Evidence has been found that a subduction event, the driver of plate tectonics, occurred exceptionally early in Earth’s formation, contradicting existing models. To explain how this occurred, planetary scientists are pointing the finger at mantle plumes set off by Earth’s collision with Theia, the Mars-sized object whose impact created the Moon.
Earth is the only planet with plate tectonics – and without the dance of the continents we wouldn’t be here, nor any other species capable of understanding the planet. Consequently, the possibility that plate tectonics might be very rare is considered one possible explanation for the Fermi Paradox.
The idea that Earth having plate tectonics is somehow linked to the Moon has been suggested many times, with different explanations proposed for the connection. Most such claims are light on evidence, but 4.3 billion-year-old crystals from Western Australia might be the missing clue.
The crystals are zircons, a type of rock prized by geologists for its resilience and internal clock. Zircons do not allow lead inside during their formation, but do incorporate uranium and thorium. These decay to lead on well-understood schedules, so the ratio of elements inside reveals a zircon’s age.
For more than 20 years, some scientists have argued the chemistry of certain very old zircons indicates they were produced during subduction events, where a tectonic plate pushes or pulls another down into the mantle.
However, plate tectonics today is driven in part by dense oceanic crust sinking into the mantle, something that should have been possible so early in Earth’s existence. A team of researchers has now offered an explanation. Their modeling suggests that when Theia smashed into Earth 4.51 billion years ago, the heat produced would have lasted a very long time, raising the temperature of the core-mantle boundary long after the magma ocean caused by the impact solidified. The extra heat at the core-mantle boundary came not just from the gravitational potential energy supplied by the impact, but from the decay of long half-life radioactive elements Theia carried.
This in turn would have provided the conditions for strong mantle plumes, where enormous hot globules rose from the boundary, weakening the crust and upper mantle. In a variety of scenarios, the models the study authors used consistently show that around 120 million years after the impact, over-heated mantle blobs make their way to near the surface, initiating subduction.
Changing certain assumptions alters the timeline somewhat, but as long as temperatures exceed 3,773 K (6,332 °F) at the core-mantle boundary for an extensive period, and pressures near the surface are not too great, subduction occurs.
“The giant impact is not only the reason for our moon, if that’s the case, it also set the initial conditions of our Earth,” study co-author Dr Qian Yuan told The Washington Post.
Even if the idea is correct, it’s not clear whether modern plate tectonics can trace its origins to the Theia impact, or if an initial burst fizzled out, before restarting from other causes.
However, if it is the case that Theia’s massive crash was the initiator for the movements we see today, then it may set another requirement for a planet to support advanced life. The recycling of crustal material through the mantle has acted as Earth’s thermal control before life developed the same capacity, moderating temperatures extremes of hot and cold.
If only a massive collision at just the right time can create such conditions, then planets that are Earthlike in the most important ways could be hundreds of times rarer than we think. In this case, civilizations may be correspondingly more unlikely and precious.
The study is published open access in the journal Geophysical Research Letters.
Source Link: Hot Blobs In Earth’s Mantle From Moon-Forming Impact Possibly Started Plate Tectonics