The earth is the only planet we know of with continents, huge tracts of land that house humans and most Earth biomass. However, we still don’t have definite answers to some basic questions about continents: how did they form, and why did they form? One theory is that they were formed by small planet crashed into the Earth’s crust long ago. This idea has been suggested many times, but so far there has been little evidence to support it.
In the new study published in the journal Nature, we studied ancient minerals from the West Australia and found clues that the giant-impact hypothesis might be true.
How do you create a continent?
The continents make up part of the lithosphere, the hard outer rocky crust of the Earth made up of ocean floors and continents, of which the top layer is the crust.
The crust beneath the ocean is thin and made up of dark, dense basalt that contains only a small amount of silica. In contrast, continental crust is thick and mainly composed of granite, a less dense, pale-colored, silica-rich rock that makes the continents “float”.
Beneath the lithosphere is a thick, slow-flowing quasi-molten rock near the top of the mantle, the layer of Earth between the crust and core.
If part of the lithosphere is removed, the mantle beneath it will melt as pressure from above is released. And the impact from the giant meteorite – magnetic rock space tens or hundreds of kilometers – is an extremely efficient way to do exactly that!
What are the consequences of a massive impact?
Colossal impacts blast out huge masses of matter almost instantaneously. Close rock face would melt hundreds of kilometers or more around the impact site. The impact also released pressure on the mantle below, causing it to melt and create a “blob-like” mass of thick basalt crust.
This mass is known as the oceanic plateau, similar to the one below present-day Hawaii or Iceland. The process is similar to what would happen if you were hit in the head by a golf ball or Pebble – the resulting tumor or “egg” resembles an ocean plateau.
Our study suggests that these oceanic plateaus may have evolved to form continents through a process known as crustal differentiation. The thick oceanic plateau that was formed from the impact could have been hot enough at its bottom, and it also melted, creating the type of granite that formed floating continental crust.
Are there other ways to create plateaus on the ocean floor?
There are other ways that oceanic plateaus can form. Thick crust beneath Hawaii and Iceland formed not through colossal impacts but “mantle plumes”, streams of hot matter rising from the edge of the Earth’s metallic core, as in lava lamp. When this plume reaches the lithosphere, it triggers a massive melting of the mantle to form the oceanic plateau.
So maybe plumes created the continents? Based on our studies, and the balance of different oxygen isotopes in the small grains of the mineral zircon, which are usually found in very small quantities in rocks from the continental crust, we do not think so.
Zircon is the oldest known mantle material, and it can remain intact for billions of years. We can also determine fairly precisely when it was formed, based on the decay of the radioactive uranium it contains.
Furthermore, we can learn about the environment in which zircon forms by measuring the relative proportions of the oxygen isotopes it contains.
We looked at zircons from one Among the oldest surviving pieces of continental crust in the world, the Pilbara Craton in Western Australia, began to form more than 3 billion years ago. Many of the oldest grains of zircon contain more light oxygen isotopes, showing shallower melting, but younger grains contain more mantle-like equilibrium isotopes, showing much deeper melting. .
This “top-down” model of oxygen isotope is what you might expect after a giant meteorite impact. In contrast, in mantle coatings, melting is a “bottom-up” process.
Sounds reasonable, but is there any other proof?
Yes, yes! The zircons from the Pilbara Craton appear to have formed in several distinct periods, rather than a continuum over time.
With the exception of the earliest grains, other grains with light isotope zircon are about the same age as the grain beds in the Pilbara Craton and elsewhere.
The spherical cushion is the deposition of “splash” material droplets due to meteorite impacts. The fact that the zircons are of the same age suggests that they may have been formed by the same events.
Furthermore, the “top-down” isotope pattern can be recognized in other regions of the ancient continental crust, such as in Canada and Greenland. However, the data from elsewhere has not yet been filtered as carefully as the Pilbara data, so it will take more work to confirm this model.
The next step in our research is to reanalyze these ancient rocks from elsewhere to confirm what we suspect – that the continents evolved at the sites of the giants. meteorite fragment impacts. Boom.
