`Hard rocks preserve evidence of past geological events`

Most rocks are robbed off their history if exposed to extreme geological conditions.

Last Updated: Jan 02, 2011, 09:38 AM IST

Mumbai: 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 Sajeev Krishnan of the Indian Institute of Science, IS 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 crustal metamorphism at over 1100 deg C, 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, 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.

PTI