On 2 February 2017, Time magazne reported on the findings of a study published in the journal Nature Communications that described how, by their telling, a lost continent was found in the Indian Ocean:
You’d think it would be hard to misplace an entire continent, what with the mountains and trees and all that other hard-to-miss stuff. Now, however, it seems that one of Earth’s continents indeed went missing. The good news is, it’s at last been found, lying below the waters of the Indian Ocean, beneath the tiny, 790 sq. mi. (2,040 sq. km) island of Mauritius.
This description, though generally accurate, is a sensationalized wording of more specific geologic discovery, which confirms a 2013 suggestion that a 2.5 billion-year-old fragment of continental crust (but not a continent itself, strictly speaking), through eons of tectonic shifts, found itself buried under the much younger volcanic rocks that make up the island of Mauritius:
A fragment of continental crust was suggested to underlie the young plume-related lavas of the Indian Ocean island of Mauritius, on the basis of gravity inversion modelling (crustal thickness) and the recovery of Proterozoic (660–1,971 Ma) zircons from basaltic beach sands.
Gravity inversion data allows scientists to estimate the thickness of Earth’s crust by mapping extremely slight local variations in the amount of gravity present in a given area, as described in a NASA Earth observatory feature about a NASA mission producing this kind of data:
Gravity anomaly maps show how much the Earth’s actual gravity field differs from the gravity field of a uniform, featureless Earth surface. The anomalies highlight variations in the strength of the gravitational force over the surface of the Earth. Gravity anomalies are often due to unusual concentrations of mass in a region. For example, the presence of mountain ranges will usually cause the gravitational force to be more than it would be on a featureless planet — positive gravity anomaly. Conversely, the presence of ocean trenches or even the depression of the landmass that was caused by the presence of glaciers millennia ago can cause negative gravity anomalies.
A zircon is a mineral prized by geologists both because they are extremely durable and because they contain trace amounts of radioactive thorium and uranium, which allow for radiometric dating over timescales longer than the age of Earth itself. Evidence of the earliest crust on Earth once came from a single grain of this mineral.
This authors of the 2017 study were able to confirm hints of this large mass of old continental crust that had already provided by those two bits of evidence. They added to this evidence by finding and dating a number of a specific kind of zircon grains called a xenocrysts in unambiguously younger volcanic rocks. Xenocrysts are relatively older grains of an ancient rock that have been incorporated (in this case by a lava flow) into a younger rock:
Here we document the first U–Pb zircon ages recovered directly from 5.7 [million year old] Mauritian trachytic rocks [rocks from lava flows that clearly indicate the direction of flow]. We identified concordant Archaean xenocrystic zircons ranging in age between 2.5 and 3.0 [billion years old] within a trachyte plug that crosscuts Older Series plume-related basalts of Mauritius.
Our results demonstrate the existence of ancient continental crust beneath Mauritius; based on the entire spectrum of U–Pb ages for old Mauritian zircons, we demonstrate that this ancient crust is of central-east Madagascar affinity, which is presently located ~700 km west of Mauritius.
The researchers, in other words, were able to identify and date extremely old zircon crystals that unambiguously were exhumed by the action of molten rock passing through ancient crust on its journey to become the island nation of Mauritius. This, they argue, makes the clearest case yet for the existence of this tectonic fragment of Earth’s deep past.
While the Time piece’s headline is suggestive of the discovery of some recently inhabited Atlantis-like lost human world, the reality is starkly different. The last time the crust that contained these zircons was exposed to the surface, around 200 million years ago, the earliest mammals had only just evolved, and flowering plants were still 40 to 80 million years away from existing.