Vaalbara
Encyclopedia
Vaalbara is theorized to be Earth's first supercontinent
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...

, beginning its formation about , completing its formation by about and breaking up by . The name Vaalbara is derived from the South African Kaapvaal craton
Kaapvaal craton
The Kaapvaal craton , along with the Pilbara craton of Western Australia, are the only remaining areas of pristine 3.6-2.5 Ga crust on Earth...

 and the West Australian Pilbara craton
Pilbara craton
The Pilbara craton , along with the Kaapvaal craton are the only remaining areas of pristine Archaean 3.6-2.7 Ga crust on Earth...

. These craton
Craton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...

s were combined during the time of the Vaalbara supercontinent.

Identical radiometric ages of have been obtained for the ejecta
Ejecta
Ejecta can mean:*In volcanology, particles that came out of a volcanic vent, traveled through the air or under water, and fell back on the ground surface or on the ocean floor...

 from the oldest impact event
Impact event
An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...

s in each craton. Remarkably similar structural sequences between these two cratons have been noted for the period between 3,500 to 2,700 million years ago.

Paleomagnetic
Paleomagnetism
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...

 data from two ultramafic complexes in the cratons showed that at the two cratons could have been part of the same supercontinent. The reconstructed apparent polar wander
Apparent polar wander
Apparent polar wander is the imaginary movement of the Earth's magnetic poles relative to a continent while regarding the continent being studied as fixed in position, as determined by paleomagnetic data...

 path for the two cratons shows marked similarities. Both the Pilbara and Kaapvaal cratons show extensional fault
Extensional fault
An extensional fault is a fault that vertically thins and horizontally extends portions of the Earth's crust and/or lithosphere. In most cases such a fault is also a normal fault, but may be rotated to have a shallower geometry normally associated with a thrust fault...

s which were active about during felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....

 volcanism and coeval with the impact layers.

Continental plates have periodically collided and assembled in geologic periods of orogenesis (mountain building) to form supercontinents. The cycle of supercontinent formation, breakup, dispersal and reformation by plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

 occurs every 450 million years or so.

Vaalbara

Vaalbara is the Earth's theorized first supercontinent
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...

. It was a complete supercontinent by . Vaalbara formed possibly beginning and was broken apart by about .

It is uncertain when Vaalbara began to break up; geochronological
Geochronology
Geochronology is the science of determining the age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent to the method used. A variety of dating methods are used by geologists to achieve this, and schemes of classification and terminology have been proposed...

 and palaeomagnetic evidence show that the two cratons had a rotational 30 ° latitudinal separation in the time period of 2.78 to 2.77 billion years ago, which indicates they were no longer joined after ~.

Vaalbara consisted of the eastern South African Kaapvaal craton joined with the Pilbara craton in northwest Western Australia. (The name Vaalbara is derived from the last four letters of each craton's name.)

Evidence for Vaalbara

South Africa's Kaapvaal craton and Western Australia's Pilbara craton are two of the best-preserved existing Archean cratons on earth; they have remarkably similar early Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...

 cover sequences. Kaapvaal's Barberton granite-greenstone
Greenstone belt
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies....

 terrane and Pilbara's eastern block show evidence of four large meteorite impacts between 3,200 to 3,500 million years ago. (Similar greenstone belts are now found at the margins of the Superior craton
Superior craton
The Superior craton forms the core of the Canadian Shield at the heart of the North American continent. It extends from Quebec in the east to eastern Manitoba in the west...

 of Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, and across the cratons of the former Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

 and Laurasia
Laurasia
In paleogeography, Laurasia was the northernmost of two supercontinents that formed part of the Pangaea supercontinent from approximately...

 continents.)

Identical single-zircon uranium-lead ages of have been obtained for the ejecta from the oldest impact events in each craton. The high temperatures created by the impact’s force fused sediments into small glassy spherules. Spherules of 3,500 million years old exist in South Africa and spherules of a similar age have been found in Western Australia, they are the oldest-known terrestrial impact products. The spherules resemble the glassy chondrule
Chondrule
Chondrules are round grains found in chondrites. Chondrules form as molten or partially molten droplets in space before being accreted to their parent asteroids...

s (rounded granules) in carbonaceous chondrites, which are found in carbon-rich meteorites and lunar soils. These deposits represent a global fallout layer which is associated with sedimentation
Sedimentation
Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...

 by an impact-generated tsunami
Tsunami
A tsunami is a series of water waves caused by the displacement of a large volume of a body of water, typically an ocean or a large lake...

.

Remarkably similar lithostratigraphic and chronostratigraphic structural sequences between these two cratons have been noted for the period between 3,500 to 2,700 million years ago. Paleomagnetic data from two ultramafic complexes in the cratons showed that at the two cratons could have been part of the same supercontinent. The reconstructed apparent polar wander path for the two cratons shows marked similarities and overlap to a large extent. Both the Pilbara and Kaapvaal cratons show extensional faults which were active about during felsic volcanism and coeval with the impact layers.

Supercontinent formation

Cratons are the masses of rock composing the basic, initial structure of continents; they have remained stable for billions of years throughout the process of the ocean crust being continually created and destroyed. Continental plates, containing the ancient cratons, have periodically collided and assembled in geologic periods of orogenesis (mountain building) to form supercontinents. A supercontinent is a landmass composed of more than one craton.

Mechanism for breakup of supercontinents

Supercontinents act as thermal lids, blocking the escape of Earth's internal heat, so the asthenosphere
Asthenosphere
The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...

 overheats. Eventually the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

 begins to dome upward, it cracks, magma wells upward and fragments of the supercontinent slide off the overswell. The east African Rift Valley is a modern-day example of this breaking apart. This cycle – the Wilson Cycle – of supercontinent formation, breakup, and dispersal, followed by convergence and patching, through plate tectonics occurs every 450 million years or so.

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