Secrets in Rocks

The highlands of Meghalaya, India have been treating us well. The people are very friendly and helpful. A couple days ago, some men and boys led us down to the river from their village, which is called "Sohbar." The hike took about an hour, so we tried to pay them for their help. They wouldn't accept our money and were embarrassed by the offer because we were their guests!

This has been Dhiman’s first experience working with hard rock because Bangladesh only has loose, young sedimentary rocks. In Meghalaya, India we find extremely old metamorphic rocks - rocks that have been altered under high heat and pressure. We also find igneous rock called basalt that formed from molten rock (magma and lava), and sedimentary rocks such as sandstone and limestone. The sandstone here had millions of years to get compacted and cemented, and it’s so strong that it forms cliffs and giant waterfalls. Limestone is made of shells that have turned into rock.

What secrets can we learn from these rocks?

You remember one: we can learn how the Earth has folded using our compasses to measure the strike and dip! Once we put all our measurements on a map we can learn about the 3-D shape.

Antje’s also been collecting lots of samples of basalt (igneous rock), and she’ll process them in a paleomagnetics laboratory in order to learn about how the rocks have moved (remember that the Earth’s crust is made of plates that move around!). If you’ve ever used a compass, you probably know that Earth has a magnetic field. Magnetic minerals in rocks align with the magnetic field when they form, kind of like little compass needles. Since the Earth’s magnetic field doesn’t vary that much, we can figure out how the rock has moved and rotated since it formed by determining where the magnetic minerals point.

I’ve been collecting samples of sand from the rivers because I’m trying to determine how fast these mountains are eroding. I measure low levels of the element Berilium, which is produced in rocks when they’re exposed to radiation from space.

Sand grains gain Beryllium at a known rate when they’re at the surface, so by measuring the Beryllium I can determine how long the sand grains sat at the surface before they ended up in the river. I assume that my bag of sand is an average from the whole area upstream and that way I can estimate an average erosion rate for the river.field doesn’t vary that much, we can figure out how the rock has moved and rotated since it formed by determining where the magnetic minerals point.

Unlike Bangladesh, Meghalaya has plenty of rocks exposed at the surface. The problem here is that the jungle makes them hard to find! We find the best outcrops along new roads and rivers. We have spent many days hiking through the canyons, climbing our way over huge boulders and getting our feet wet in search of outcrops. The rivers are at the bottom of deep canyons, so we often have to hike many thousands of steps to reach the water.

Crossing the rivers where there isn’t a bridge is very challenging because the rocks are so slippery! A couple days ago we hiked a river where the Khasi people have built living root bridges. There’s a special kind of Indian rubber tree that puts out long roots and can grow on top of huge boulders. Over about 200 years, they have trained these roots to cross the river, and woven them into beautiful, strong bridges without using any wire or rope. I got to walk across the longest living root bridge in the world!