Earth & Space Science Chapter 8: Geologic Time

A Brief History of Geology

• Earth processes and materials have been a focus of investigation for centuries • Aristotle's explanations were not based on observation but rather arbitrary ideas, e.g., rocks were created under the "influence" of the stars, earthquakes developed when air in the ground was heated by fires in the Earth, and fossilized fish lived motionless in the Earth until discovered • Aristotle's explanations were continued for centuries, hindering a more modern approach to Earth processes • James Usher constructed a chronology of human and Earth history in the mid-1600s, determining the Earth was created in 4004 B.C. • The idea of catastrophism developed in the 17th and 18th centuries, stating that the Earth's landscape was shaped primarily by great catastrophes • James Hutton published the Theory of the Earth in 1795, proposing the principle of uniformitarianism, which states that the physical, chemical, and biological laws that operate today also operated in the geologic past

Principles of Geology

• Rocks can be viewed as the pages of Earth's history, but many have been removed, destroyed, or modified by the same processes that formed them • Interpreting Earth history is one of the main goals of geology • Scientists who developed the geologic time scale revolutionized how people perceive our planet and think of "time" • Early attempts to determine the Earth's age in the late 1800s and early 1900s were unsuccessful • Today, scientists use radiometric dating methods to determine numerical dates

Principles of Relative Dating

• To understand and apply relative dating techniques, several principles had to be discovered and applied • The principle of superposition states that in an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below • The principle of original horizontality states that layers of sediment are generally deposited in a horizontal position • The principle of cross-cutting relationships states that when a fault cuts through other rocks or when magma intrudes and crystallizes, we can conclude that the fault or intrusion is younger than the rocks affected • The principle of inclusions states that the rock unit containing the inclusion is the younger of the two

Unconformities

• Unconformities represent a long period during which deposition ceased, erosion removed the material, and deposition resumed • There are three basic types of unconformities: angular unconformity, disconformity, and nonconformity • Angular unconformity consists of folded or tilted sedimentary rocks that are overlain by younger, more flat-lying strata • Disconformity often resembles a bedding plane, and there is little evidence of erosion • Nonconformity separates older metamorphic or igneous rocks from younger sedimentary strata

Fossils: Evidence of Past Life

• Fossils are essential for interpreting the geologic past • The study of fossils is called paleontology • Fossils help scientists understand environmental conditions in which the organism lived • Fossils are crucial in correlating rocks from different locations • There are several ways in which fossilization can occur, including permineralization, molds and casts, carbonization and impressions, and amber

Correlation of Rock Layers

• To develop a geologic time scale for the entire Earth, rocks of similar age in different regions must be matched • Correlation over short distances is relatively easy, but correlation over great distances is far more complex • Correlation of rock units separated by great distances can be accomplished by using fossils • The principle of fossil succession states that fossil organisms succeed one another in a definite and determinable order • Index fossils are widespread geographically but limited to a short span of geologic time

Dating with Radioactivity

• Radiometric dating is a reliable method for determining the numerical ages of Earth materials and events • Radioactive isotopes decay at a fixed rate, and the time required for one-half of the radioactive isotopes to decay into the daughter products is termed the half-life • There are many radioactive isotopes in nature, but five have proven to be very useful in providing radiometric ages • Potassium-40 is the most versatile radioactive isotope due to its abundance in silicate minerals

The Geologic Time Scale

• The geologic time scale is divided into units of varying magnitude: eons, eras, periods, and epochs • The geologic time scale was developed before radiometric dating • The detail of the geologic time scale does not begin until after the Precambrian (4 billion years of Earth history) • The geologic time scale is dynamic, and as new evidence is uncovered, it will be modified to reflect the new conclusions