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Earth’s History. Geologic Events. “The present is the key to the past”. This statement summarizes one of the basic principles of geologic history – that the geologic processes going on today, such as weathering, erosion, volcanism, and earthquakes, also went on in the past.
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Geologic Events “The present is the key to the past”. This statement summarizes one of the basic principles of geologic history – that the geologic processes going on today, such as weathering, erosion, volcanism, and earthquakes, also went on in the past. This concept is known as the principle of uniformitarianism.
Sequence of Geologic Events Relative Age versus Absolute Age The relative age of a rock or event is the age as compared to other rocks or events. The absolute age of a rock or event is the actual age.
Chronology of Layers There are two major principles used when interpreting geologic history: • Principle of original horizontality; • Principle of superposition.
Principle of Original Horizontality • The principle of original horizontality states that sediments are deposited in horizontal layers that are parallel to the surface on which they were deposited. • This implies that tilted or folded layers indicate that the crust has been deformed.
Principle of Superposition • The principle of superposition states that, in a series of undisturbed layers, the oldest layer is on the bottom and each overlying layer is progressively younger with the youngest layer on the top.
Evidence of Events • Several things provide geologists with evidence of events: • Igneous intrusions and extrusions; • Faults, joints and folds; • Internal characteristics.
Igneous Intrusions and Extrusions When magma forces its way into cracks or crevices in crustal rock and solidifies, it forms a mass of igneous rock called an intrusion. When lava solidifies at the surface it forms a mass of igneous rock called an extrusion. Since the rock that the magma moved through, or over, existed prior to the intrusion, (or extrusion), it must be older.
Faults, Joints, and Folds • Features such as faults, joints, and folds must be younger than the rocks in which they are found. • A joint is a crack in a rock formation, similar to a fault but without any displacement.
Internal Characteristics • Cracks, veins, and mineral cement are younger than the rocks in which they appear. • A vein is a mineral deposit that has filled a crack, or permeable zone, in existing rock.
Correlation Techniques • Correlation is the process of matching rocks and geologic events in one location to the rocks and events in another location. • Methods used for correlation include: • Continuity of rocks • Fossil evidence in rocks • Volcanic time markers in rocks
Bedrock and Outcrops • Bedrock is the solid, unbroken rock of the crust. • An outcrop is bedrock that is exposed at the Earth’s surface. • Outcrops provide opportunity for geologists to directly study the layers of the bedrock, tracing them from one location to another – called “walking an outcrop”.
Fossil Evidence • Index fossils are fossils or organisms that lived over an extensive area, preferably over the entire Earth, for relatively short periods of time. • Index fossils are useful in correlating the sedimentary rocks in which they are found.
Volcanic Time Markers • Severe volcanic eruptions can deposit a thin layer of volcanic ash over the surface of the entire Earth. • These layers within a rock sequence may remain distinguishable and provide a time marker. (Similar to index fossils)
Anomalies • Anomalies are differences from what is expected.
Determining Geologic Ages • Without the rock record there would be no geologic history. • The older the rock the more difficult to determine absolute age. • The geologic history of an area is determined primarily by fossil evidence, the age of the rocks, and the erosional record in the rocks.
Geologic Time Scale • Geologic time is subdivided divided into units based on fossil evidence. • There are 4 major divisions: • Precambrian – represents the first 85% of Earth’s history (mostly devoid of fossils). • Paleozoic Era – represents ~ 8.5% of Earth’s history (invertebrates, fishes, amphibians, vertebrates and land plants first appear). • Mesozoic Era – rep. ~ 3.5% of Earth’s history (dinosaurs, earliest birds, and mammals). • Cenozoic Era – rep. ~ 1.4% of Earth’s history (humanoids show up late ~0.04% of history).
Erosional Record • Buried erosional surfaces, called unconformities, indicate gaps or breaks in the geologic time record. • 4 steps combine in sequence to form unconformities: • Uplift, erosion, submergence and deposition • 3 most common types of unconformities are: angular unconformities, parallel unconformities, and nonconformities.
Radioactive Decay • Radioactive decay occurs when the nuclei of unstable atoms break down, changing the original atoms into atoms of another element. • The rate of radioactive decay is measured in terms of half-life. • Half-life is the amount of time it takes for half the atoms of a substance to decay into another element. • Different substances have different half-life’s • Examples are Uranium 238 and Carbon 14.
History and Evolution of the Earth’s Atmosphere • Earth is about 4.5 billion years old. • The primary source of gases for the earliest atmosphere are thought to be from outgassing by volcanoes. • Water vapor in the outer atmosphere would have been broken down into hydrogen and oxygen.
Fossil Record • Fossils preserved in the rocks provide evidence that many kinds of animals and plants have lived on Earth in the past under a variety of different conditions.