Scientists have claimed that hard rocks can have ‘long memories’, preserving evidence of various geological events that occurred millions of years ago, piecing together the evolution of earth’s crust.
Most rocks are robbed off their history if exposed to extreme geological conditions, but there are rare cases where particular rocks derived from the earth’s lower crust have preserved, in their distinctive mineralogy, evidence of very high temperatures at depth, a report published in leading international journal ‘Geology’ said.
The report on ‘Sensitive high-resolution ion U-Pb dating of pro-grade and retrograde ultrahigh-temperature metamorphism as exemplified by Sri Lankan granulates’ was prepared by scientists Sanjeev Krishnan of the Indian Institute of Science, I S Williams from Research School of Earth Sciences at Canberra (Australia) and Osanai Y of Division of Earth Environments in Japan’s Kyushu University.
The rocks of the Central Highland Complex in Sri Lanka, parts of Antarctica and southern India were subject to some of the highest temperatures of crystal metamorphism at over 1100 deg C, they said.
At such temperatures, most rocks will turn into molten magma but recently it was discovered that rocks from near Kandy (Sri Lanka) not only survived such temperatures, but also has crystals of zircon in which a uranium-lead isotopic record of their provenance and thermal history were found.
The sediments were heated to over 1100 deg C at a depth of about 25 km below earth surface about 570 million years ago, and then rapidly lifted towards the surface, while still hot, about 550 million years ago, they said.
The Sri Lankan rocks were probably trapped in the violent collision between the two halves of the Gondwana supercontinent about 600 million years ago, superheated by basalt magmas rising from the earth’s interior, then forced to the near surface again as the tectonic pressures relaxed.
The preservation of the isotopic record of these events is remarkable and remains to be fully explained.
One of the best ways to understand the geological history of our 4,500 million-year-old planet is to study rocks formed under a wide variety of geological conditions, Mr. Krishnan said, adding, geologists can identify the rocks that can best preserve evidence of past geological events.
The scientists said the reconstruction of the continents that existed in the past is an important part of understanding the dynamic evolution of earth.
The ancient supercontinent of Gondwana once consisted of what are now the smaller continents of South America, Africa, Madagascar, Southern India, Sri Lanka, Antarctica and Australia.
