There is a belief that continents and ocean basins were formed as a result of isostatic readjustments of the continental rock that Pangaea was composed of rock of varying densities where some rocks were very dense and heavy while some areas had lighter rocks. The differences resulted into the upwarping of areas with lighter rocks and formed the continents (sialic). The dense and heavier rocks led to down warping some areas that formed ocean basins.

Rock expansion theory:

It is believed that the Pangaea split as a result of excessive environmental temperatures which was followed by the cracking of the Pangaea into several continental landmasses and their eventual drifting apart.

Tetrahedral hypothesis:

This theory does not involve horizontal crystal land mass movement but vertical movements. It releases on the belief that the earth is contracting . It is stated that any given surface expresses greatest volume in a sphere and small volume on a tetrahedron. That the earth would shrink on Geli a way that the ocean basin would be depressed on the tetrahedral faces while the continental crusts would occupy the remaining faces. The arrangement of continents into 3 pairs and ocean basins make the earth resemble a tetrahedron.

Gravitational pull of the earth so thought to be the basis for the earth contraction whereby materials are pulled to the earth’s interior which cause the earth to become smaller. The theory was criticised.

The expanding earth theory:

This emphasises horizontal movement of the crustal landmasses. That the earth was originally smaller and as the interior of the earth expanded, thus forced outer crustal materials to break forming continental plates and as the earth continued to expand, the bigger the gaps became and further plates drifted from earth other.

Polar Wandering:

In the 1950s, physicists in England became interested in the observation that certain kinds of rocks produced a magnetic field. They soon decided that the magnetic fields were remnant, or left over, magnetism acquired from the earth’s magnetic field as the rocks cooled and solidified from the hot magma that formed them.

Scientists measured the orientation and direction of the acquired magnetic fields and, from these orientations, calculated the direction of the rock’s magnetism and the distance from the place the rock was found to the magnetic poles. As calculations from rocks of varying ages began to accumulate, scientists calculated the position of the earth’s magnetic poles over time.

The position of the poles varied depending on where the rocks were collected, and the idea of a polar wander path began to form. When sample paths of polar wander from two continents, such as North America and Europe, were compared, they coincided as if the continents were once joined. This new science and methodology became known as the discipline of paleomagnetism. As a result, discussion of the theory of continental drift increased, but most earth scientists remained skeptical.

In summary, Polar wandering theory explains latitudinal changes through which the continents have passed. It suggests that the earth crust with the mantle migrate as a whole relative to the earth’s axis but remains fixed relative to the star eg if Africa moves a metre in a certain direction, even other continents follow the same distance on the same direction. It was highly criticized too.

The moon theory:

The theory tried to explain why there is missing sialic crust on the floor of the ocean basins, that the moon separated itself from the earth early in the history of the solar system. The theory formed on the earth’s surface before the moon was born, then much of the crust must have been carried away by the moon when the great rapturre took place.

It was Osmand Fisher (1881) who first suggested that the huge Pacope basin must be a scar left behind when the moon separated itself from the earth. It was supported for two reasons.

Materials of the moon reflect sunlight in the same way that powdered sialic materials do. They are therefore similar materials with possibly the same origins.

The Pacific basin is the largest region from which sialic materials are lacking. If 1/3 of the earth’s surface and the size of the moon is about 1/3 of the earth. It is possible that the pacific basin is the huge hole (scar) left behind when the moon separated from the earth.

Evidence Of Continental Drift.

To justify his continental drift theory, Alfred Wegner Marshalled an impressive amount of evidence, facts and opinions, most of which have proved too true to be denied. Other researchers have added on the amount of evidence.

Geometrical fitting (jig – saw /visual fit).

The present of continental margins is such that most of the continents have geometric shapes that would fit into each of forced close. Thus jig – saw fit of continents were at one time together forming Pangea but they merely split to other parts eg fitting the Eastern coast of South America in the West Africa.

Geological similarities.

Similarity on rocks is another evidence used to justify continental drift. There are remarkable similarities between rocks found in different landmasses of the southern continents. In Africa the rocks of Ghana and Cameroon share common characteristics with those of Brazil on South America. It therefore suggests that those areas were once of the same continent. This evidences constituents what is commonly known as “matching geology”.

Presence of fold mountains.

