Topic 2-Lesson 1. Plate Tectonics-The Birth of a Theory. The Theory of Plate Tectonics. If we look at a globe carefully, most of the continents seem to fit together like a puzzle. The West African coastline seems to fit nicely into the east coast of South America.
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Plate Tectonics-The Birth of a Theory
If we look at a globe carefully, most of the continents seem to fit together like a puzzle. The West African coastline seems to fit nicely into the east coast of South America.
The fit is even more striking when the submerged continental shelves are compared rather than the coastlines.
Alfred Wegener (1880-1930) noticed the same thing and proposed that the continents were once compressed into a singe protocontinent which he called Pangea (meaning ‘All lands’) and over time they have drifted apart.
Wegener’s Hypothesis lacked a geological mechanism to explain how the continents could drift across the Earth’s surface as he had proposed.
In 1929 Arthur Holmes elaborated one of Wegener’s many hypothesis:
Arthur Holmes stated that the mantle undergoes thermal convection. And that this thermal convection was like a conveyor belt and that the upwelling pressure could break apart a continent and then force the broken continent in opposite directions carried by the convection currents.
Not until the 1960’s did Holmes’ idea receive any attention. Greater understanding of the ocean floor and the discoveries of features like mid-ocean ridges, geomagnetic anomalies parallel to the mid-ocean ridges, and the association of island arcs and oceanic trenches occurring together and near continental margins, suggested convection might indeed be at work.
To understand this theory better we need to look at the geological processes taking place and the evidence which supports it.
We’ll start by looking
at the crust.
The crust covers the mantle and is the Earth’s hard outer shell, the surface we are living on .
Compared to the
other layers the
crust is much
The crust is made up of solid material but this material is not the same everywhere. There is an oceanic crust and a continental crust.
This crust is below the ocean and is 6-11 km thick. The rocks of the oceanic crust are very young compared with rocks of the continental crust; not older than 200 million years. The main rock type is basalt and the average density is 3g/cm^3
Continental crust is the part of Earth’s crust not covered by water. This is much thicker than oceanic crust with an average of about 30-40km and a maximum of 70km. Continental crust is much older, some rocks are 4.1 billion years old. Continental crust consists of igneous rocks such as Granite. The average density is 2.7g/cm^3
Evidence for Continental Drift
The Theory of Plate Tectonics builds on Wegener’s Theory of Continental Drift. This theory states that the Earth’s crust or Lithosphere is broken up into “tectonic plates” each of which are moving as a result of convection currents in the mantle.
Evidence for Continental Drift
When lava erupts onto the surface, the iron minerals within it, such as magnetite, align themselves with the Earth’s magnetic field in the same way as a compass.
Once the lava cools and solidifies, the minerals are locked into position. By analysing the alignment of magnetic minerals within lava, geologists are able to determine the directions of the North and South poles when the lava erupted.
In addition to the magnetic bearings, the distance at which the lava formed from either pole can also be estimated by the angle of tilt of these magnetic minerals.
If you hold a hand compass vertically to the ground, it tells us the angle of the Earths magnetic field.
At the equator, the compass needle will be parallel with the ground. This is how the minerals would align themselves.
At the magnetic South Pole the south end of the compass needle will point straight at the ground.
Midway between the equator and the poles, the compass needle will point down at a 45-degree angle
If lava erupted today in Sydney, the south end of the magnetic minerals (compass needle) would point down at 34 degrees.
By measuring the angle of the iron minerals formed in rocks, scientists are able to determine both the direction of the Pole and the distance from the pole when the mineral solidified.
For reasons not fully understood, the polarity of Earth’s magnetic field reverses every few million years. In other words, the magnetic North Pole becomes the magnetic South Pole and vice versa.
Such magnetic reversals are believed to be caused by the changes in the circulation patterns of molten iron in the Earth’s outer core.
This flipping is rapid and the locations of the poles of the globe stay much the same. If this were to happen today, the north end of our compass would point towards Antarctica.
A series of magnetic reversals were first identified in lave fields on land. Using radiometric dating scientists were able to measure dates for the reversals.
But why does this matter?
The importance of magnetic reversals to continental drift can be seen when examining the polarity of rocks across mid ocean ridges.
The pattern of magnetic reversals on one side of oceanic ridge is a mirror image of that on the other. This can only be explained if the crust is being pulled apart at these ridges.
As the crust breaks open in the central valley of an oceanic ridge, lava erupts, forming a long and narrow plug. Further spreading at the ridge will cause the old plug to tear down the middle and fresh lava to create a new plug.
