EARTH SCI Q2

• The Geologic Time Scale is a chronological model that uses the stratigraphy of the earth, or the layers of rock, in relation to time.
• The scale is studied by earth scientists, such as geologists and paleontologists, to study the events that occurred in our earth’s history.
• The scale is read from the most recent events at the top, to the oldest events at the bottom.
• The scale is further broken down into time periods which are explained below.
• Eon: Two or more eras compose an Eon, this is the largest division of time, lasting hundreds of millions of years.
• Era: Two or more periods compose an Era, one Era is hundreds of millions of years in duration.
• Period: This is the basic unit of geologic time, a Period lasts tens of millions of years, which is the time it takes to form one type of rock system.
• Epoch: This is the smallest unit of geologic time, an _____ lasts several million years.
• Age: This is a measurement of time which describes an event, such as an Ice Age.
• Geologic time scale describes the history of the Earth through geologic time.
• Relative geologic time places rocks in order relative to each other.
• In the late 1700s, geologists noticed that layered rocks always appeared in the same order, and that fossils in deeper rocks were always more primitive.
• Geologists concluded that younger rocks pile up on top of older rocks.
• Geologists used the changes in plant and animal fossils over time and the Principles of Stratigraphy to determine the relative ages of rocks.
• For sedimentary rocks, strata on the bottom of a sequence are older and were deposited before any strata on top of them.
• The sequence allows scientists to label layers from oldest to youngest.
• Absolute geologic time assigns ages to rocks in millions of years before present.
• Absolute ages are discovered through chemistry, and the invention of a tool called the Mass Spectrometer in the 1900s advanced modern dating techniques, providing the ability to measure the ratios of radioactive isotopes in rocks.
• Because the half-life, or how long it takes for half of a radioactive isotope to decay, is known, the age of the rock can be determined.
• Scientists now agree that Earth is approximately 4.6 billion years old.
• The divisions of the geologic timescale are updated every few years as new evidence and new technologies improve the precision of the dates.
• The geologic time scale is the “calendar” for events in Earth history.
• The geologic time scale subdivides all time into named units of abstract time called eons, eras, periods, epochs, and ages
• One way to distinguish and define each segment of time is by the occurrence of major geologic events and the appearance (and disappearance) of significant life-forms.
• The oldest rocks found so far on Earth (based on Zircon grains from Jack Hills of Australia) have been dated at 4.1 - 4.3 billion years.
• Meteorites have also been dated at 4.6 billion years.
• The oldest fossils are preserved remains of stromatolites, which are layers of lithified blue-green algae, dating to approximately 3.5 billion years before present.
• What epoch do we live in? Holocene
• The idea that younger rock layers are deposited on top of older rocks layers is called the? Principle of Superposition
• Which era was the age of the mammals? Cenozoic
• Which era had no life on land but plants evolved in shallow seas? Precambrian
• Which era were fishes dominant? Paleozoic
• Which era did dinosaurs exist during? Mesozoic
• The geologic-time unit representing the smallest interval of time is the Epoch
• The geologic-time unit representing the largest interval of time is the Eon
• The geologic-time unit representing the second largest interval of time is the Era
• The geologic-time unit representing the third largest interval of time is the Period
• The geologic-time unit that represents the smallest interval of time is the epoch
• The first life forms on planet earth occur during the Arhaean are called Prokaryotes
• Radiometric age is often reffered to as Absolute Age