South Atlantic Tectonic Plates: Angola-Brazil Spreading Timeline

Angola and Brazil have been tectonically spreading for millions of years, ever since the supercontinent Gondwana, which included the present-day western coast of Africa and the eastern coast of South America, broke off from the larger landmass of Pangea. This split, which began around 140 million years ago, caused gashes in the Earth's crust, known as rifts, to form along pre-existing weaknesses. As the tectonic plates beneath South America and Africa moved apart, magma from the Earth's mantle rose to the surface, creating a new oceanic crust and pushing the two continents away from each other. This eventually led to the formation of the South Atlantic Ocean, which now separates Angola and Brazil.

The prehistoric split of South America and Africa

The western coast of Africa and the eastern coast of South America were once nestled together as part of the supercontinent Gondwana, which in turn was part of the larger landmass of Pangea. Africa and South America began to split around 140 million years ago, causing gashes in the Earth's crust, known as rifts, to open up along pre-existing weaknesses. This process, known as continental drift, saw the formation of the South Atlantic Ocean, which now separates the two continents.

Evidence from Angola

A research team from Southern Methodist University (SMU) has found evidence in Angola that clearly illustrates this prehistoric split. The southern coast of Angola provides a complete geological record of the two continents moving apart. The rocks and fossils found in this region date back from 130 million years ago to 71 million years ago. According to Louis L. Jacobs, an SMU professor emeritus of Earth Sciences, "it's all laid out in one place". The fossils discovered in Angola include those of large marine reptiles that lived during the Cretaceous Period, right after the Atlantic Ocean was formed.

Tectonic Movement

As the tectonic plates beneath South America and Africa moved apart, magma from the Earth's mantle rose to the surface, creating a new oceanic crust and pushing the continents away from each other. This movement of tectonic plates, known as plate tectonics, resulted in the formation of new landmasses and the opening of the South Atlantic Ocean. The process of continental drift has been described as "a grand, slow-motion bumper-car game", with ridges rising between the plates and new earth appearing as the plates spread apart.

Historical Connections

The connection between Brazil and Angola is not just geological but also historical. Both countries were once united as part of the Portuguese Empire, with Angola being a major source of slaves for Brazil during the Transatlantic Slave Trade. Additionally, the geology of Brazil includes ancient craton basement rock from the Precambrian period, which was joined with Africa. This further highlights the prehistoric connection between the two continents, even before the formation of the supercontinents Gondwana and Pangea.

The formation of the South Atlantic Ocean

The South Atlantic Ocean was formed as a result of the separation of South America and Africa, which began around 140 million years ago. This separation was part of the final major phase of the break-up of the supercontinent Pangaea, which had three major phases of separation. The first phase began in the Early-Middle Jurassic between North America and Africa, and the second phase began in the Early Cretaceous, during which the South Atlantic Ocean was formed.

The separation of South America and Africa caused gashes in the Earth's crust, known as rifts, to open up along pre-existing weaknesses. As the tectonic plates beneath the two landmasses moved apart, magma from the Earth's mantle rose to the surface, creating a new oceanic crust and pushing the continents away from each other. This process, known as seafloor spreading, led to the formation of the Mid-Atlantic Ridge, a submarine mountain range that divides the Atlantic Ocean into two halves. The Mid-Atlantic Ridge rises 2-3 km above the surrounding ocean floor and its rift valley is the divergent boundary between the South American and African plates in the South Atlantic.

The opening of the South Atlantic Ocean has been studied by scientists through geophysics and well cores drilled through the ocean floor. However, the most complete geological record of this event has been found on the southern coast of Angola, where ancient rocks and fossils from long-extinct marine reptiles clearly show the splitting of the two continents and the subsequent formation of the ocean. By studying the rock formations and fossils found in eight different locations along the coast, researchers have been able to walk through each phase of this significant chapter in Earth's history.

The supercontinent Gondwana

Gondwana was made up of landmasses that we recognise today as Africa, South America, Australia, Antarctica, the Indian subcontinent, and the Arabian Peninsula. The continent was named by Austrian scientist Eduard Suess, after the region in central India.

The first stage of Gondwana's break-up began in the Early Jurassic Period, around 180 million years ago, with the western half (Africa and South America) separating from the eastern half (Madagascar, India, Australia, and Antarctica). The South Atlantic Ocean opened around 140 million years ago as Africa and South America continued to move apart.

The exact mechanisms behind Gondwana's split are still unknown. Some theorists believe that "hot spots", where magma is very close to the surface, caused the break-up. However, in 2008, researchers from the University of London suggested that Gondwana split into two tectonic plates, which then separated.

Tectonic plates and their movement

The Earth's lithosphere, the rigid outer shell of the planet, is divided into seven or eight major plates and many minor plates or "platelets". These tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own type of crust. The plates are relatively rigid and float across the ductile asthenosphere beneath. The relative movement of the plates typically ranges from zero to 10 cm annually.

The movement of tectonic plates is believed to have begun around 3 to 3.8 billion years ago. The driving forces behind this movement are convection currents in the Earth's mantle, the excess density of the oceanic lithosphere sinking in subduction zones, and gravitational sliding away from a spreading ridge. The plates move apart at seafloor spreading ridges, where new oceanic crust is formed, and converge at subduction zones, where one plate moves beneath the other.

The four main types of interactions between tectonic plates are:

• Collision: When two plates crash into each other and fold up.
• Subduction: When one plate sinks beneath another.
• Spreading: When two plates move apart from each other.
• Transform faulting: When two plates slide horizontally past each other.

The movement of tectonic plates results in the formation of mountains, volcanoes, oceanic trenches, and earthquakes. It also plays a role in the creation of oceans and continents. The presence of the same fossils on now-separated continents is evidence of their past connection and subsequent movement.

Regarding the specific case of Angola and Brazil, research has shown that the two landmasses were once part of the supercontinent Gondwana, which broke off from the larger landmass of Pangea. The southern coast of Angola provides evidence of the splitting of South America and Africa and the formation of the South Atlantic Ocean around 140 million years ago.

The discovery of fossils in Angola

The research team, led by Southern Methodist University, focused their efforts on the southern coast of Angola, where they unearthed ancient rocks and fossils dating back from 130 million to 71 million years ago. These fossils, including those of long-extinct marine reptiles, provided clear evidence of the two continents moving apart and the formation of the South Atlantic Ocean.

One notable discovery during their fieldwork was the fossil of a large sauropod dinosaur, Angolatitan adamastor, found near Iembe in the province of Bengo. This discovery, made by Octávio Mateus in May 2006, marked the first non-avian dinosaur found in Angola. The specimen, stored in the Museu de Geologia of the Universidade Agostinho Neto in Luanda, includes a partial right forelimb with a shoulder blade, upper arm bone, ulna, radius, and three metacarpals.

Another significant finding was the fossil of a turtle, Angolachelys mbaxi, and mosasaurs, Angolasaurus bocagei and Tylosaurus iembeensis. These fossils, along with those of plesiosaurs, provided further evidence of the marine environment that existed during the Cretaceous Period.

The fossils discovered in Angola filled in crucial gaps in our understanding of Earth's geological history. As Louis L. Jacobs, the lead author of the study, noted, "There are places that you can go to in South America, for instance, where you can see this part of the split or that part of it, but in Angola, it's all laid out in one place." The discovery highlighted the significance of Angola in unraveling the complex story of continental drift and the formation of our planet's oceans.

Overall, the discovery of fossils in Angola has provided invaluable insights into the prehistoric split between continents and the formation of the South Atlantic Ocean, contributing to our understanding of Earth's geological past.

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