, erosion, and tectonic plate motion constantly reshape Earth over millions of years.
• Present-day geologic processes can offer insights into those of the past, such as volcanoes erupting and glaciers carving valleys.
• Some changes can happen relatively quickly, such as meteorite impacts and volcanic eruptions. Other processes are much slower.
• The movement of tectonic plates is slow, but can build tall mountain ranges and form ocean basins over millions of years.

Scales of Time

• The Himalayan Mountains, for example, grow at a rate of about 1 cm per year due to tectonic plate movement.

Compiling Evidence of Earth's Past

• Rocks and fossils clues about how they formed, past conditions and environments.
• Comparing rocks globally helps scientists organize the timing of events in Earth's history.
• The rock record contains major geologic events, used as benchmarks for specific time periods.
• Absolute dating of events, like a supervolcano eruption, provides evidence for the ages of surrounding rocks and fossils.
• Volcanic tuff deposits from a supervolcano in Wyoming are dated to 640,000 years ago.

Fossil Evidence

• Fossils, provide clues about Earth's past.
• The fossil record catalogues all known fossils and their Earth's history information.
• Matching fossil sequences globally helps apply relative dating principles to determine life changes over time.

Analysis of Rock Layers

• Field geologists analyze undisturbed rock sequences to describe geologic history.
• The oldest rock in a study location is shale, while the youngest is igneous rock.
• There was a period of no rock deposition or active erosion from 450 to 120 million years ago.
• The area was covered by a shallow sea between 120 and 50 million years ago, evidenced by sea turtle fossils.
• Brittle stars lived long before the sea turtle.

Correlating Rock Sequences

• Fossil correlations in undisturbed rock sequences can construct diagrams and interpret the geological history of a region.
• Extinct fossils are generally older than fossils resembling living organisms.

Organizing Earth's History

• Rock and fossil records offers evidence of change, construct timelines that describe Earth's history.
• Geologic timelines can detail geological events or focus on life changes.
• The oldest mineral is 4 billion years old.
• First flowering plants appeared 145 million years ago.
• Dinosaurs became extinct 65 million years ago.
• The supercontinent Pangaea broke up 220 million years ago.

Divisions in Geologic Time

• Earth's history is divided into four eons, eras, periods, and epochs
• Divisions in the geologic time scale are not equal; it is based on major events like extinctions.
• Divisions differ from how time is organized on a clock due to these major events and changes in Earth's history.
• Precambrian time represents almost 90% of Earth's 4.6-billion-year history.

Evidence

• Photosynthetic organisms released oxygen into shallow iron-rich seas over 3 billion years ago.
• Photosynthetic sea creatures existed about 2 billion years ago, enriching the air with oxygen after the seas' iron was depleted.
• Crinoids flourished during the Paleozoic Era, anchored to the ocean floor, and some species exist now.

Examples

• Pterodactylus (Late Jurassic) was a flying meat-eating reptile.
• Horse-like mammal fossil from Paleogene time.
• Members of the genus Homo appeared about 200,000 years ago.