History of Geologic Time

History of Studying Geologic Time

• The history of geologic time is connected to the human interest in mining.
• Commercial mining in the 1500s and 1600s led to increased interest in rock units.
• Strata, or rock layers, began to be identified by the types of fossils found within them.
• Nicolaus Steno and James Hutton are two scientists credited with significantly contributing to this area of study.

Nicolaus Steno

• Steno adapted two geologic principles.
• Law of Superposition: Sedimentary rocks' ages can be determined by their position; the oldest are on the bottom, and the youngest are on top.
• Principle of Original Horizontality: Undisturbed rock layers stay in horizontal order.

James Hutton

• Hutton was an 18th-century Scottish physician and farmer.
• He studied his farmland for geologic changes.
• "The Present is the Key to the Past".
• Principle of Uniformitarianism: Geologic processes from the past are the same as the current geologic processes.

Relative Age

• Relative Age determines the age of a rock layer based on the ages of the layers around it (i.e. younger or older).

Law of Crosscutting Relationships

• A fault or body of rock is younger than any other body of rock that it cuts through.
• Igneous intrusions that are uplifted to the surface are younger than the rock they push through.
• Unconformities: Breaks in geologic time where rock layers erode away or sediment isn't deposited.

Paleontology

• Paleontology is the study of fossils.
• Fossils: remains of plants and animals that lived in a previous geologic time.
• Organisms have changed throughout the geologic past.
• Fossils provide clues behind environmental changes of the past.

Fossilization

• Organisms with hard parts are typically preserved.
• Those without hard parts are fossilized by quick burial.
• Fossils are formed through:
  • Mummification
  • Amber
  • Tar seeps
  • Freezing
  • Petrification

Mummification

• Mummification is the drying of organisms; found in deserts.

Amber

• Amber is hardened tree sap where insects get stuck and the sap hardens.

Tar Seeps

• Tar seeps are thick petroleum where animals get stuck in tar, preserving them.

Freezing

• Freezing preserves animals because bacteria cannot survive in a cold climate to decay the bodies.

Petrification

• Petrification occurs when organic material is replaced with minerals.

Types of Fossils

• Trace Fossils: evidence that an animal existed (e.g., footprints)
• Molds: Animal remains dissolve away, but the shape of the animal remains.
• Cast: When a mold fills in with sediment and becomes hardened.
• Imprints: Carbonized imprints
• Coprolites: Fossilized animal droppings that are used to learn eating habits.
• Gastroliths: Stones found in dinosaur stomachs to help them digest food; generally round and polished.
• Index fossil: A unique fossil that lived during a specific time.
• Trilobites lived 750 million years ago.

Geologic Time Scale

• The Geologic Time Scale is a calendar geologists use to put events in their proper order.

Calculating Earth's Age

• Earth's age was estimated by measuring the radioactive decay of uranium to lead.
• Earth's age is about 4.5 - 4.6 billion years old, based on improved techniques and evidence from tree rings and glaciers.

Geologic Column

• By applying relative ages to sedimentary rocks, scientists look at fossils to learn to date them.
• A single place does not contain a complete record of geologic time.
• Geologic Column: An ordered arrangement of rock layers based on the relative ages of the rock.
• Geologists can fill in gaps once they have dated rock layers.

Divisions of Geologic Time

• Geologic time is typically divided by clear changes, such as extinctions and dominant fossils.
  • MYA: Millions of years ago.
  • BYA: Billions of years ago.
• Eons are the largest unit of geologic time, with four total eons.
• The first three eons are the Precambrian time, which has little fossil evidence.
• Microbe-rich stromatolite reefs in Shark Bay, Australia, are the first sources of oxygen on Earth; they flourished throughout the planet's shallow waters 3.5 billion years ago and are extremely rare today.

Eras

• Era: A unit of time that includes two or more periods.
• The Phanerozoic Eon can be divided into three eras:
  • Paleozoic
  • Mesozoic
  • Cenozoic
  • "Zoic = "animal life"
• Different fossils represent each of these eras: marine life, birds, reptiles, and mammals.

Paleozoic Era

• Beginning: Oceans.
• Middle to End: Forests and jungles, and land animals evolved.
• Cambrian Period: Development of organisms having the ability to secrete calcium carbonate and calcium phosphate.
• Ordovician Period: Earliest appearance of vertebrates, and fungi, plants, and animals colonize the land.
• Silurian Period: Emergence of terrestrial life; first vascular plants appear.
• Devonian Period: The Age of Fishes.
• Carboniferous Period: Warm, moist climate conditions, lash vegetation and dense forest, appearance of giant cockroaches and dragonflies, and evolution of first reptiles (development of amniotic egg and porous shell).
• Permian Period: Reptiles diversify, major extinctions of many marine organisms; two major groups of reptiles include Diapsids (gave rise to dinosaurs) and Synapsids (gave rise to mammals).

Mesozoic Era

• Mesozoic Era: The Age of Reptiles
  • Triassic Period: Dinosaurs evolve, origin of mammals, gymnosperms dominate.
  • Jurassic Period: Dinosaurs are abundant, the first birds appear, and gymnosperms dominate.
  • Cretaceous Period: Angiosperms diversify, and dinosaurs become extinct at the end of the period.

Cenozoic Era

• Cenozoic: The Age of Mammals
• Tertiary Period (Neogene and Paleogene). Subdivided into Paleocene, Eocene, Oligocene, Miocene, and Pliocene Epochs.
  • Paleocene Epoch: Mammals, birds, and insects diversify.
  • Eocene Epoch: Angiosperms diversify, and mammals diversify.
  • Oligocene Epoch: Origin of primates.
  • Miocene Epoch: Mammals and angiosperms continue to diversify.
  • Pliocene Epoch: Bipedal human ancestors appear.
• Quaternary Period: Subdivided into Pleistocene and Holocene Epochs.
  • Pleistocene Epoch: Ice ages; origin of Homo.
  • Holocene Epoch: Historical time.
• Eras are divided into shorter time units called Periods.
• Each period is characterized by specific fossils, and is named for the location they were discovered.
• Epochs are created when the fossil record is so complete that periods can be divided further.