by Florence Mui
In the most recent issue of Earth & Planetary Science Letters (vol. 133), geologists confirmed that the hereto "Indo-Australian" plate is actually made up of two separate plates: Indian and Australian. Direct evidence was reported by scientists at Columbia University's Lamont-Doherty Earth Observatory. According to the theory of plate tectonics, the lithosphere is broken into rigid plates that move together and apart from each other like pieces of a jigsaw puzzle. The newly discovered crack of the Indo-Australian plate would mean that now the Earth is made up of 13, not 12, major plates.
Scientists believe that the Indian subcontinent has been moving northward into Eurasia for the past 50 million years, compressing the nearby crust to form the Tibetan Plateau and the Himalayas. Drilled samples taken in the past 20 years show that the Indo-Australian plate started breaking (just south of the Equator beneath the Indian Ocean) about 8 million years ago when the accumulated mass became too great. At Northwestern University, more recent data collected by researchers on seafloor spreading along the mid-Indian Ocean Ridge Rift, suggested that a distinct plate boundary must exist at this area. In addition, the scientists theorized that the lower part of the plate (Australian) is moving counterclockwise in relation to the upper part of the plate (Indian). This means that the plates at the newly claimed boundary are compressing with increasing intensity from the west to the east across the Central Indian Ocean.
To prove that the new boundary does really exist, scientists at the Lamont-Doherty Earth Observatory took actual measurements of the compression intensity of the seafloor. Using sound waves to probe oceanic rock layers, they created images of subseafloor structures. The research "gives insight into how strong and rigid plates are, how they respond to stress and what their limits are before they break," Dr. James Cochran who is a LDEO scientist and coauthor of the report said. Data was taken during two expeditions, one aboard the 1991 "Phedre" cruise of the French research vessel Marion Dufresne, and the other aboard the 1986 Robert D. Conrad vessel, which traveled along a north to south line 185 miles farther west. The images showed more than two hundred faults in the area were created as the once-whole plate split. As the two newly distinct plates continue to converge, parts of the ocean floor slides upward along the faults to alleviate the strain. When compared, the two sets of data revealed that the thrusting observed on the "Phedre" line was twice that formed along the Conrad's line, thus validating the Northwestern scientists' prediction of mid-ocean ridges' seafloor spreading rate and direction, consequently revealing the compression was more intense to the east.
From the notion of unchanging geological features to the theory of plate tectonics, geologists have come a long way. While the discovery of the new plates is certainly an accomplishment, there remain even greater challenges such as the search for the underlying cause for movement and formation of the plates. Knowing the driving mechanism of plate tectonics might be the fundamental key to understanding the Earth.
