Geology 11

Skip to Archean Backstop, 2. Here, I sketch the big picture in Colorado, as best I can put it together, from past to present. Subsequent sections will flesh out the details, also in chronological order. The mobile belt added to the continent during this time is known as the Colorado Province.

Geology 11

Relative dating Cross-cutting relations can be used to determine the relative ages of rock strata and other geological structures. Methods for relative dating were developed when geology first emerged as a natural science.

Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events. The principle of uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time.

In geology, when an igneous intrusion cuts across a formation of sedimentary rockit can be determined that the igneous intrusion is younger than the sedimentary rock. Different types of intrusions include stocks, laccolithsbatholithssills and dikes.

The principle of cross-cutting relationships pertains to the formation of Geology 11 and the age of the sequences through which they cut. Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault.

Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.

A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix.

As a result, xenoliths are older than the rock that contains them. The Permian through Jurassic stratigraphy of the Colorado Plateau area of southeastern Utah is an example of both original horizontality and the law of superposition. These strata make up much of the famous prominent rock formations in widely spaced protected areas such as Capitol Reef National Park and Canyonlands National Park.

From top to bottom: Rounded tan domes of the Navajo Sandstonelayered red Kayenta Formationcliff-forming, vertically jointed, red Wingate Sandstoneslope-forming, purplish Chinle Formationlayered, lighter-red Moenkopi Formationand white, layered Cutler Formation sandstone.

The "Rock Cycle" (revisited)

The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds.

Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal.

Geology 11

Logically a younger layer cannot slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.

As organisms exist during the same period throughout the world, their presence or sometimes absence provides a relative age of the formations where they appear. Based on principles that William Smith laid out almost a hundred years before the publication of Charles Darwin 's theory of evolutionthe principles of succession developed independently of evolutionary thought.

The principle becomes quite complex, however, given the uncertainties of fossilization, localization of fossil types due to lateral changes in habitat facies change in sedimentary strataand that not all fossils formed globally at the same time.

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Absolute datingradiometric datingand geochronology Geologists also use methods to determine the absolute age of rock samples and geological events.

These dates are useful on their own and may also be used in conjunction with relative dating methods or to calibrate relative methods. This changed the understanding of geologic time. Previously, geologists could only use fossils and stratigraphic correlation to date sections of rock relative to one another.Buy The Geology of Ore Deposits on attheheels.com FREE SHIPPING on qualified orders.

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Plate Tectonics