Continental Drift
(source: USGS at http://pubs.usgs.gov/publications/text/historical.html)
In geologic terms, a plate is a large, rigid slab of solid rock. The word tectonics comes from the Greek root "to build." Putting these two words together, we get the term plate tectonics, which refers to how the Earth's surface is built of plates. The theory of plate tectonics states that the Earth's outermost layer is fragmented into a dozen or more large and small plates that are moving relative to one another as they ride atop hotter, more mobile material. Before the advent of plate tectonics, however, some people already believed that the present-day continents were the fragmented pieces of preexisting larger landmasses ("supercontinents"). The diagrams below show the break-up of the supercontinent Pangaea (meaning "all lands" in Greek), which figured prominently in the theory of continental drift -- the forerunner to the theory of plate tectonics.
According to the continental drift theory, the supercontinent Pangaea began to break up about 225-200 million years ago, eventually fragmenting into the continents as we know them today. Plate tectonics is a relatively new scientific concept, introduced some 30 years ago, but it has revolutionized our understanding
of the dynamic planet upon which we live. The theory has unified the study of the Earth by drawing together many branches of the earth sciences, from paleontology (the study of fossils) to seismology (the study of earthquakes). It has provided explanations to questions that scientists had speculated upon for centuries -- such as why earthquakes and volcanic eruptions occur in very specific areas around the world, and how and why great mountain ranges like the Alps and Himalayas formed.
The belief that continents have not always been fixed in their present positions was suspected long before the 20th century; this notion was first suggested as early as 1596 by the Dutch map maker Abraham Ortelius in his work Thesaurus Geographicus. Ortelius suggested that the Americas were "torn away from Europe and Africa . . . by earthquakes and floods" and went on to say: "The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents]." Ortelius' idea surfaced again in the 19th century. However, it was not until 1912 that the idea of moving continents was seriously considered as a full-blown scientific theory -- called Continental
Drift -- introduced in two articles published by a 32-year-old German meteorologist named Alfred Lothar Wegener. He contended that, around 200 million years ago, the supercontinent Pangaea began to split apart. Alexander Du Toit, Professor of Geology at Johannesburg University and one of Wegener's staunchest supporters, proposed that Pangaea first broke into two large continental landmasses, Laurasia in the northern hemisphere and Gondwanaland in the southern hemisphere. Laurasia and Gondwanaland then continued to break apart into the various smaller continents that exist today.
Wegener's theory was based in part on what appeared to him to be the remarkable fit of the South American and Africancontinents, first noted by Abraham Ortelius three centuries earlier. Wegener was also intrigued by the occurrences of unusualgeologic structures and of plant and animal fossils found on the matching coastlines of South America and Africa, which arenow widely separated by the Atlantic Ocean. He reasoned that it was physically impossible for most of these organisms to have swum or have been transported across the vast oceans. To him, the presence of identical fossil species along the coastalparts of Africa and South America was the most compelling evidence that the two continents were once joined.
In Wegener's mind, the drifting of continents after the break-up of Pangaea explained not only the matching fossil occurrences but also the evidence of dramatic climate changes on some continents. For example, the discovery of fossils of tropical plants (in the form of coal deposits) in Antarctica led to the conclusion that this frozen land previously must have been situated closer to the equator, in a more temperate climate where lush, swampy vegetation could grow. Other mismatches of geology and climate included distinctive fossil ferns (Glossopteris) discovered in now-polar regions, and the occurrence of glacial deposits in present-day arid Africa, such as the Vaal River valley of South Africa.
The theory of continental drift would become the spark that ignited a new way of viewing the Earth. But at the time Wegener introduced his theory, the scientific community firmly believed the continents and oceans to be permanent features on the Earth's surface. Not surprisingly, his proposal was not well received, even though it seemed to agree with the scientific information available at the time. A fatal weakness in Wegener's theory was that it could not satisfactorily answer the most fundamental question raised by his critics: What kind of forces could be strong enough to move such large masses of solid rock over such great distances?
