Dating Rocks and Fossils: A Comprehensive Guide
Dating Rocks and Fossils
Types of Dating
One objective of geology is dating rocks and fossils to place events in time. There are two types of dating:
- Absolute Dating: Sets a precise date for events.
- Relative Dating: Sorts events from oldest to most recent.
The most commonly used method for absolute dating is radiometric dating. Drawbacks of this method include:
- Applies almost exclusively to magmatic rocks.
- Expensive.
- Potential for error.
Relative dating is based on the principle of superposition of strata. This principle doesn’t apply when the original position of strata has been modified. In such cases, the principle of superposition of events applies.
Fossils in Geology
Fossils provide two types of information:
- Temporal: Indicates the period in which the organism existed.
- Paleoecological: Reveals characteristics of the environment where the organism lived.
Index fossils are species that existed for short periods but colonized large areas, making them useful for dating rock layers.
Earth Changes
Major Earth changes include:
- Climate: Alternating warm and cold phases.
- Eustatic: Changes in sea level.
- Paleogeographic: Changes in continent distribution and ocean movements due to plate tectonics.
- Biodiversity: Increases in the number of species and extinctions.
Theories of Earth Changes
- Catastrophism: Proposed by Cuvier, this theory suggests sudden catastrophes cause rapid changes.
- Gradualism: Proposed by Charles Lyell, this theory posits slow, gradual changes over millions of years.
- Neo-Catastrophism: Combines gradual processes with abrupt changes and catastrophes.
Geological Time Scale
Earth’s history is divided into eons, eras, and periods.
Nebular Theory
The most accepted theory of solar system formation is the nebular theory. It proposes that the solar system formed from a nebula (cloud of gas and dust) originating from a star explosion. Supporting evidence includes:
- Planets revolve around the sun in the same direction.
- Inner planets are metal-rich.
- Outer planets are gas giants.
- The sun rotates in the same direction as the planets.
Eons
Hadean (4500-3800 Ma)
- Formation of Earth’s layers.
- Formation of the atmosphere and oceans from volcanic gases (methane, sulfur, CO2), creating a reducing atmosphere. Water vapor condensation formed oceans.
Archean (3800-2500 Ma)
- End of meteorite bombardment.
- Start of plate tectonics.
- Formation of an oxidizing atmosphere due to oxygen-producing cyanobacteria, enabling the appearance of microorganisms.
Proterozoic (2500-542 Ma)
- Cold period.
- Continents formed a supercontinent called Rodinia.
Phanerozoic (542 Ma-Present)
Eras of the Phanerozoic
Paleozoic (542-251 Ma)
- Appearance of organisms with exoskeletons.
- Continents continued reorganizing.
This era includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods.
Characteristic Paleozoic Fossils
- Trilobites: Marine arthropods with chitin exoskeletons.
- Graptolites: Marine organisms living in colonies.
- Giant ferns (e.g., Lepidodendron).
- Armored fish: The first fish species.
Mesozoic (251-65 Ma)
- Breakup of Pangea and formation of current continents.
- Warm period with no glaciation.
- Two major extinctions: Permo-Triassic extinction and the Cretaceous-Tertiary extinction (dinosaurs).
This era includes the Triassic, Jurassic, and Cretaceous periods.
Characteristic Mesozoic Fossils
- Ammonites: Swimming cephalopods with coiled shells.
- Belemnites: Elongated shelled cephalopods.
- Reptiles: Dominant animals of the Mesozoic.
Cenozoic (65 Ma-Present)
- Continents continued dispersing.
- Formation of the Alps, Andes, Himalayas, and Pyrenees.
- Cooling climate.
- Appearance of mammals, birds, and angiosperms.
This era includes the Tertiary and Quaternary periods.
Characteristic Cenozoic Fossils
- Nummulites: Single-celled organisms with a disc-shaped shell.
- Mammals: Rodent-like creatures.
- Mesohippus: Ancestor of the horse with three toes.
- Smilodon (Sabertooth cat): Known for its long teeth.