I. CONTINENTAL DRIFT

What?

• The Theory of Continental Drift is an early attempt to understand the processes controlling the origin, shapes, locations, and features of continents.
• In 1908, Taylor proposed the existence of two continents: Northern Angaraland and Southern Gondwanaland.
• Alfred Wegener, a meteorologist, aimed to reconstruct Earth’s climatic history and find indirect climatic evidence such as fossil types, geomorphological evidence, and features suggesting past climate.
• Wegener observed matching coastlines of South America and Africa, which had been previously noted by Francis Bacon, Benjamin Franklin, and Snieder.
• Snieder also observed similarities between the coal fields of western Europe and North America, including age, composition, and type of coal, as well as the distribution of Mesozoic reptiles.

Postulates of Wegener’s Continental Drift Theory:

• The supercontinent Pangaea existed during the late Paleozoic – Carboniferous times.
• Pangaea consisted of all continents surrounded by a super-ocean called Panthalassa.
• Wegener accepted Edward Suez’s idea that continents were SIALic blocks floating over SIMAtic oceans.
• Pangaea was located in the Southern Hemisphere during the mid-Mesozoic times before breaking up and drifting apart.
• All processes and features can be explained by Wegener’s theory.

Explaining the features of Wegener’s Theory:

• Fold Mountains: Wegener challenged Herald Jeffrey’s theory of the permanency of oceans and continents, explaining that as the continents’ SIALic blocks drifted north and westwards, they formed fold mountains.
• Islands: According to Wegener, as continental SIALic blocks moved eastwards, the trailing edge of the continents scraped, leaving behind islands such as Madagascar, Tasmania, New Zealand, Japanese Islands, South East Asian Islands, Caribbean Islands, and Cuba.
• Earthquakes: As the SIAL moves over SIMA, the hard rigid SIAL scrapes, cuts, and ruptures, leading to earthquakes.

DISPLACEMENT HYPOTHESIS

Who?

• Alfred Wegener proposed the displacement hypothesis during the Carboniferous Period.

Based on Postulation:

• If the climate zones remained stationary, landmasses would have displaced and drifted.

Purpose:

• The displacement hypothesis aimed to explain the origin of mountains, island arcs, festoons, continents, and ocean basins.

What?

• According to Wegener, the Earth consists of a three-layer system: SIAL, SIMA, and Nife.
• All landmasses were once united as one landmass called Lauratia (North America, Europe, and Asia) and Gondwanaland (South America, South Africa, Madagascar, Peninsular India, Australia, and Antarctica), forming the supercontinent Pangaea.
• Pangaea was surrounded by a primitive ocean called Panthalassa.

Evidences in support of the Theory:

• Matching Coastline and Continental Fit Evidence:

1. Fit Evidence: First observed by Benjamin Franklin, Francis Bacon, and Snieder, the coasts of the Atlantic Ocean fit together like a jigsaw puzzle.
   • Examples include Cape of Sao Roque of Brazil and the Plateau of Borborema fitting into the Gulf of Guinea coast in North America.
2. Structural Match Evidence: Wegener proved that not only the shape but also the role, type, and composition of structures match.
   • Edward Bullard confirmed Wegener’s jigsaw fit evidence by studying shape fit and structure match.

• Fossil Evidences:
  • Similar fossil and vegetation remains were found on the eastern coast of South America and the western coast of Africa.
  • Distribution of glossopteris flora in India, South Africa, Australia, Antarctica, and the Falkland Islands.

• Paleo-Climatic Evidence:
  • Tropical areas showed glacial evidence in places like Talchar (Orissa) and Durban (South Africa).
  • Temperate areas had extensive coal resources like the Ruhr Valley and Appalachian mountain range.
  • Glacial evidences in tropical regions matched the age of coal fields in mid-latitudes.

Driving Mechanism of Continental Drift:

• Wegener’s initially flawed mechanism was later revised.
• Three possible forces were suggested: imbalance between the center of gravity and center of buoyancy, relative motion due to Earth’s rotation, and gravitational pull of the moon and sun.
• Herald Jeffery disproved the three forces, weakening Wegener’s explanation.
• Wegener also mentioned the pole-fleeing force that drives continents northwards away from the southern poles.

Evaluation of the Theory:

• Criticism: Gravitational and tidal forces were deemed insufficient to cause the drift of continents. Contrasting views on the resistance offered by SIMA against SIALic continents and the lack of complete fit between both coasts of the Atlantic Ocean were raised. The direction of displacement and chronological sequence were not elaborated.
• Arguments in favor: The horizontal displacement of continents and the subsequent development of the Plate Tectonic Theory owe their postulation to Wegener’s Continental Drift Theory.

Conclusions:

• Even if all the details of Wegener’s theory are incorrect, the postulation of the Plate Tectonic Theory resulted from the Continental Drift Theory proposed by Wegener.

II. Plate Tectonics

Plate tectonics is a scientific theory that explains the movement and interaction of lithospheric plates, which make up the Earth’s surface. It is based on several key developments and observations.

Continental Drift and Reconsideration

The development towards the tectonic theory began with the idea of continental drift proposed by Alfred Wegener in the early 20th century. However, Wegener’s hypothesis was initially discarded due to a lack of supporting evidence. It was not until the 1950s and 1960s that continental drift was reconsidered and gained acceptance.

Convection Current Hypothesis

One significant contribution to the development of plate tectonics was made by Arthur Holmes, who proposed the convection current hypothesis. According to this hypothesis, thermal convective currents within the Earth’s mantle are responsible for the movement of tectonic plates. As the material in the mantle heats up, it becomes less viscous and rises, while cooler material sinks, creating a cyclical motion that drives the movement of plates.

