Geologic Time Scale: Rock Layers & Earth's History

• Geologic time represents the 4.6 billion years since Earth's formation.
• The geologic time scale is a timeline dividing Earth's history into time units based on significant events.
• Fossils and rock layers are used to organize Earth's history.

Rock Layers

• In places like the Grand Canyon, the oldest rock layers are at the bottom, with younger layers on top.
• Stratigraphy studies rock layers to understand Earth's history.
• Relative age dating uses the position of rock layers and fossils to determine their age.
• Absolute age dating calculates the actual age of rocks, using radiometric methods.
• Both absolute and relative age dating determine when different periods began and ended.

History of Geologic Time Understanding

• Aristotle (c. 350 B.C.) suggested the locations of land and water could change over long periods.
• Avicenna (early 1000s) proposed mountains formed layer by layer over time.
• Steno (mid-1600s) established that rock layers form horizontally and chronologically, laying the basis for the law of superposition.
• Werner (late 1700s) incorrectly thought all rocks came from a vast ocean but created a timeline of rock changes.
• Lyell (early to mid-1800s) proposed changes on Earth occurred incrementally, forming the basis for uniformitarianism.

Geologic Time Symbols

• a: annus, year
• Ka: 1,000 years
• Kya: thousands of years ago
• Ma: 1,000,000 years
• Mya: millions of years ago
• Ga: 1,000,000,000 years
• Gya: Billions of years ago

Understanding the Geologic Time Scale

• The geologic time scale divides 4.6 billion years of Earth's history, using fossil evidence and events to define time units.
• Eras are divisions spanning hundreds of millions of years.
• Periods are subdivisions within eras, spanning tens of millions of years.
• Epochs are smaller subdivisions within periods, spanning several million years.
• Ages are subdivisions of Epochs

Current Geological Period

• The current geologic time division is the Holocene Epoch, within the Quaternary Period of the Cenozoic Era.
• The Quaternary Period has two Epochs: Pleistocene and Holocene.
• Scientists debate whether a new epoch should be identified due to human influence on Earth.

Eon Geologic Time Scale

• Eons are the largest divisions of the geologic time scale, spanning millions to billions of years.
• The oldest three eons (Hadean, Archean, and Proterozoic) comprise about 90% of geologic time, known as Precambrian Time.
• The Phanerozoic Eon is the current eon.
• There is minimal fossil evidence for the oldest three eons, as organisms were microscopic.

Decoding the Cenozoic Era Epochs

• The Cenozoic Era contains three periods: Quaternary, Neogene, and Paleogene.
• The Paleogene Period (66 to 23 Mya) includes the Paleocene, Eocene, and Oligocene Epochs.
• The Neogene Period (23 Mya to 2.6 Mya) includes the Miocene and Pliocene Epochs.
• The Quaternary Period (2.6 Mya to current) includes the Pleistocene and Holocene Epochs.

Visualizing the Geologic Time Scale

• The geologic time scale compresses Earth's history into a diagram.
• The four eons from oldest to youngest are: Hadean, Archean, Proterozoic, and Phanerozoic.
• The first three eons constitute the Precambrian time, making up 88% of geologic time.
• The Precambrian time has limited fossil evidence, mainly of simple, microscopic life in the Proterozoic Eon.
• The Phanerozoic Eon began with a proliferation of multicellular life about 542 million years ago.

Eras of the Phanerozoic Eon

• The Paleozoic Era is the oldest era of the Phanerozoic, where life began in the oceans and moved to land.
• A mass extinction marked the end of the Paleozoic and the start of the Mesozoic Era.
• The Mesozoic Era is known as the era of reptiles and dinosaurs with flourishing like on land.
• Another mass extinction 65 million years ago marked the end of the Mesozoic and the start of the Cenozoic Era.
• During the Cenozoic Era, birds, flowering plants, and mammals flourished.
• The Cenozoic Era includes the present time and is when humans evolved.
• Fossil records show when living organisms evolved, flourished, and went extinct, marking the ends and beginnings of eras, periods, and epochs.

Explore geologic time, the 4.6 billion-year history of Earth. Learn about the geologic time scale, rock layers, and the use of fossils in understanding Earth's past. Discover relative and absolute age dating methods.