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Geology - Geologic Time

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    Study Guide Geology – Geologic Time 1. Geologic Correla Ɵ ons Geologists often need to match rock layers from different locations to figure out their age and history. This process is called geologic correlation . Because Earth’s rock record is rarely complete or fully exposed, geologists rely on several clues to decide whether rocks in different places formed at the same time. 1.1 Physical Con Ɵ nuity: Matching Rocks by Loca Ɵ on and Layers 1.1.1 Tracing Rock Layers The simplest way to correlate rocks is through physical continuity . If a rock layer or boundary can be seen continuously from one place to another, it is easy to prove that the rocks are the same. However, this is rarely possible over long distances. Erosion, vegetation, and burial often break up rock exposures. As a result, geologists usually must compare rock sequences in detail rather than trace them directly. 1.1.2 Comparing Rock Sequences Geologists examine: • The rock type • The order of layers • The rocks found above and below the layer being studied For example, if you find this sequence in two different places: • Red shale • Sandstone • Green shale • Limestone • Chert

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    Study Guide and the order is the same in both locations — even 20 kilometers apart — it is reasonable to conclude that the green shale layer formed at the same time and under similar conditions in both places. 1.1.3 Marker Beds Sometimes, a rock layer has a unique appearance that makes it easy to recognize. These special layers are called marker beds . Even if only parts of a sedimentary sequence are exposed, marker beds help geologists: • Match layers across distances • Reconstruct the complete geologic history of an area 1.2 The Fossil Record: Using Life to Track Time 1.2.1 Fossils and Time Another powerful way to correlate rocks is by studying fossils . The study of fossils is called paleontology . Because plant and animal species existed only during certain periods of Earth’s history: • Rocks with the same fossils formed during the same time interval • Fossils act as natural time markers 1.2.2 Index Fossils Some fossils are especially useful. Index fossils : • Lived during a short, well - defined time span • Were widespread • Are easy to recognize If an index fossil is found in a rock layer, geologists can narrow down when that rock formed .

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    Study Guide 1.2.3 Fossil Assemblages A fossil assemblage is a group of different fossil species found together. Assemblages are often more reliable than single fossils because they: • Provide a clearer age range • Help identify the environment of deposition (such as ocean, reef, or swamp) Using fossil assemblages, geologists can correlate rock layers across continents and even worldwide . 1.3 The Geologic Time Scale 1.3.1 Dividing Earth’s History By combining fossil evidence from around the world, geologists created the standard geologic time scale . This scale divides Earth’s history into: • Eons • Eras • Periods • Epochs Each division represents major changes in life, climate, or tectonic activity.

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    Study Guide

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    Study Guide 1.3.2 The Precambrian When the time scale was first developed, scientists believed that the earliest fossil life appeared during the Cambrian Period , which began about 570 million years ago . All time before the Cambrian — from about 4.5 billion to 570 million years ago — is called the Precambrian . Because fossils from this time are rare, the Precambrian is divided mainly using: • Radiometric age dating • Major tectonic and crustal events 2. Absolute Age Geologists first built the geologic time scale using relative dating , which compares the order of rock layers and events. However, knowing which rock is older is different from knowing how old it actually is . Finding the actual numerical ages of rocks became possible only after scientists discovered radioactivity . Thanks to radiometric dating, geologists can now assign real dates to major divisions of Earth’s history, such as: • The boundary between the Precambrian and Paleozoic : about 570 million years ago • The Mesozoic Era (the “Age of Dinosaurs”): from about 245 million to 66 million years ago 2.1 Radioac Ɵ vity and Atomic Clocks What Is Radioac Ɵ ve Decay? Some chemical elements naturally break down over time. This process is called radioactive decay , and it happens at a constant and predictable rate . Radioactive decay begins once atoms are locked into crystalline mineral structures as a rock forms.

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    Study Guide Isotopes Explained Many elements exist in different forms called isotopes . Isotopes are atoms of the same element that have: • The same number of protons • Different numbers of neutrons For example: • Uranium - 238 (U - 238) has three more neutrons than Uranium - 235 (U - 235) Some isotopes are unstable and decay over time. 2.2 Parent and Daughter Products As a radioactive isotope decays, it transforms into new elements called daughter products . For example: • Uranium - 238 decays through a series of steps • Eventually, it becomes lead - 206 , which is stable Because the decay rate is constant, geologists can calculate how long decay has been happening by measuring the ratio of parent isotope to daughter product in a mineral. 2.3 Common Radiometric Da Ɵ ng Systems Several decay systems are used to determine absolute age, including: • Carbon - 14 → Nitrogen - 14 • Potassium - 40 → Argon - 40 • Rubidium - 87 → Strontium - 87 • Thorium - 232 → Lead - 208 • Uranium - 235 → Lead - 207 • Uranium - 238 → Lead - 206 Each system is useful for different rock types and age ranges.

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