Supporting Observations and Concepts

New observations and concepts further supported the theory of plate tectonics. One important observation was the discovery of sea floor spreading, which provided evidence for the continuous creation of new oceanic crust. This process occurs at mid-oceanic ridges, where molten lava wells up and solidifies, forming new crust. This observation was supported by the identification of alternating magnetic anomalies on either side of mid-oceanic ridges, indicating periods of normal and reverse magnetism in the Earth’s history.

Definition of Plate Tectonics

Based on these developments, plate tectonics is defined as the movement and interaction of rigid lithospheric plates or solid crustal layers. There are six major plates: Eurasian plate, Indian-Australian plate, American plate, Pacific plate, African plate, and Antarctic plate, along with several minor plates. The collective movement of these plates is referred to as plate tectonics.

Plate Margins and Their Types

Observations have revealed that the ocean floor is not flat and featureless but rather exhibits various topographic features. Tectonic activities, such as earthquakes and volcanic eruptions, predominantly occur along plate margins. One prominent feature associated with plate margins is the mid-oceanic ridge, an extensive underwater mountain system. These ridges are formed at constructive or divergent plate boundaries, where plates move apart, allowing molten lava to well up and create new crust.

Plate margins can be categorized into three types: constructive/divergent/accreting boundaries, destructive/convergent boundaries, and conservative/shear/transform boundaries.

1. Constructive/Divergent/Accreting Boundaries
   Constructive/divergent/accreting boundaries represent zones of divergence, where two plates move away from each other. This movement leads to a continuous upwelling of molten lava, forming new oceanic crust. Examples of constructive boundaries include the Mid-Atlantic Ridge and the East Pacific Rise.
2. Destructive/Convergent Boundaries
   Destructive/convergent boundaries involve two plates colliding. One plate overrides the other, and the overridden plate is subducted and lost into the mantle. This process occurs at subduction zones and leads to the formation of features such as trenches, volcanic arcs, and folded mountains. The collision of the Indian-Australian plate with the Eurasian plate, resulting in the formation of the Himalayas, is an example of a destructive boundary.
3. Conservative/Shear/Transform Boundaries
   Conservative/shear/transform boundaries occur when two plates slide past one another horizontally along transform faults. Unlike the other types of boundaries, no new crust is created or destroyed at these boundaries. The San Andreas Fault in California is a well-known example of a transform boundary.

Plate Motion and Causes

Plate motion refers to the constant movement of lithospheric plates with respect to each other. The causes of plate motion are attributed to thermal convective currents inside the Earth’s mantle, driven by high temperatures and pressure conditions. The high temperature decreases the viscosity of the material, leading to an upward flow of hot and liquid matter that splits below the crust and diverges in opposite directions, resulting in plate movements.

Additionally, the high gravity force of lava and magma on either side of mid-oceanic ridges, along with the intrusion of magma in these ridges, can cause the separation and displacement of oceanic plates. The exact cause of plate motion is still a subject of ongoing scientific research, as there is no universally accepted explanation.

Applications of Plate Tectonics

The concept of plate tectonics has various applications in understanding Earth’s processes. It has provided a framework to explain the reality of continental drift and has shed light on the formation of folded mountains, volcanic activity, earthquakes, and tsunamis.

Formation of Folded Mountains
Plate tectonics has helped explain the origin of folded mountains by demonstrating how convergent plate boundaries lead to the collision and uplift of crustal rocks.

Volcanic Activity
Volcanic activity is closely associated with plate boundaries, with about 15% of the world’s active volcanoes occurring at constructive plate margins and 80% at destructive plate boundaries. The intensity of volcanic activity is related to plate boundaries, as convergent boundaries often experience more explosive eruptions.

Earthquakes
Earthquakes are another consequence of plate tectonics. Constructive plate boundaries exhibit moderate earthquakes due to the slow rate of rupture, upwelling of lavas, and plate movements. Destructive plate boundaries, on the other hand, are associated with high magnitude earthquakes, such as those found in the circum-Pacific region known as the “Ring of Fire.” Different types of faults, such as normal faults, reverse faults, and strike-slip faults, are linked to plate tectonics.

Tsunamis
Tsunamis, which are destructive oceanic waves, can be triggered by high magnitude earthquakes, volcanic eruptions, landslides, and submergence of continental shelves and slopes. Plate tectonics provides an understanding of the processes that can lead to tsunamis.

Sea Floor Spreading

Sea floor spreading is a concept within plate tectonics, first proposed by Harry Hess. It explains the continuous creation of new oceanic crust at mid-oceanic ridges and the destruction of crust at oceanic trenches. As molten lavas cool down and solidify along the ridge crests, they form alternating bands or strips of magnetic anomalies parallel to the ridges. These magnetic anomalies provide evidence for the continuous spreading of the sea floor. The rate of sea floor spreading can be calculated using isochrons, which are lines connecting points of equal ages of magnetic strips. By measuring the age of isochrons and the distance between them, scientists can estimate the rate of sea floor spreading.

In summary, plate tectonics is a comprehensive theory that explains the movement of lithospheric plates and various geological phenomena. It is supported by observations such as sea floor spreading, volcanic activity, earthquakes, and mountain building. Plate boundaries play a crucial role in these processes, and the understanding of plate tectonics has revolutionized our knowledge of Earth’s dynamic nature.