Earth’s continents formed when our planet was bombarded with giant meteorites about 3.5 billion years ago, a new study has found.
Researchers at Curtin University in Perth, Australia, analyzed crystals of the mineral zircon from the Pilbara Craton to investigate its origins.
A craton is a piece of the planet’s crust that has remained intact for billions of years while other pieces broke off and moved to form the continents.
Zircon composition revealed information about how the Pilbara Craton formed.
They suggest that the rocks first melted near the Earth’s surface before moving deeper. consistent with the geological effects of meteorite impacts.
Dr Tim Johnson, from Curtin’s School of Earth and Planetary Sciences, said: “Our research provides the first strong evidence that the processes that eventually formed the continents began with giant meteorite impacts, similar to responsible for the extinction of the dinosaurs, but that happened billions of years earlier.
Continents formed when Earth was bombarded with giant meteorites about 3.5 billion years ago, a new study from Curtin University, Australia has found (stock image)
The three-stage evolution of the Pilbara craton. Stage 1 (ad) – A giant meteorite disrupted the crust, reducing the pressure and causing the mantle to melt more and more into the Earth. Stage 2 (e) – Granitic magmas (TTG) begin to form at the base of the newly formed plate. Stage 3 (f) – TTGs melt to form granite and are pushed to the surface
WHAT IS A CRATON?
A craton is an old, stable part of the Earth’s crust that has survived the merging and splitting of continents and supercontinents for at least 500 million years.
Some are over 2 billion years old.
Cratons are generally found in the interior of continents and are made up of ancient igneous rocks such as granite.
They have a thick crust and deep roots that extend into the mantle below to a depth of 200 km.
Over the course of Earth’s 4.5 billion year history, land masses have broken up, separated, and rejoined.
This is the result of heat from radioactive processes inside the planet that cause these plates to move.
However, large and particularly strong parts of the crust have remained stable over time despite these processes, which are known as cratons.
A new study of the Pilbara craton, published yesterday in Nature, has provided evidence of how the structure formed and why it is so strong.
Geologists analyzed zircon crystals within the igneous rock of the craton, which had been dated to have formed between 3.6 billion and 2.9 billion years ago.
Johnson said: “The study of the oxygen isotope composition in these zircon crystals revealed a ‘top-down’ process that started with the melting of the rocks near the surface and progressed deeper, d ‘according to the geological effect of giant meteorite impacts’.
The three types of oxygen isotopes – different forms of the element – found within zircon indicate that the craton formed in three stages.
The first stage was a giant impact about 3.6 billion years ago that destroyed part of the planet’s crust and, as a result, reduced the pressure on the lower mantle.
The approximate locations of Archean cratons, which are over 2.5 billion years old and include the Pilbara Craton and surrounding Proterozoic regions of the world
A geological map of the Pilbara Craton in Western Australia. The rocks exposed here range from 2.5 billion to 3.5 billion years old, providing a unique and well-preserved window into Earth’s deep past.
This mantle then began to melt and seep upward through the crust to form what is known as an “oceanic plateau.”
The high temperatures at the base of the plateau give rise to the formation of granites, which are stable and of low density; the second stage.
In the final stage, the granites move up with the magma and add strength to the forming craton.
The oldest rocks were dated between 3.9 and 3.5 billion years old, which coincides with the late heavy bombardment.
This was when a large number of smaller asteroids, up to 40 km across, collided with the newly formed planets of the inner Solar System.
Images of zircon grains, showing targeted areas and ratio of oxygen-18 to oxygen-16 isotope in parentheses
The Pilbara Craton is an ancient, stable part of the Earth’s outer crust located in the Pilbara region of Western Australia (stock image)
Researchers say this is the strongest evidence yet for the decades-old theory that Earth’s continents were formed by giant meteorite impacts.
It undermines a competing theory that cratons formed as a result of ancient volcanic activity.
Dr Johnson said understanding the formation and ongoing evolution of the continents is crucial as humanity is heavily dependent on them.
They occupy most of the Earth’s biomass, all humans and important mineral deposits.
He added: “Not least, the continents are home to critical metals such as lithium, tin and nickel, commodities that are essential for the emerging green technologies needed to meet our obligation to mitigate climate change.
“These mineral deposits are the end result of a process known as crustal differentiation, which began with the formation of the first land masses, of which the Pilbara craton is just one of many.”
In future work, the team would like to see if the findings from this Pilbara craton study are applicable to other areas of the ancient continental crust.
Earth’s tectonic activity began more than 3.2 billion years ago, study finds
Earth’s tectonic plates began moving more than 3.2 billion years ago, just over 1.3 billion years after Earth formed and earlier than originally thought.
Earth’s outermost shell is divided into seven major tectonic plates and many smaller ones that move between 0.4 inches and 6.2 inches per year.
A team of geologists from Harvard University set out to find out how early in Earth’s history tectonic activity, that is, plate movement, began.
Rocks in Western Australia, one of the oldest pieces of the Earth’s crust, show evidence of drift of about an inch per year, starting 3.2 billion years ago.
Read more here
Study author Roger Fu poses at an outcrop of Honeyeater basalt in Western Australia’s Pilbara Craton. The ancient rocks exposed here showed the study authors that the Pilbara craton moved over the Earth’s surface about 3.2 billion years ago.
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