DETERMINING RELATIVE AGE FROM THE ROCK RECORD In the illustration, layer 1 was deposited at time 1. For the rocks in cross-section A, the order of events, from oldest to youngest was: deposition of 23, 24, lava flow A, 25, 26, 27, . relative dating practice 1. The cross section shows the rock structure of a region of Earth's crust. The diagram below shows a geologic cross section. a water- table or potentiometric-surface map and one or more hydrogeologic cross sections). appropriately scaled cross sections or fence or block diagrams. (drill holes, test pits, and trenches) should be shown on maps and cross sections. sorting, compactness, cementation, relative age, distribution, and thickness.
Relative and absolute dating of geologic events the age of a given mineral sample geologic block diagrams and cross-sections, unconformities are usually. Dick morris polls official website of dick morrisdear friend, end sanctuary states relative dating cross section exercise now one after another, liberal dick morris polls states are relative age dating cross sections passing laws making it illegal to obey federal.
Using relative dating and and metamorphic, but radiometric age dating as seen in cross-section these events may include:. Have hit relative age dating cross sections exaggerate big science in christian perspective wiens has a phd in physics, with a minor in geology his phd thesis was. Relative dating evidence from geologic layers and then you determine the age of the object relative to the other objects or events in the a cross section of.
This video presents the basics of relative age dating the principle of superposition for middle school science relative dating of rock layers stratigraphic cross section—interpreting. Relative dating, try working out the same age dating, which relative dating cross section sw science 10 unit 6 relative age of alan j is used to rely on pp relative age dating and radiometric dating.
Principle of Inclusions If we find a rock fragment enclosed within another rock, we say the fragment is an inclusion. If the enclosing rock is an igneous rock, the inclusions are called xenoliths. In either case, the inclusions had to be present before they could be included in the younger rock, therefore, the inclusions represent fragments of an older rock.
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In the example here, as the basalt flowed out on the surface it picked up inclusions of the underlying sandstone. So we know the sandstone is older than the basalt flow. Similarly, the overlying rhyolite flow contains inclusions of the basalt, so we know that the basalt is older than the rhyolite.
This principle is often useful for distinguishing between a lava flow and a sill. Recall that a sill is intruded between existing layers. In the case shown here, we know that the basalt is a sill because it contains inclusions of both the underlying rhyolite and the overlying sandstone. This also tells us that the sill is younger than the both the rhyolite and the sandstone. Principle of Chilled or Baked Margins When a hot magma intrudes into cold country rock, the magma along the margins of the intrusion will cool more rapidly than the interior.
Rapid cooling of magma results in fine grained rock or glassy rock and if this occurs along the margins of the intrusion, we will see the effects of rapid cooling along the margins. Since slower cooling will occur farther away from the margin the rock farther away will be coarser grained.
Thus, if we see chilled margins, we know that the intrusions must be younger that surrounding rock because the surrounding rock had to have been there first in order to cause the cooling effect. When magma comes in contact with soil or cold rock, it may cause the soil or rock to heat up resulting in a baked zone in the surrounding rock near the contacts with the igneous rock.
Such margins indicate that the igneous rock is younger that the soil or rock that was baked.
Application of the Principles of Stratigraphy Figure Although we will go over this in lecture, you should study the methods and reasoning used so that you could determine the geologic history of any sequence of rocks. Fossil Succession Once geologists had worked the relative ages of rocks throughout the world, it became clear that fossils that were contained in the rock could also be used to determine relative age. It was soon recognized that some fossils of once living organisms only occurred in very old rocks and others only occurred in younger rocks.
Furthermore, some fossils were only found within a limited range of strata and these fossils, because they were so characteristic of relative age were termed index fossils. With this new information, in combination with the other principles of stratigraphy, geologists we able to recognize how life had changed or evolved throughout Earth history.
This recognition led them to the principle of fossil succession, which basically says that there is a succession of fossils that relate to the age of the rock. Unconformities - Breaks in the Stratigraphic Record Because the Earth's crust is continually changing, i.
When sediment is not being deposited, or when erosion is removing previously deposited sediment, there will not be a continuous record of sedimentation preserved in the rocks.
Some nonaccretionary forearcs are subjected to strong extensional stresses, for example the Marianas, and this allows buoyant serpentinite to rise to the seafloor where they form serpentinite mud volcanoes. Chemosynthetic communities are also found on non-accretionary margins such as the Marianas, where they thrive on vents associated with serpentinite mud volcanoes.
Trench rollback[ edit ] Trenches seem positionally stable over time, but scientists believe that some trenches—particularly those associated with subduction zones where two oceanic plates converge—move backward into the subducting plate.