Alfred Wegener proposed his theory of continental drift in the early 20th century. It offered an explanation of the existence of similar fossils and rocks on continents that are far apart from each other. But it took a long time for the idea to become accepted by other scientists.
Wegener said continental drift happens when tectonic plates - huge segments of the Earth’s crust and upper mantle, sometimes forming continents - move. They shift at a few centimetres per year, with earthquakes and volcanoes often occurring around their edges.
Wegener’s theory
Alfred Wegener proposed the theory of continental drift at the beginning of the 20th century. His idea was that the Earth's continents were once joined together, but gradually moved apart over millions of years.
Wegener’s evidence for continental drift was:
- The same types of fossilised animals and plants are found in South America and Africa.
- The shape of the east coast of South America fits the west coast of Africa, like pieces in a jigsaw puzzle.
- Matching rock formations and mountain chains are found in South America and Africa.
Before Wegener developed his theory, it was thought mountains formed because the Earth was cooling down, and in doing so contracted. This was believed to form wrinkles, or mountains, in the Earth's crust. If the idea was correct, however, mountains would be spread evenly over the Earth's surface. We know this is not the case.
Wegener suggested mountains are formed when the edge of a drifting continent collides with another, causing it to crumple and fold. For example, the Himalayas were formed when India came into contact with Asia.
Problems
Alfred Wegener (1880 - 1930)
Wegener’s theory of continental drift was rejected by many geologists. Some of the reasons for this were:
- The movement of continents could not be detected - because they only move by a few centimetres per year.
- no-one could provide a good explanation of how whole continents could move apart
- Wegener was not a geologist - he trained as an astronomer and meteorologist
- there were other, simpler, explanations for the same evidence
- it was felt his idea was too big for the evidence at hand
It was only in the 1960s, long after Wegener’s death in 1930, that the theory of continental drift was accepted by scientists.
Consequences of continental drift
Plate tectonics
The Earth’s crust and upper part of the mantle are broken into large pieces called tectonic plates, sometimes forming continents. These are moving constantly, at the rate of a few centimetres each year. Although this does not sound like a lot, over millions of years continents shift thousands of kilometres.
Seafloor spreading
New seafloor material is created when tectonic plates move apart. Molten rock rises between the plates, cools, and solidifies. This process is called seafloor spreading - a consequence of the solid mantle moving.
At the edges of tectonic plates
When tectonic plates meet, the Earth’s crust becomes unstable as they push against each other, or ride under or over one another. Earthquakes and volcanic eruptions occur at the boundaries between plates, and the crust may ‘crumple’ to form mountain ranges. It is difficult to predict exactly when an earthquake will happen, or how destructive it will be, even in places that are known for them.
These are some of the things public authorities can do to reduce the damage caused by geohazards such as earthquakes and volcanoes:
- assess how vulnerable local buildings are
- provide education and training for emergencies
- make risk assessments
- monitor natural hazards in the local area
- research the hazards themselves
Watch
You may wish to view this BBC News item from 2006 about the 100th anniversary of San Francisco’s great earthquake.
Seafloor spreading - higher
Magnetic field reversal
Seafloors spread by about 10cm per year. This spreading leaves a characteristic pattern of magnetism in the rocks on the ocean floor that can be "read" by scientists.
The Earth’s magnetic field has not always had the same North-South alignment. Every so often it reverses direction - for thousands or millions of years. Iron-rich minerals in molten magma line up in the magnetic field, and this alignment is preserved when the magma solidifies. Scientists can then 'read' the pattern of magnetic field reversal that forms.
The patterns on either side of a mid-ocean ridge are mirror images of each other. This provides evidence for seafloor spreading and plate tectonics in general.
Tectonic plates - higher
There are three types of boundary between tectonic plates. They can cause earthquakes and/or volcanoes:
Types of plate boundary and their effects
| cause earthquakes? | cause volcanoes? | what they do | |
|---|---|---|---|
| constructive plate boundary | yes | yes | make new crust |
| destructive plate boundary | yes | yes | make mountains |
| conservative plate boundary | yes | no | do not make or destroy land |
Constructive plate boundary
Destructive plate boundary
Now try a Test Bite
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