If Earth’s magnetic field reversed between these eruptions, the different lava flows can be identified.
Think about it:
If new crust is being created at mid-ocean ridges and the planet isn’t growing in size. What’s happening to the old crust??
If new crust is continuously formed at oceanic ridges and is then pulled apart sideways, rocks should get older the further they are from the ridge.
Radioisotope dating has shown this to be the case at all oceanic ridges.
Ocean basins formed at ridges are only 200 million years old.
Organic debris and other sediments rain down onto the deep ocean floor at a steady rate. The fact that little or no sediment covers rocks at the ridge, while sediment layers get thicker as you move away from it. This is further evidence that ocean rocks increase with age away from the ridge.
The break up of Pangaea
Characteristics and features of mid-ocean ridges
If we analyse the distribution of earthquakes around the world, we are able to identify the locations of the plate boundaries.
Why would we associate earthquakes with plate boundaries?
If we imagine the tectonic plates like blocks of wood, there are three ways they can interact.
Remember what Holmes said about convection currents? He stated that the mantle undergoes thermal convection. And that this thermal convection was like a conveyor belt and that the upwelling pressure could break apart a continent and then force the broken continent in opposite directions carried by the convection currents.
What type of plate boundary would assimilate with Holmes’ idea?
Divergent plate boundaries are located where plates are moving away from one another. This occurs above rising convection currents as Holmes suggested. Mid-ocean ridges are found at this boundary.
Convergent plate boundaries are locations where plates are moving towards one another. The plate collisions that occur in these areas can produce earthquakes, volcanoes and crustal deformation. Subductinos zones are found at this boundary.
There are three
different types of
which we will discuss
Transform plate boundaries are locations where two plates slide past one another. The fracture zone that forms a transform plate boundary is known as a transform fault. Most transform faults are found in the ocean basin and connect offsets in the mid-ocean ridges.
Sea floor spreading is happening at this very moment. The mid-ocean ridge is a continuous chain of volcanoes on the ocean floor where lava erupts and the crust of the Earth is created.
Basalt and Gabbro are two common rocks which form at mid-ocean ridges.
Rocks which form along mid-ocean ridges are composed of mostly mafic minerals such as olivine, pyroxene and amphibole.
Mafic: is used to describe silicate minerals, magmas and rocks which are relatively high in heavier elements.
The following geological features can be found at mid-ocean ridges:
(water of course)
Black smokers Pillow lava (basalt)
Here the Earth’s crust is spreading, creating new ocean floor and literally renewing the surface of our planet. Older crust is recycled back into the mantle elsewhere on the globe, typically where plates collide.
Using today's technology we can measure this plate movement
With technologies like computers, laser measurement and satellite remote sensing, the slow movement of the Earth’s plates can be measured directly.
By calculating the distance
between two points on
either side of a plate
boundary, the rate and
direction of relative
movement can be
The plates move in different directions which means they collide, separate and rub past one another. Individual plates have one general direction of movement but the rates at which different parts of the plate move can vary.
These variations in movement cause the tearing or folding of plates where the pressures are greatest or rocks are weakest. These forces cause fault lines.
From the geological evidence available, scientists have been able to piece together the previous positions of the continents for the last 200 million years.
At the beginning of this period (200mya), the Earth’s continental crust was joined into one supercontinent that stretched from pole to pole on one side of the planet.
Pangaea means ‘all lands’
The massive ocean on the other side is called Panthalassa meaning ‘all seas’
180 mya Pangaea had begun to break up into two main slabs.
The northern slab
contained what was to
become North America,
Greenland, Europe and
The southern slab, Gondwana contained land that would form South America, Africa, Antarctica, India and Australia
By 135 MYA South America had begun to separate from Africa; North America and Greenland from Europe; and Australia and Antarctica from the rest of Gondwana.
The explanation as to why previously joined continents begin to break up and travel in different directions lies in the convection currents that drive them.
Australia has the longest geological record of any landmass: over 90% of Earth’s complete time span is recorded in our rocks.
By analysing many rocks of various ages and plotting the latitude at which they formed against time, continental drift is apparent.
The oldest Australian rocks have drifted at least 100,000 kilometres (twice around the world).
Australia’s journey included two trips to each of the polar regions and crossed the equator a number of times.
A movement of only a few centimetres per year over the huge amount of geological time amounts to great distances
It has taken 200 million years for Pangaea to break up. This is less than 5% of the Earth’s life span!! Equal to about 8 months of your life span.
Given the rate and direction of movement of the continents, another supercontinent is likely to form in around 250 million years