You're walking along a river and a pebble catches your eye. When you pick up that unsuspecting pebble in your hand, you have just picked up a page in the book of the earth's history. Everything about that pebble in your palm, from it's colour, the minuscule grains that make up the pebble, it's shape and the fact that you found it on this river bank, tells a story. A story not only about the pebble, but also the Earth you stand on.
Let me break it down for you. Here is our pebble:
It's a pretty pebble I found along the banks of the Nubra river in Ladakh, India, on the evening of 21st July 2024. It's nested in the ground along the river with its other pebble friends. Most of which are a similar size.
As I dramatically tear it from its current context (the aforementioned paragraph) and into the palm of my hands to examine it further, I notice a couple of things:
Colour : It's salt and pepper coloured. Often this means it has a mixture of different grains. Looking closely I notice: white grains, some transparent grains and black grains.
Grains: Grains are well defined and large enough to be visible to the naked eye. Elements come together to form stable chemical compounds called minerals. The grains here are different minerals that have come together to form a rock. Minerals have unique physical properties like colour, hardness and texture. Recognizing the right cocktail of minerals is what helps identify the rock itself. Now because I'm a geologist, I am able to identify the white mineral as feldspar, the transparent/greyish mineral as quartz, and the black mineral as mica.
Inference: The mineral cocktail of this rock indicates its: Granite! Granite comes under the igneous category. Or as I like to call it, one of the fire rocks. It's origin is magma, straight from the belly of the Earth. Magma is molten rock soup that feeds the lava of all the volcanoes on Earth.
Granite, specifically, is a type of rock that was cooled slowly under the surface of the earth, within the belly of a volcano. The slow cooling is what allows for the mineral crystals to grow. Our pebble has some very beautiful large crystals which tells me that this magma had enough time to cool.
The fact that I can hold the belly of a volcano in my hand tells me, this volcano was very old and most of it has been eroded leaving just the belly on the surface.
Updated inference: This is a granite rock that came from the belly of a volcano. Which means there is an ancient volcano somewhere in the vicinity. But where ? The shape and the size of the pebble can tell us more. Lets investigate!
Shape: Rounded. The river is our shaping agent here. Water has a habit of polishing rocks into rounded shapes. The more rounded the shape the more water erosion action the rock as seen.
Size: Pebble sized. Alot smaller than the cobble. but a little larger than a granule. The water has had alot of time to work on this rock which means it probably come a long way from original source.
Inference: The volcano we are looking for, is somewhere far upstream. If I look up at the mountains around me I notice one side of the river bank has a slightly grey rock face. Matching the salt pepper granite texture of the pebble in my hand. Upon further googling, I realize I am standing next to the belly of the mighty Karakoram volcanic range, that 50 million years ago, had been spewing lava and ash into its surroundings. However, its source magma dried out and the volcanic range was left to be weathered away until all you can see is its belly as the formidable Karakoram range. Though weathered, it still stands as one of the tallest mountain ranges in the world and probably the most treacherous part along the ancient silk route into India.
This granite pebble probably got pulled away from its parent rock high up in the Karakorum mountains upstream in a landside event. It tumbled down a ravine and into a young stream that flows down one of these mountains to join the river. The strong water action of the young stream polished the rock as it tumbled along its way down to the river. And further down it would go until the river decided to leave it along its bank here, where it would have sat for another millennia if I hadn't picked it up and held it in the palm of my hand to tell you story.
Credits: The images in this post have been generated by AI
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