The thick oceanic shelf formed by the impact can become hot enough at its base to also melt, producing the type of granitic rock that forms floating continental crust.
Earth is the only planet we know of with continents, the giant landmasses that are home to humanity and most of the Earth’s biomass.
However, we still don’t have definitive answers to some fundamental questions about continents: how did they arise and why did they form where they formed?
One theory is that they were formed by giant meteorites that crashed into the earth’s crust a long time ago. This idea has been proposed many times, but until now there has been little evidence to support it.
In new research published in Nature, we studied ancient minerals from Western Australia and found tantalizing clues to suggest the giant impact hypothesis might be right.
How do you make a continent?
Continents are part of the lithosphere, the rigid outer rocky envelope of the Earth made up of the ocean floors and continents, the top layer of which is the crust.
The crust beneath the oceans is thin and made up of dark, dense basalt rocks that contain only a little silica. On the other hand, the continental crust is thick and mainly made up of granite, a less dense rock, pale in color and rich in silica which makes the continents “float”.
Beneath the lithosphere is a thick, slow-moving mass of nearly molten rock, which sits near the top of the mantle, the layer of Earth between the crust and the core.
If part of the lithosphere is removed, the mantle below will melt when the pressure from above is released. And the impacts of giant meteorites – rocks from space tens or hundreds of miles in diameter – are an extremely effective way to do just that!
What are the consequences of a giant impact?
Giant impacts blast huge volumes of material almost instantaneously. Rocks near the surface will melt for hundreds of miles or more around the impact site. The impact also releases pressure on the mantle below, causing it to melt and produce a “teardrop-shaped” mass of thick basalt crust.
This mass is called an ocean shelf, similar to the one underlying Hawaii or Iceland today. The process is much like what happens if you are hit hard in the head by a golf ball or a pebble – the resulting bump or “egg” is like the ocean shelf.
Our research shows that these ocean shelves could have evolved to form continents through a process known as crustal differentiation. The thick oceanic shelf formed by the impact can become hot enough at its base to also melt, producing the type of granitic rock that forms floating continental crust.
Are there other ways to make ocean trays?
Ocean shelves can form in other ways. The thick crusts beneath Hawaii and Iceland were formed not by giant impacts but by “mantle plumes”, streams of hot material rising from the edge of Earth’s metallic core, much like in a lamp to lava. When this rising plume reaches the lithosphere, it triggers massive melting of the mantle to form an ocean shelf.
Could the plumes have created the continents? Based on our studies and the balance of different oxygen isotopes in the tiny grains of the mineral zircon, which are commonly found in small amounts in rocks of continental crust, we don’t think so.
Zircon is the oldest known crustal material, and it can survive intact for billions of years. You can also determine quite precisely when it was formed, based on the decay of the radioactive uranium it contains.
In addition, one can know the environment in which the zircon was formed by measuring the relative proportion of oxygen isotopes it contains.
We examined zircon grains from one of the world’s oldest pieces of continental crust, the Pilbara Craton in Western Australia, which began forming over 3 billion years ago. Many of the older zircon grains contained more light oxygen isotopes, indicating shallow melting, but younger grains contain more mantle-like equilibrium isotopes, indicating much deeper melting. deeper.
This “falling down” pattern of oxygen isotopes is what you might expect after a giant meteorite impact. In mantle plumes, on the other hand, melting is a “bottom-up” process.
Sounds reasonable, but is there any other evidence?
Yes there is! Zircons in the Pilbara craton appear to have formed in a handful of distinct periods, rather than continuously over time.
Except for the oldest grains, the other grains containing isotopically light zircon are the same age as the spherule beds in the Pilbara Craton and elsewhere.
The beds of spherules are deposits of droplets of material “splattered” by the impacts of meteorites. The fact that the zircons are the same age suggests that they may have been formed by the same events.
Additionally, the “top-down” pattern of isotopes can be recognized in other regions of ancient continental crust, such as in Canada and Greenland. However, data from elsewhere has not yet been carefully filtered like the Pilbara data, so more work will be needed to confirm this pattern.
The next step in our research is to re-analyze these ancient rocks from elsewhere to confirm what we suspect – that continents developed on the sites of giant meteorite impacts. Boom.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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