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22 Cards in this Set

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Relative Age
The age of a rock, fossil, geologic feature. or event relative to that of another rock, fossil, geologic feature or event. Relative age is ot an exact age; it only tells you htat one thing is older or younger that another.
Extreme pressure within Earth's crust can cause rock layers to bend and fold, especially in areas of mountian building. Under sufficient pressur these folds can overturn the rock layers so that in certain regions older rocks are on top of younger rocks. If some of the exposed surfaces of these folds have been weathered and eroded it can be tricky to identify what happened and correctly identify realive age of the rock layer.
other types of pressure within Earths curst can cause rock layers to break due to movement of the layers, usually associated with earthquakes. The location of this fracture is called a fault. If rock layers facture and then move along that fault, they become offset from each other or displaced. This displacemnet may cause older layers to eventually be pushed up so that they are above younger layers.
Sometimes molten rock magma squeezeds into preexisting rocks from deeper withing Earth and cools. This cooled magma forms and igneous rock body called an intrusion.
Contact Metamorphism
As the magma squeezes through or between existing layers of rock, it is so hot that it changes the rock immediately above, below, and next to it by contact metamorphism. In a narrow zone surrounding the intrusion, that adjacent rocks become metamorphic rocks.
Similarly, molten rock can rise up to Earths surface flow and solidify. When molten rock reaches Earths surface it is not called magma, it is called lava, and as it cools, it forms a mass of igneous rock called and extrusion. Extrusions incude lava flow and volcanoes. The extrusion is younger than any rock beneath it but will be older than any subsequent rock layers that may form on top of it. The rock beneath an extrusion will also show a zone of contact metamorphism where the hot lave baked it. Any rock layers deposited on top of an extrusion will not show contact metamorphism.
An inclusio is a body of older rock withing igneous rock. Often when magma rises toward Earths sufrface, pieces of the rock that the magma is intruding pushing through will fall into the magma. Usually these pieces of rock will melt to become part of the magma but sometimes the magma is too cool and just about to solidify. When this occurs the older pice of rock will not melt but will form an inclusion within the igneous rock; the inclusion will be older than the surrounding bedrock.
Processes used to show that rocks or geoloic eventts from differentplaces are the some or similar in age. In this way rocks in one location may fill gaps in the record in another location and the realative ages of rocks in widely separated outcrops can be determined.
Walking the Outcrop
Sometimes rock layers can be followed from one loaction to antoer by physically tracing the layers. This is difficult because an areas local rock or bedrock is usually covered with layers of loose rock and soil but on occasionlayers of rock will be exposed due to natural causes or human activities such as road building. Directly tracking the continuity of a rock is easiest in arid regions where there is less soil or vegatation.
Matching Physical Characteristics
Sometimes rock layers can be correlated based on the similarity characteristics such as composition, color, thcikness, rock sequence or pattern and fossil remains. Geologist have to be careful when they do this type of correlation because similar rocks can frorm in similar enviroments though they may be separated by millions of years. For this reason correlation using rock simialrities is usually only valid over short distances.
Index Fossils
Fossils are the preserved remains or traces of living things. Typically the hard parts of an organism are presered after the soft parts decay.
Type of fossilization process. The soft parts of an organism may decay and the hard parts dissolve but the compacted sediments around the organism retain the shape and create a mold. Minerals in ground water may fill the space and harden into a cast that duplicates the original shape and becomes the fossil.
Type of fossilization process. Mineralized water fills the pores and cavities in buried trees or other organic material. The minerals are deposited and the organic material is slowly turned into rock.
Type of fossilization process. Overlying sediments flatten a plant and turn it into a thin carbon film.
Index Fossil Characteristics
-lived over a broad geographical area
-relatively short period of time lived
Key Beds
Key beds are well defined easily identified rock layers. They are not confused with other rock layres because they have very distinctive characteristics that allow them to be used correlation. Key beds are created after a large volcanic eruption whtn vast amounts of sand and clay sized particles of extrusive igneous rock are shot into the air and dispersed broadly. Different volcanic reuptions have diferentt chemical compositions, and these can be detected in the resulting rock layers. Key beds were also created from the imapct of meteorite or asteroid on Earths surface. Debeis can be scattered over much of the earth inculding particles of the asteroid forming deposits that can easily identified and used for correlation.
Geologic Time Scale
Recorded History thru fossils and rock remains. Mainly on the basis of changing fossil evidence geologist were ale to create a model for geologic time call the Geologic Time Scale. The Geologic Time Scal shown in the Earth Science Reference Tables show the different eons, eras, periods, and epochs used by geologist and some of the characteristics of the environment and lif on Earth during those times.
When investigating rock layers geologist often find evidence of an unconformity, a buried eroded surface that indicates part of the rock record is missing. Unconformities typically occur because a region experienced uplift during a mountain building phase of Earths crust in that area. The region may have been underwater but becomes exposed to weathering and erosion afer uplift and roc layers are slowly worn away.
Steps for Unconformity
Emergence- area must be uplifted out of a water enviroment or the water level must fall

Weathering and Erosion- previously formed rock layers are worn away

Submergence- the area must be under water again to allow new layers to be depostied; this can occur because the land sinks or the water level rises
An atom is the smallest piece of an element that still retains all the elements propertties. An atom is made of protons and neutrons in a nucleus and electrons "orbiting" around the outside of the nucleus. As far as radioactivity goes, the electrons are not part of the story.
Absolute Age
Geologist did a good job of reading the rock record to establish a sequence of geologic events, but still needed a means of determining the actual age of Earth and its rock layers, not just relative age. The abiltiy to do this is dependent on understanding radioactivity.
The identitiy of an element comes from the number of protons within the nucleus. Hydrogen has one proton, carbon has 6 and Oxygen has 8. In order for a nucleus to rmain stable it also needs some neutrons in the nucleus. Stable nuclei will usually have about the same number of protons and neutrons; if the number of protons and neutrons get too far out of balance, the aton falls apart or "decays".