Geologic Time Scale – Flashcards

A record of Earth's history from its origin 4.6 billion years ago (BYA) to the present. This history is divided into blocks of time distinguished by geologic and evolutionary events. This allows scientists to correlate the geologic events, environmental changes and development of life-forms that are preserved in the fossil record. Since the naming of the first time unit in 1797 (Jurassic Period) this has continuously been refined. The names of the periods do not change, but the years marking the beginning and end of each time period are constantly being updated. This happens because technology is improving. The oldest division of time is at the bottom of the scale. Therefore, moving upward on the scale, each division is younger. The time scale is divided into units called eons, eras, periods and epochs.

The largest formal unit of geologic time; it is measured in billions of years. There are three eons: the Archean, Proterozoic, and Phanerozoic.

A major division of geological time, hundreds of millions to a billion of years long, usually distinguished by significant changes in the plant and animal kingdoms. There are four major divisions: Precambrian, Paleozoic, Mesozoic, and Cenozoic. Greek: paleo= "Old"; meso= "Middle"; ceno= "Recent"; zoic= "of life" The Precambrian is the oldest and longest of the this, comprising 90% of geologic time. It began when Earth was first formed 4.6 billion years ago to the evolution of abundant macroscopic hard-shelled fossils, which marked the beginning of the Cambrian, some 542 million years ago (MYA). Few fossils of this era have been found.

Geologic time units defined by the life-forms that were abundant, or became extinct during the time in which specific rocks were deposited. Usually measured in tens to hundreds of millions of years.

The smallest division of geologic time, lasting several million years. Different groups of organisms have been used to distinguish various epochs. In the late 18th century, James Hutton, the father of modern geology, began to observe and attempt to explain Earth's landscapes. He was looking for a way to explain the forces behind the mountain building, erosion, earthquakes and sea-level changes that contribute to the changing of Earth's surface features.

The fundamental geological principle that the processes that are operating now to shape the world around us have been operating throughout the geologic past as well. This principle was first proposed by Hutton (and named by Lyell), and is often explained by, "the present is the key to the past." It implied that the Earth had to be billions of years old to create all of the rocks and structures we see today.

The age of one geologic feature with respect to another, without the use of exact dates.

Principle stating that sediments are usually placed in horizontal layers. While we may not know the exact ages of the different layers of rocks, we can assume that the oldest rocks are at the bottom, and that each successive layer going towards the top is younger.

Principle stating that within a sequence of undisturbed sedimentary rocks, the oldest layers are on the bottom, the youngest on the top

A break in the succession of sedimentary deposition, commonly associated with erosion of underlying rock units. This often leads to difficulty in relative dating of rock layers. MISSING INFORMATION

The actual age of a rock, fossil or other object. Some other ways to determine age include: • Dendrochronology: counting tree rings • Seasonal climate changes: summer sediment deposits tend to be lighter in color and thicker than thinner, darker-colored winter deposits • Key bed: a sedimentary layer formed by an instantaneous or short-lived event (ex: asteroid 65 MYA)

• Offers the most direct evidence that evolution takes place • Will forever remain incomplete due to the fact that very few fossils actually form—conditions must be perfect • Evolutionary biologists must do a lot of "reading between the lines" when interpreting the fossil evidence

The preserved or mineralized parts or traces of a once living organism that lived long ago. These are made when settling particles build up over time in many layers, in places where conditions are perfect for their survival over time (places with a lot of sediment accumulation: ocean floors, tar pits, mud, swamps, areas near volcanoes, etc.). Therefore, fossils are most often found in sedimentary rocks. Older, less complex organisms are in the bottom layers. Newer, more complex organisms are in the top layers.

A fossil of an organism known to have lived in a particular geologic age that can be used to date the rock layer in which it is found. Fossils can be dated to determine their absolute age using a process called Radiometric Dating.

Determination of the age in years of a rock or mineral by measuring the proportions of an original radioactive material and its decay product. In radiometric dating (such as carbon dating) the half-life of radioactive isotopes are used.

The time needed for one-half of a given amount of radioactive substance to decay. Radioactive atoms breakdown from a less stable atom into a more stable atom. Scientists compare the amount of radioactive atoms remaining in a sample with the number of atoms of the new stable element it breaks down into. For example: the half-life of carbon-14 is 5,730 years. If an initial sample contained 12 grams of C-14, after 5,730 years, how much would be left? (6 grams). What about after 11,460 years? (3 grams). After 17,190 years? (1.5 grams) Eventually, the amount becomes too small to measure. Other radioactive isotopes are good for dating things of different ages—really old, to really new.

Life first evolved in oceans to protect against harmful UV radiation. The ozone layer allows organisms to develop and use oxygen for energy, as well as providing protection from UV radiation. Why is ozone protection from UV radiation important if organisms were already protected by living in the oceans? Allow animals to come up to land. Ozone enables survival on land (2.5 BYA) Ozone is such an important part of Earth's evolutionary history for two main reasons: 1. It blocks harmful UV radiation. 2. That protection allowed organisms to leave the ocean and come onto land.

Life-forms undergo an "explosion" The geologic time period (approximately 500 MYA) that most of these major groups of organisms originated is called the Cambrian Period. The sudden appearance, 520 million years ago, of many major groups of animals, as witnessed in the fossil record.

Jawless fish Jawed fish Amphibians Reptiles Mammals Birds

The death of all members of many different species. They are usually caused by a large ecological disaster. Approximate dates of these five mass extinctions: 1. 440 MYA 2. 360 MYA 3. 245 MYA (96% of all species die) (PANGEA) 4. 210 MYA 5. 65 MYA (2/3 of all land species die—including dinosaurs)

A warm-blooded animal; one that can keep its core body temperature at a nearly constant level regardless of the temperature of the surrounding environment. (ex: birds and mammals) A cold-blooded animal; one having a body temperature determined primarily by the temperature of its surrounding environment. (ex: amphibians, reptiles, and fish)