Geologic Time Scale


Dating techniques are procedures used by scientists to determine the age of a specimen. Relative dating methods tell only if one sample is older or younger than another sample; absolute dating methods provide a date in years. The latter have generally been available only since 1947. Many absolute dating techniques take advantage of radioactive decay, whereby a radioactive form of an element is converted into another radioactive isotope or non-radioactive product at a regular rate. Others, such as amino acid racimization and cation-ratio dating, are based on chemical changes in the organic or inorganic composition of a sample. In recent years, a few of these methods have undergone continual refinement as scientists strive to develop the most accurate dating techniques possible.

Absolute dating is the process of determining an age on a specified chronology in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word “absolute” implies an unwarranted certainty of accuracy. Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events. In archaeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates (coins and written history). Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped charge dating methods such as thermoluminescence dating of glazed ceramics.

Relative dating methods determine whether one sample is older or younger than another. They do not provide an age in years. Before the advent of absolute dating methods, nearly all dating was relative. The main relative dating method is stratigraphy .



Hadean, Archaen, and Proterozoic Eons made up the Precambrian Period. This was the time to which no living organisms existed yet with the exception of a bacteria known as stromatolites.

The Phanerozoic Eon brought about three Eras. The Paleozoic Era is known to be start up for life. Explosion of life started with Cambrian Period to which most organisms present had hard shell covering.

Ordovician Period’s typical marine community consisted of organisms such as green algae, primitive fishes, cephalopods, corals and gastropods. One important creature that existed in this time are the nautiloids which is a tentacled mollusk.

Silurian Period was the age where terrestrial plants started to flourish thus pushing some marine organisms out of the water and into the land.

Devonian Period is also known as the age of fishes. The first amphibians breathed through simple lungs and their skin. They may have spent most of their lives in the water, leaving it only to escape the attentions of predatory fish.

Carboniferous Period is famous for its vast swamp forests. Vegetation included giant club mosses, tree ferns, great horsetails and towering trees with strap-shaped leaves. Insects also flourished during this period.

The Permian period, which ended in the largest mass extinction the Earth has ever known, began about 299 million years ago. Early reptiles were well placed to capitalize on the new environment. Shielded by their thicker, moisture-retaining skins, they moved in where amphibians had previously held sway.

Mesozoic Era was divided into 3 periods; Triassic, Jurassic, and Cretaceous. Triassic Period was a time of tremendous change and rejuvenation. Life that survived the so-called Great Dying repopulated the planet, diversified into freshly exposed ecological niches, and gave rise to new creatures, including rodent-size mammals and the first dinosaurs.

The Triassic closed in the same way it began. Something—perhaps a volcanic belch or an asteroid collision—caused another mass extinction. Dinosaurs, however, survived and went on to dominate the Jurassic.

The Jurassic period (199.6 million to 145.5 million years ago) was characterized by a warm, wet climate that gave rise to lush vegetation and abundant life. Many new dinosaurs emerged—in great numbers. Among them were stegosaurs, brachiosaurs, allosaurs, and many others.

Long before the carnage began, the Cretaceous picked up where the Jurassic left off: Gigantic sauropods led parades of dinosaurs through the forests, over the plains, and along the coasts; long-necked and toothy marine reptiles terrorized fish, ammonites, and mollusks in the seas; pterosaurs and hairy-feathered birds filled the skies.



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