DEFINITION OF PLATE TECTONICS

Definition of plate tectonics

The study of the large-scale evolution of

the lithosphere of the Earth. In the 1960s the Earth Sciences

experienced a scientific revolution, when the paradigm of

plate tectonics was formulated from a number of previous

hypotheses that attempted to explain different aspects about

the evolution of continents, oceans, and mountain belts. New

plate material is created at mid-ocean ridges and destroyed

when it sinks back into the mantle in deep-sea trenches. Scientists

had known for some time that the Earth is divided

into many layers defined mostly by chemical characteristics,

including the inner core, outer core, mantle, and crust. The

plate tectonic paradigm led to the understanding that the

Earth is also divided mechanically and includes a rigid outer

layer, called the lithosphere, sitting upon a very weak layer

containing a small amount of partial melt of peridotite,

termed the asthenosphere. The lithosphere is about 78 miles

(125 km) thick under continents, and 47 miles (75 km) thick

under oceans, whereas the asthenosphere extends to about

155 miles (250 km) depth.

Plate tectonics has been a unifying science, bringing

together diverse fields such as structural geology, geophysics,

sedimentology and stratigraphy, paleontology,

geochronology, and geomorphology, especially with respect

to active tectonics (also known as neotectonics). Plate

motion almost always involves the melting of rocks, so

other fields are also important, including igneous petrology,

metamorphic petrology, and geochemistry (including isotope

geochemistry).

The base of the crust, known as the Mohorovicic discontinuity,

is defined seismically and reflects the difference

in seismic velocities of basalt and peridotite. However, the

base of the lithosphere is defined rheologically as where the

same rock type on either side begins to melt, and it corresponds

roughly to the 2,425°F (1,330°C) isotherm. The

main rock types of interest to tectonics include granodiorite,

basalt, and peridotite. The average continental crustal composition

is equivalent to granodiorite (the density of granodiorite

is 2.6 g/cm3; its mineralogy includes quartz,

plagioclase, biotite, and some potassium feldspar). The

average oceanic crustal composition is equivalent to that of

basalt (the density of basalt is 3.0 g/cm3; its mineralogy

includes plagioclase, clinopyroxene, and olivine). The average

upper mantle composition is equivalent to peridotite

(the density of peridotite is 3.3 g/cm3; its mineralogy

includes olivine, clinopyroxene, and orthopyroxene). Considering

the densities of these rock types, the crust can be

thought of as floating on the mantle; rheologically, the

lithosphere floats on the asthenosphere.

The plate tectonic paradigm states that the Earth’s outer

shell, or lithosphere, is broken into 12 large and about 20

smaller rigid blocks, called plates, that are all moving with

respect to each other. The plates are rigid and do not deform

internally when they move, but only deform along their

edges. The edges of plates are therefore where most mountain

ranges are located and are where most of the world’s earthquakes

occur and active volcanoes are located. The plates are

moving as a response to heating of the mantle by radioactive

decay, and are somewhat analogous to lumps floating on a

pot of boiling stew.

The movement of plates on the spherical Earth can be

described by a rotation about a pole of rotation, using a theorem

first described by Euler in 1776. Euler’s theorem states

that any movement of a spherical plate over a spherical surface

can be described by a rotation about an axis that passes

through the center of the sphere. The place where the axis of

rotation passes through the surface of the Earth is referred to

as the pole of rotation. The pole of rotation can be thought

of analogous to a pair of scissors opening and closing. The

motions of one side of the scissors can be described as a

rotation of the other side about the pin in a pair of scissors,

either opening or closing the blades of the scissors. The

motion of plates about a pole of rotation is described using

an angular velocity. As the plates rotate, locations near the

pole of rotation experience low angular velocities, whereas

points on the same plates that are far from the pole of rotation

experience much greater angular velocities. Therefore,

oceanic spreading rates or convergence rates along subduction

zones may vary greatly along a single plate boundary.

