Transcript SGES 1302 Lecture6 - Department Of Geology

SGES 1302
INTRODUCTION
TO EARTH SYSTEM
LECTURE 6: Evidences of Plate Tectonics
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
2
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.
3
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.
4
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.
5
6
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.
7
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.
8
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.
9
Evidences of Plate Tectonics




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.
10
Evidences of Plate Tectonics

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.
11
12
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.
13
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.
14
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.
15
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.
16