A Compass Frozen in Stone
Deep within the Gangdise Mountains of southern Tibet, volcanic rocks hold a silent record of Earth's ancient geography. When these rocks first erupted and cooled millions of years ago, tiny, iron-rich mineral grains within the molten lava acted like microscopic compass needles, aligning themselves with the planet's magnetic field. This magnetic orientation, including its inclination or "dip," was permanently locked into the rock as it solidified. The inclination indicates of latitude; the magnetic field is horizontal at the equator and points straight down at the poles. By measuring this "paleomagnetism," scientists can determine the latitude at which the rock formed.
Studies in the Gangdise Mountains focus on a sequence of volcanic and sedimentary layers known as the Linzizong Group. These rocks erupted between approximately 69 and 43 million years ago, during the final chapter of the age of dinosaurs and the dawn of the age of mammals. Paleomagnetic measurements from these formations consistently show that the southern edge of what is now Asia was located at a latitude of about 10° to 11.5° North. Today, this same region sits at around 29°N. This discrepancy provides a fixed point against which the colossal northward movement of the Indian subcontinent can be measured.
The Greater India Hypothesis
The collision of the Indian and Eurasian tectonic plates, which began around 55 to 60 million years ago, is the most significant continental collision in the last half-billion years. This ongoing tectonic event buckled the crust to form the Himalayan Mountains and the vast Tibetan Plateau. However, geological reconstructions of the amount of crustal shortening in the mountain ranges account for only a fraction of the total convergence between the two plates, which is estimated to be over 3,600 km. This has led to the "Greater India" hypothesis: the idea that the Indian continental plate was once much larger, with a vast northern extension that has since been driven down, or subducted, beneath the Eurasian plate.
The paleomagnetic data from the Gangdise Mountains is important evidence for this model. By establishing the precise latitude of the southern Asian margin just before the collision, geologists can calculate the original size of the ocean basin—the Neo-Tethys Ocean—that separated India from Asia. Comparing this with the known paleolatitude of India at the time allows for an estimate of the total landmass that has disappeared under Tibet. Some reconstructions suggest that a piece of the Indian plate extending 2,000 to 3,000 km beyond its modern northern edge has been consumed in this process. The rocks of the Gangdise Mountains, therefore, do not tell a story of their own movement, but are a stationary benchmark to measure one of the most powerful geological events on the planet.