The Earth's major plates (Image courtesy of US Geological Survey)
When viewed from the perspective of geological time, the Earth is a very dynamic place. Over the course of millions of years, the face of the Earth has changed as continents move and mountain ranges are formed and eroded.
The Earth’s surface does not consist of a motionless crust but rather of large crustal plates which move and jostle against each other. There are seven large plates and many smaller plates (100 to 150 km thick) that drift around the Earth’s surface, highlighted in the diagram.
The continents move as a consequence of volcanic processes in oceanic areas known as mid oceanic ridges where basalt oozes out onto the sea floor, forcing adjacent plates apart. As the oceanic crust moves away from the ridge it cools, becoming denser and may eventually sink back into the mantle at a subduction zone, pulling the plate along with it. A further mechanism driving the movement of the Earth’s plates are large convection currents within the Earth’s mantle.
What evidence is there for continental drift?
As early as 1596, the Dutch map maker Abraham Ortelius suggested that the Americas, Eurasia and Africa were once joined and have since drifted apart "by earthquakes and floods" His "evidence" was the jigsaw fit of the continents. This fit is especially close when the continental shelves of the continents are considered.
Alfred Wegener, a German polar meteorologist, proposed the theory of continental drift in 1912, after noticing that there were similar glacial deposits in the southern continents, which had a rational distribution if these continents were once joined. The theory also helped explain the distribution of fossils, living plant and animal species and the occurrence of matching rock types in continents that were once contiguous.
Wegener's theory was not accepted by the scientific community of the day, as there was little evidence to reveal the processes which drove the movements of the continents. Indeed, Wegener spent much of his life subject to the derision of scientists from around the world for proposing and defending his theory of continental drift. The theory was discredited for decades until the 1960s, but, with a growing body of evidence to support both the movement of the continents and the mechanisms which drive the movement, the theory is now widely accepted.
Advances in technology which allowed scientists to gain data on sea floor spreading, and the use of laser to actually measure the speed at which plates move (some move at about the same speed at which your fingernail grows), have added to the increasing weight of evidence for the theory of continental drift. Evidence from oceanography has shown that the seafloor has regions of normal and reverse polarity magnetism that occur in bands parallel to the ridge crest producing sea floor spreading. Because the Earth's magnetic field periodically reverses polarity (the north and south poles switch), rocks crystallizing during one of these periods of magnetic reversal will be magnetized with a polarity opposite of rocks that crystallize today. As new seafloor is created at the ridge, it is added in equal amounts to both trailing edges of the spreading seafloor, with the polarity of the magnetic particles within the rock occurring in a mirror image away from the ridge crest.
The breakup of Gondwana and the movement of the southern continents over the past 200 million years. Click and hold on the slider and move it to any position to see the configuration of the continents over time, or press play animation.
What was Gondwana?
Gondwana was the great southern landmass that formed as a result of the division of a much larger supercontinent known as Pangaea about 250 million years ago. This Gondwanan supercontinent consisted of present day landmasses: Africa, South America, India, Madagascar, Australia and New Zealand. The other section of Pangaea, known as Laurasia, comprised what are now Europe, Asia and North America.
The breakup of Gondwana
Dolerite is a common rock
type throughout Tasmania. It
is derived from the breakup of
The Gondwanan landmass started to disperse between 170 - 180 million years ago. The dispersal caused great tension in the Earth’s crust and molten rock intrusion followed as conduits were created in the continental crust, tapping the molten rocks (magma) in the Earth’s mantle. The dolerites that outcrop over extensive parts of central and eastern Tasmania, together with similar igneous rocks in South Africa, South America and Antarctica, are the solidified evidence of the magma from the break up of Gondwana.
India was the first to break away, followed by Africa, and then New Zealand, which started to drift north. By the end of the Cretaceous (65 million years ago), South America and Australia were still joined to Antarctica.
The Australian plate took a very long time to separate from the Antarctic plate. In fact they were the last of the major Gondwanan continental plates to split, only 45 million years ago. As a result, Tasmania has many geological similarities with Antarctica.
By this time Tasmania had also taken on its triangular shape and developed many of its valley and mountain systems, which formed as a result of similar faulting events which created the Bass Strait.
Australia, along with its freight of Gondwanan plants and animals, started being dragged northwards about 45 million years ago while the first glaciers were forming in Antarctica. However, it still took until about 5 million years ago for Antarctica to completely freeze over. This was the first extensive ice to have formed in Antarctica since the Carboniferous - Permian glaciations 300 million years before, which also affected parts of the other southern continents.
Are continents still on the move?
Yes. The Australian plate is moving northwards at about 3cm per year. The Himalayan mountain belt is forming today as a result of India colliding with Asia. Indeed, Mt Everest is marginally higher today than it was when first climbed by Sir Edmund Hillary and Tenzing Norgay.
Volcanoes and earthquakes all provide evidence for the Earth’s continually wandering continents, as they generally occur along the margins of plates. There are three distinct types of boundaries between plates:
Divergent — where plates move away from one another, such as along mid-ocean ridges or rift valleys such as the East African Rift Valley.
Convergent — where plates move into one another and denser oceanic crust is subducted, as in the Andes and Aleutian Islands, or where there are continental collisions.
Transform — where plates grind past one another, such as along the St Andreas fault in California.
Each plate may be bounded by a variety of plate boundaries.
The biological legacy of Gondwana
The breakup of Gondwana as the result of continental drift played a major role in determining the present day distribution of southern hemisphere plants and animals. Tasmania has a rich diversity of species that are derived from Gondwanan ancestral stock. The species within cool temperate rainforest that is found in Tasmania were once widespread across Gondwana. Consequently, it is not surprising that closely related species are found in those continents that once formed Gondwana - South America, New Zealand, and prior to freezing over, Antarctica.
This Gondwanan connection is true of many other species of plants and animals found in Tasmania. Further details of this biological legacy can be found on our page on the biogeography of Tasmania.