This seems to suggest that continents have been moving towards each other because when the tectonic plates collide with other, land masses on them may fold and lead to the formation of fold mountains. Atlas and Alps ranges, Himalayas in India due to collision of the Indian Sub continent and the Laurasia rock.

Paleotological Evidence(Historical Fossils).

The existence of historical fossils and other rock types where climatic conditions do not favour theory formation also suggests that continental drift took place. The following are the examples: In the Greenland, there is coral rock development while develops only on tropical regions with warm conditions.

In the same way, rocks of Greenland, there are Lichans and swamps of tropical trees embedded within the rocks. There are also bones of tropical animals in the rocks all of which prove that Green land could be in the tropical region but later lifted.

Furthermore, the existence of Petroleum, coal and other organic minerals in the extreme Northern Hemisphere as an indication of continental drift. This is because coal, petroleum are meant for tropical areas.

Vegetation.

The similarity of the vegetation and animals in different continents, which seem to have been attached to each other suggests the occurrence of continental drift, eg the Brazilian tropical rainforests and the Congo basin. Rain forests also suggest that those regions must have been attached together one time.

Occurrence of Laterite.

The occurrence of Laterite on North America and certain parts of Europe shows that those regions once upon a time enjoyed tropical climatic conditions which were conducive for the formation of Laterites.

Dwyka Tillites:

Studies from Palaeclimatology have provided further evidence that justifies continental drift. This is in convection with glacial deposits. Dwyka Tillites are the glacial carboniferous rocks are found on many parts of the former Gondwanaland which proves that these continents were once together.

Palaegmanetism evidence.

Palemagnetism refers to the ancient or fossils magnetism on rocks. It is established from the geological laboratory that research or rocks of the earth crust which are always magnetized by the earth’s magnetic field at the time of formation. They show real North and South.

Magnetic Field Reversals

In 1963, British scientists Fred J. Vine and Drummond H. Matthews combined their observations of the marine magnetic anomalies with the concept of reversals of the earth’s magnetic field. They proposed that the marine magnetic anomalies were a “tape recording” of the spreading of the ocean floor as the earth’s magnetic field reversed its direction. At the same time, other geophysicists were studying lava flows from many parts of the world to see how these flows revealed the record of reversals of the direction of the earth’s magnetic field. These studies showed that nearly four reversals have occurred over the past 5 million years. The concept of magnetic field reversals was a breakthrough that explained the magnetic polarity switches seen in seafloor spreading as well as the concept of similar magnetic patterns in the rocks used to demonstrate continental drift.

However when Pangea split up, the rock changed position and the direction of the magnetic north of the individual pieces of rock changed.

Sea floor spreading evidence:

Research has sown that most sea floors are expanding and many oceans and seas are becoming bigger. This is probably because of the movement of continents eg the Red sea enlarged to 300 km wide in the 200 million years. The ocean ridges on the middle oceans are reported to be expanding eg the Great Atlantic ridge is said to be expanding and the Atlantic ocean’s enlarging

Expansion of the E. African Rift valley and the Jordan Valley.

The E. African rift valley is expanding proving that continents are in the state of motion as a result of compressional and tensional forces within the earth’s crust. About 1000 million years the rift valley was not there. It widens at a rate of about 2 – 3 cm/ year.

Similarity of oil beds.

It has been found out that the oil beds of Brazil are the same as those of Angola . This has been an indicator that Angola and Brazil the must have been one geological unit with common oil beds that drifted apart from each other carrying along with it once shared oil beds among others

Criticisms of evidence of continental drift.

Some researchers have given different views discrediting the contribution of Wegner and people who contributed to the theories of continental drift. They argue that:

1. The spreading of ocean floors might be because of continuous erosion of the mainland, which makes the oceans appear as if they are expanding.

2. The similarity in the rocks might because of the same climatic conditions eg Equatorial climatic for the case of Brazil and Congo.

3. The presence of coral rock on different continents and the existence of identical vegetation on different continents might be because f the increasing world temperatures.

4. The presence of glacial tillites material on the Congo Basin and the Karoon basin might be because there existed very high mountains on those regions and they were covered by snow. But with time, snow was eroded down those mountains through glaciation leaving behind glacial deposits.

5. The above criticisms show that there are some weaknesses on theories of Wegner. However some are real and therefore the theory should not be under estimated.

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