This type of relationship is similar to a marching band going

around a corner. The musicians near the corner have to

march in place and pivot (acting as a pole of rotation) while

the musicians on the outside of the corner need to march

quickly to keep the lines in the band formation straight as

they go around the corner.

Rotations of plates on the Earth lead to some interesting

geometrical consequences for plate tectonics. We find that

mid-ocean ridges are oriented so that the ridge axes all point

toward the pole of rotation and are aligned on great circles

about the pole of rotation. Transform faults lie on small circles

that are concentric about the pole of rotation. In contrast,

convergent boundaries may lie at any angle with respect

to poles of rotation.

Since plates do not deform internally, all the action happens

along their edges, and we can define three fundamental

types of plate boundaries. Divergent boundaries are where

two plates move apart, creating a void that typically becomes

filled by new oceanic crust that wells up to fill the progressively

opening hole. Convergent boundaries are where two plates

move toward each other, resulting in one plate sliding beneath

the other (when a dense oceanic plate is involved), or collision

and deformation (when continental plates are involved).

Transform boundaries form where two plates slide past each

other, such as along the San Andreas Fault in California.

Since all plates are moving with respect to each other, the

surface of the Earth is made up of a mosaic of various plate

boundaries, and the geologist has an amazing diversity of different

geological environments to study. Every time one plate

moves, the others must move to accommodate this motion,

creating a never-ending saga of different plate configurations.

Divergent Plate Boundaries and the

Creation of Oceanic Crust

Where plates diverge, new oceanic crust is produced by

seafloor spreading. As the plates move apart, the pressure on

deep underlying rocks is lowered, which causes them to rise

and partially melt by 15–25 percent. Basaltic magma is produced

by a partial melting of the peridotitic mantle, leaving a

“residue” type of rock in the mantle known as harzburgite.

The magma produced in this way upwells from deep within

the mantle to fill the gap opened by the diverging plates. This

magma forms a chamber of molten or partially molten rock

that slowly crystallizes to form a coarse-grained igneous rock

known as gabbro, which has the same composition as basalt.

Before crystallization, some of the magma moves up to the

surface through a series of dikes and forms the crustal sheeted

dike complex, and basaltic flows. Many of the basaltic

flows have distinctive forms with the magma forming bulbous

lobes known as pillow lavas. Lava tubes are also common,

as are fragmented pillows formed by the implosion of

the lava tubes and pillows. Back in the magma chamber,

other crystals grow in the gabbroic magma, including olivine

and pyroxene, which are heavier than the magma and sink to

the bottom of the chamber. These crystals form layers of

dense minerals known as cumulates. Beneath the cumulates,

the mantle material from which the magma was derived gets

progressively more deformed as the plates diverge and form a

highly deformed ultramafic rock known as a harzburgite or

mantle tectonite. This process can be seen on the surface in

Iceland along the Reykjanes Ridge.

TRANSFORM PLATE BOUNDARIES AND TRANSFORM FAULTS

In many places in the oceanic basins, the mid-ocean ridges

are apparently offset along great escarpments or faults, which

fragment the oceanic crust into many different segments. In

1965 J. Tuzo Wilson correctly interpreted these not as offsets

but as a new class of faults, known as transform faults. The

actual sense of displacement on these faults is opposite to the

apparent “offset,” so the offset is apparent, not real. It is a

primary feature of Wilson’s model, proven correct by earthquake

studies.

These transform faults are steps in the plate boundary

where one plate is sliding past the other plate. Transform

faults are also found on some continents, with the most

famous examples being the San Andreas Fault, the Dead Sea

Transform, North Anatolian Fault and Alpine Fault of New

Zealand. All of these are large strike-slip faults with horizontal

displacements and separate two different plates.

Title Post:
Rating: 100% based on 99998 ratings. 99 user reviews.
Author:

Terimakasih sudah berkunjung di blog SELAPUTS, Jika ada kritik dan saran silahkan tinggalkan komentar