SGES 1302INTRODUCTION TO EARTH SYSTEM

LECTURE 6: Evidences of Plate Tectonics

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Evidences of Plate Tectonics Palaeomagnetism Distribution of earthquakes Ages of seafloor and sediments on the floor

deep ocean basins Presence of island groups over hot spots

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Paleomagnetism: Polar Reversal Like all magnets, Earth's magnetic field has two opposing regions, or poles,

positioned approximately near geographical North and South Poles.

During a period of normal polarity the region of attraction corresponds with the North Pole.

Today, a compass needle, like other magnetic materials, aligns itself parallel to the magnetizing force and points to the North Pole.

During a period of reversed polarity, the region of attraction would change to the South Pole and the needle of a compass would point south.

Studies of the magnetism retained in rocks at the time of their formation (like small compasses frozen in time) have shown that the polarity of the magnetic field has reversed repeatedly throughout geological time.

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Paleomagnetism: Polar Reversal The reason for polar reversals is not known. Although the average

time between reversals over the last 10 million years has been 250,000 years, the rate of reversal has changed continuously over geological time.

The most recent reversal was 780,000 years ago; scientists have no way of predicting when the next reversal will occur.

The reversal process probably takes a few thousand years. Dating rocks using distinctive sequences of magnetic reversals is called magnetic stratigraphy.

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Palaeomagnetism: Polar Reversal The geophysicists recognized that there was

a striped pattern of alternating normal polarity, reversed polarity, normal polarity in the basalts of the oceans.

The basalt lavas marked by reverse polarity magnetism must have crystallized during a period when the Earth's magnetic field was the opposite of what it is today.

The discovery that the linear pattern of magnetism was symmetrical across the submarine mountain range, and that the basalt lavas became increasingly older on either side of the ridge with distance suggest that new oceanic crust in the form of basaltic lavas was produced at the ocean ridges, cooled, crystallized and moved away from the ridges as newer oceanic crust replaced it at the ridges.

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Paleomagnetism: Polar Wandering In the 1950's it was discovered that when iron-rich minerals in lavas

cooled, they became magnetized in the direction parallel to the existing magnetic field.

Scientific evidence indicates that the magnetic poles have slowly and erratically wandered across the surface of the Earth.

Pole locations calculated from measurements on rocks younger than about 20 million years do not depart from the present pole locations by very much, but successively greater "virtual pole" distances are revealed for rocks older than 30 million years, indicating that substantial deviations occurred.

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Palaeomagnetism: Polar Wandering Plotting the apparent position of the magnetic north pole over the past 500 million

years showed that either the the magnetic poles migrated through time and the continents had drifted.

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Evidences of Plate Tectonics Earthquakes and volcanic activity on the Earth is concentrated in a linear band that

snakes around the world. This band is particularly evident around the edge of the Pacific Ocean where it is known as the Ring of Fire.

Within the ocean basins near these bands are some of the deepest oceanic waters on Earth. These linear areas of anomalously deep water are called trenches.

In the late 1920s, seismologists had identified earthquake zones parallel to the trenches that were inclined 40 to 60° from the horizontal and extended several hundred kilometers into the Earth. These Wadati-Benioff zones, named for the scientists that first recognized them, marked the descent of the oceanic plates back into the mantle at the oceanic trenches.

The Princeton University geologist, Henry Hess, realized that at the same time that new sea floor is being created at the ridges, old sea floor is being consumed by subduction at the trenches.

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Ages of seafloor and sediments on the floor deep ocean basins The basaltic lavas that make up the ocean floor were discovered to be less than 180

million years old, and the amount of sediments in the ocean basins were much thinner than expected.

The ocean floor was not flat and featureless as expected. Numerous oceanographic surveys revealed that a great submarine mountain range (MOR) more than 50,000 km long, up to 800 km across and 4,500 m or more high virtually encircled the Earth.

Rocks at the mid-oceanic ridge is the youngest and they become progressively older away from the ridge.

Evidences of Plate Tectonics

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The final supporting evidence is the information that scientists get from hot spots. For example, the Hawaiian Island and the seamounts that extend from Hawaii to the Aleutian trench show the movement of the Pacific plate as it moved over the hot spot. Radiometric dating shows that the volcanic activity decreases in age toward the island of Hawaii, which is now over the hot spot.

Evidences of Plate Tectonics

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Driving forces of plate motion Tectonic plates are able to move because of the relative density of oceanic

lithosphere and the relative weakness of the asthenosphere. Dissipation of heat from the mantle is known to be the original source of energy

driving plate tectonics, but it is no longer thought that the plates ride passively on asthenospheric convection currents.

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Driving forces of plate motion It is accepted that the excess density of the oceanic lithosphere sinking in

subduction zones drives plate motions. When it forms at mid-ocean ridges, the oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes more dense with age, as it conductively cools and thickens.

The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate motions. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.

Mantle convection is the slow creeping motion of Earth's rocky mantle in response to perpetual gravitationally unstable variations in its density.

Material near the surface of Earth, particularly oceanic lithosphere, cools down by conduction of heat into the oceans and atmosphere, then thermally contracts to become dense, and then sinks under its own weight at plate boundaries.

This subducted material sinks to some depth in the Earth's interior where it is prohibited, by inherent density stratification, from sinking further. This stoppage creates a thermal boundary layer where sunken material soaks up heat via thermal conduction from below, and may become buoyant again to form upwelling mantle plumes.

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Isostacy In addition to the large crustal movements caused by

plate tectonics, gradual vertical movements of the continental crust that are not related to plate margins are recorded.

These vertical movements of the crust are to establish a gravitational balance.

Isostasy is used to refer to the state of gravitational equilibrium between the Earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density.

In the simplest example, isostasy is the principle observed by Archimedes, where he saw that when an object was immersed, an amount of water equal in volume to that of the object was displaced.

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Gravity & Isostacy On a geological scale, isostasy can be observed where

the Earth's strong lithosphere exerts stress on the weaker asthenosphere which, over geological time flows laterally such that the load of the lithosphere is accommodated by height adjustments.

When large amounts of sediment are deposited on a particular region, the immense weight of the new sediment may cause the crust below to sink.

Similarly, when large amounts of material are eroded away from a region, the land may rise to compensate.

Therefore, as a mountain range is eroded down, the (reduced) range rebounds upwards (to a certain extent) to be eroded further. Some of the rock strata now visible at the ground surface may have spent much of their history at great depths below the surface buried under other strata, to be eventually exposed as those other strata are eroded away and the lower layers rebound upwards again.