Unit 1: Many Planets, One Earth // Section 3: Reading Geologic Records
Scientists have divided Earth's history into a series of time segments that are collectively referred to as the geologic time scale (Figure 4). Each of these units is defined based on geologic and fossil records, with divisions between the units marking some major change such as the appearance of a new class of living creatures or a mass extinction. Geologic time phases become shorter as we move forward from Earth's formation toward the present day because records grow increasingly rich. Newer rocks and fossils are better preserved than ancient deposits, so more information is available to categorize recent phases in detail and pinpoint when they began and ended.
Figure 4. The geologic time scale
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Source: © United States Geological Survey.
Most of what we know about our planet's history is based on studies of the stratigraphic record—rock layers and fossil remains embedded in them. These rock records can provide insights into questions such as how geological formations were created and exposed, what role was played by living organisms, and how the compositions of oceans and the atmosphere have changed through geologic time.
Scientists use stratigraphic records to determine two kinds of time scales. Relative time refers to sequences—whether one incident occurred before, after, or at the same time as another. The geologic time scale shown in Figure 4 reads upwards because it is based on observations from sedimentary rocks, which accrete from the bottom up (wind and water lay down sediments, which are then compacted and buried). However, the sedimentary record is discontinuous and incomplete because plate tectonics are constantly reshaping Earth's crust. As the large plates on our planet's surface move about, they split apart at some points and collide or grind horizontally past each other at others. These movements leave physical marks: volcanic rocks intrude upward into sediment beds, plate collisions cause folding and faulting, and erosion cuts the tops off of formations thrust up to the surface.
Geologists have some basic rules for determining relative ages of rock layers. For example, older beds lie below younger beds in undisturbed formations, an intruding rock is younger than the layers it intrudes into, and faults are younger than the beds they cut across. In the geologic cross-section shown in Figure 5, layers E, F, G, H, I, and J were deposited through sedimentation, then cut by faults L and K, then covered by layers D, C, and B. A is a volcanic intrusion younger than the layers it penetrates.
Figure 5. Sample geologic cross-section
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Scientists also use fossil records to determine relative age. For example, since fish evolved before mammals, a rock formation at site A that contains fish fossils is older than a formation at site B that contains mammalian fossils. And environmental changes can leave telltale geologic imprints in rock records. For example, when free oxygen began to accumulate in the atmosphere, certain types of rocks appeared for the first time in sedimentary beds and others stopped forming (for more details, see section 6, "Atmospheric Oxygen"). Researchers study mineral and fossil records together to trace interactions between environmental changes and the evolution of living organisms.
Until the early twentieth century, researchers could only assign relative ages to geologic records. More recently, the expanding field of nuclear physics has enabled scientists to calculate the absolute age of rocks and fossils using radiometric dating, which measures the decay of radioactive isotopes in rock samples. This approach has been used to determine the ages of rocks more than 3.5 billion years old (footnote 3). Once they establish the age of multiple formations in a region, researchers can correlate strata among those formations to develop a fuller record of the entire area's geologic history (Fig. 6).
Figure 6. Geologic history of southern California
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Source: © United States Geological Survey, Western Earth Surface Processes Team.
Our understanding of Earth's history and the emergence of life draws on other scientific fields along with geology and paleontology. Biologists trace genealogical relationships among organisms and the expansion of biological diversity. And climate scientists analyze changes in Earth's atmosphere, temperature patterns, and geochemical cycles to determine why events such as ice ages and rapid warming events occurred. All of these perspectives are relevant because, as we will see in the following sections, organisms and the physical environment on Earth have developed together and influenced each other's evolution in many ways.