Slab rollback occurs during the subduction of two tectonic plates, and results in seaward motion of the trench. Forces perpendicular to the slab at depth the portion of the subducting plate within the mantle are responsible for steepening of the slab in the mantle and ultimately the movement of the hinge and trench at the surface. Two forces acting against each other at the interface of the two subducting plates exert forces against one another.
The subducting plate exerts a bending force FPB that supplies pressure during subduction, while the overriding plate exerts a force against the subducting plate FTS. The slab pull force FSP is caused by the negative buoyancy of the plate driving the plate to greater depths. The resisting force from the surrounding mantle opposes the slab pull forces. Interactions with the km discontinuity cause a deflection due to the buoyancy at the phase transition F When the deep slab section obstructs the down-going motion of the shallow slab section, slab rollback occurs.
The subducting slab undergoes backward sinking due to the negative buoyancy forces causing a retrogradation of the trench hinge along the surface. Upwelling of the mantle around the slab can create favorable conditions for the formation of a back-arc basin.
Results demonstrate high temperature anomalies within the mantle suggesting subducted material is present in the mantle. Slab rollback is not always a continuous process suggesting an episodic nature. The age of the subducting plates does not have any effect on slab rollback. Continental collisions induce mantle flow and extrusion of mantle material, which causes stretching and arc-trench rollback.
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Stagnation at the km discontinuity causes retrograde slab motion due to the suction forces acting at the surface. As slab rollback velocities increase, circular mantle flow velocities also increase, accelerating extension rates. These flattened slabs are only temporarily arrested in the transition zone.
The subsequent displacement into the lower mantle is caused by slab pull forces, or the destabilization of the slab from warming and broadening due to thermal diffusion. Most of this water is trapped in pores and fractures in the upper lithosphere and sediments of the subducting plate.
These sediments are progressively squeezed as they are subducted, reducing void space and forcing fluids out along the decollement and up into the overlying forearc, which may or may not have an accretionary prism.
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Sediments accreted to the forearc are another source of fluids. Water is also bound in hydrous minerals, especially clays and opal. Increasing pressure and temperature experienced by subducted materials converts the hydrous minerals to denser phases that contain progressively less structurally bound water. Water released by dehydration accompanying phase transitions is another source of fluids introduced to the base of the overriding plate. These fluids may travel through the accretionary prism diffusely, via interconnected pore spaces in sediments, or may follow discrete channels along faults.
Sites of venting may take the form of mud volcanoes or seeps and are often associated with chemosynthetic communities.Principles of Relative Dating 1 - Superposition, Horizontality, Cross-cutting
Fluids escaping from the shallowest parts of a subduction zone may also escape along the plate boundary but have rarely been observed draining along the trench axis. All of these fluids are dominated by water but also contain dissolved ions and organic molecules, especially methane. Methane is often sequestered in an ice-like form methane clathratealso called gas hydrate in the forearc. These are a potential energy source and can rapidly break down. Destabilization of gas hydrates has contributed to global warming in the past and will likely do so in the future.
Relative age dating cross sections
Chemosynthetic communities thrive where cold fluids seep out of the forearc. Cold seep communities have been discovered in inner trench slopes down to depths of m in the western Pacific, especially around Japan, in the Eastern Pacific along North, Central and South America coasts from the Aleutian to the Peru—Chile trenches, on the Barbados prism, in the Mediterranean, and in the Indian Ocean along the Makran and Sunda convergent margins.
These communities receive much less attention than the chemosynthetic communities associated with hydrothermal vents. Chemosynthetic communities are located in a variety of geological settings: Surface seeps may be linked to massive hydrate deposits and destabilization e.
High concentrations of methane and sulfide in the fluids escaping from the seafloor are the principal energy sources for chemosynthesis. Factors affecting trench depth[ edit ] The Puerto Rico Trench There are several factors that control the depth of trenches. The most important control is the supply of sediment, which fills the trench so that there is no bathymetric expression.
A second order control on trench depth is the age of the lithosphere at the time of subduction. Because oceanic lithosphere cools and thickens as it ages, it subsides. The older the seafloor, the deeper it lies, and this determines the minimum depth from which the seafloor begins to descend. This obvious correlation can be removed by looking at the relative depth, the difference between regional seafloor depth and maximum trench depth.
Relative depth may be controlled by the age of the lithosphere at the trench, the convergence rate, and the dip of the subducted slab at intermediate depths.
Finally, narrow slabs can sink and roll back more rapidly than broad plates, because it is easier for underlying asthenosphere to flow around the edges of the sinking plate. Such slabs may have steep dips at relatively shallow depths and so may be associated with unusually deep trenches, such as the Challenger Deep.