Plate Tectonics and Plate Boundaries This theory is
Plate Tectonics and Plate Boundaries This theory is based on a simple model of Earth. The rigid lithosphere, composed of both oceanic and continental crust as well as the underlying upper mantle, consists of numerous variable-sized pieces called plates (7 major plates & numerous smaller ones). Plates vary in thickness; the continental plates are as much as 250 km thick, whereas the oceanic plates are up to 100 km thick. The lithosphere overlies the hotter and weaker semi-plastic asthenosphere. The asthenosphere causes the overlying plates to move.
As plates move over the asthenosphere, they ; 1 - separate, mostly at oceanic ridges. 2 - collide and are sub-ducted back into the mantle in other areas, such as at oceanic trenches. There are three major types of plate boundaries: 1 - divergent 2 - convergent 3 - transform. Along these boundaries, new plates (crust) are: formed, consumed, or slide laterally past one another.
1 - Divergent boundaries (formed) : (spreading ridges) occur where plates are separating and new oceanic lithosphere is forming. Divergent boundaries are places where the magma derived from the partial melting of the mantle rises to the surface (the magma is almost basaltic). 2 - Convergent Boundaries (consumed): Whereas new crust forms at divergent plate boundaries, older crust must be destroyed and recycled in order for the entire surface area of Earth to remain the same. Such plate destruction occurs at convergent plate boundaries, where two plates collide and the leading edge of one plate descends beneath the margin of the other plate by subduction. Three types of convergent plate boundaries are recognized: oceanic–oceanic, oceanic–continental and continental–continental.
3 - Transform Boundaries (slide laterally past one another): These mostly occur along fractures in the seafloor (transform faults), where plates slide laterally past one another, parallel to the direction of plate movement. Lithosphere is neither created nor destroyed along a transform boundary.
A world map showing Earth’s plates, their boundaries, their relative motion and average rates of movement in centimeters per year, and hot spots.
How Is Geologic Time Measured? Geologists use two different frames of reference when discussing geologic time, relative dating and absolute (accurate) dating. Relative dating is placing geologic events in a sequential order as determined from their positions in the geologic record and will not tell us how long ago a particular event took place; only that one event preceded another. Absolute dating (accurate) provides specific dates for rock units or events. Radiometric dating is the most common method of obtaining absolute ages.
Principles of Relative Dating : Relative dating means placing rocks in their proper sequence of formation, first, second, third, and so on. Relative dating cannot tell us how long ago something took place, only that it followed one event and preceded another. A few basic principles or rules had to be applied. 1 -Law of Superposition: (Nicolaus Steno 1936 -1986) The law simply states that in an underformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below. 2 -Principle of Original Horizontality: Simply stated, it means that layers of sediment are generally deposited in a horizontal position.
Most layers of sediment are deposited in a nearly horizontal position. Thus, when we see rock layers that are folded or tilted we can assume that they were moved into that position by crustal disturbances after their deposition.
3 -Principle of Cross. Cutting Relationships: When a fault cuts through other rocks, or when magma intrudes and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected.
fault A occurred before the conglomerate was laid down, because that layer is unbroken, dike B and its associated sill are older than dike A, because dike A cuts the sill. batholith was emplaced after movement occurred along fault B, but before dike B was formed.
4 -Inclusions: Inclusions are pieces of one rock unit that are contained within another.
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