Himalayan rocks maintain magnetic clues about their origins. Craig Robert Martin, CC BY-ND

Respiratory shortly within the skinny mountain air, my colleagues and I set down our gear. We’re on the base of a jagged outcrop that protrudes upwards out of a steep gravel slope.

The muffled soundscape of the spectacular Himalayan wilderness is punctuated by a army convoy roaring alongside the Khardung-La street beneath. It’s a reminder how shut we’re to the long-disputed borders between India, Pakistan and China which lie on the ridgelines just some miles away.

This space additionally accommodates a distinct kind of boundary, a slender sinuous geological construction that stretches alongside the size of the Himalayan mountain vary. Often called a suture zone, it’s only some kilometers huge and consists of slivers of various kinds of rocks all sliced collectively by fault zones. It marks the boundary the place two tectonic plates fused collectively and an historic ocean disappeared.

Our group of geologists traveled right here to gather rocks that erupted as lava greater than 60 million years in the past. By decoding the magnetic information preserved inside them, we hoped to reconstruct the geography of historic landmasses – and revise the story of the creation of the Himalayas.

Sliding plates, rising mountains

At a subduction zone, two tectonic plates collide, with one slowly sliding beneath the opposite.

VectorMine/iStock through Getty Pictures Plus

Tectonic plates make up the floor of Earth, and so they’re continuously in movement – drifting on the imperceptibly gradual tempo of just some centimeters every year. Oceanic plates are colder and denser than the mantle beneath them, so that they sink downward into it at subduction zones.

The sinking fringe of the ocean plate drags the ocean flooring alongside behind it like a conveyor belt, pulling the continents towards one another. When your entire ocean plate disappears into the mantle, the continents on both aspect plow into one another with sufficient power to uplift nice mountain belts, just like the Himalayas.

Geologists usually thought that the Himalayas shaped 55 million years in the past in a single continental collision – when the Neotethys Ocean plate subducted beneath the southern fringe of Eurasia and the Indian and Eurasian tectonic plates collided.

However by measuring the magnetism of rocks from northwest India’s distant and mountainous Ladakh area, our group has proven that the tectonic collision that shaped the world’s largest mountain vary was really a fancy, multi-stage course of involving at the least two subduction zones.

Earth’s magnetic area is generated by motion throughout the planet’s outer core. Magnetic north and south drift and generally flip over time.

VectorMine/iStock through Getty Pictures Plus

Magnetic messages, preserved all the time

Fixed motion of our planet’s metallic outer core creates electrical currents which in flip generate Earth’s magnetic area. It’s oriented in another way relying the place on the planet you might be. The magnetic area all the time factors towards the magnetic north or the south, which is why your compass works, and averaged over 1000’s of years it factors towards the geographic pole. But it surely additionally slopes downward into the bottom at an angle which varies relying on how far you might be from the equator.

When lava erupts and cools to kind rock, the magnetic minerals inside lock within the course of the magnetic area of that location. So by measuring the magnetization of volcanic rocks, scientists like me can decide what latitude they got here from. Primarily, this methodology permits us to unwind tens of millions of years of plate tectonic motions and create maps of the world at completely different instances all through geologic historical past.

Geologist collects core samples utilizing a water-cooled electrical core drill.

Craig Robert Martin, CC BY-ND

Over a number of expeditions to the Ladakh Himalayas, our group collected a whole lot of 1-inch diameter rock core samples. These rocks initially shaped on a volcano energetic between 66 and 61 million years in the past, across the time that the primary levels of collision started. We used a hand-held electrical drill with a specifically designed diamond coring bit to drill roughly 10 centimeters down into the bedrock. We then fastidiously marked these cylindrical cores with their authentic orientation earlier than chiseling them out of the rock with nonmagnetic instruments.

A couple of rock core samples, with the pattern orientation line marked on their sides.

Craig Robert Martin, CC BY-ND

The goal was to reconstruct the place these rocks initially shaped, earlier than they had been sandwiched between India and Eurasia and uplifted into the excessive Himalayas. Preserving monitor of the orientation of the samples in addition to the rock layers they got here from is crucial to calculating which means the traditional magnetic area pointed relative to the floor of the bottom because it was over 60 million years in the past.

The magnetometer sits inside a magnetically shielded room on the MIT Paleomagnetism Laboratory.

Craig Robert Martin, CC BY-ND

We introduced our samples again to the MIT Paleomagnetism Laboratory and, inside a particular room that’s shielded from the modern-day magnetic area, we heated them in increments as much as 1,256 levels Fahrenheit (680 levels Celsius) to slowly take away the magnetization.

Totally different mineral populations purchase their magnetization at completely different temperatures. Incrementally heating after which measuring the samples on this means permits us to extract the unique magnetic course by eradicating newer overprints that may disguise it.

Black traces mark boundaries between tectonic plates. Black traces with triangular tick marks present subduction zones, with the course of subduction. The Trans-Tethyan Subduction Zone is the extra subduction zone not accounted for within the single-stage collision mannequin. The Trans-Tethyan Subduction Zone is the place the volcanic island chain shaped earlier than the Indian continent collided into it and pushed it into Eurasia, forming the Himalaya.

Martin et al ‘Paleocene latitude of the Kohistan-Ladakh arc signifies multi-stage India-Eurasia collision,’ PNAS 2020, CC BY-NC-SA

Magnetic traces construct a map

Utilizing the typical magnetic course of the entire suite of samples we will calculate their historic latitude, which we consult with because the paleolatitude.

The unique single-stage collision mannequin for the Himalaya predicts that these rocks would have shaped near Eurasia at a latitude of round 20 levels north, however our information reveals that these rocks didn’t kind on both the Indian or the Eurasian continents. As an alternative, they shaped on a sequence of volcanic islands, out within the open Neotethys Ocean at a latitude of about eight levels north, 1000’s of kilometers south of the place Eurasia was positioned on the time.

This discovering might be defined provided that there have been two subduction zones pulling India quickly towards Eurasia, quite than only one.

[You’re smart and curious about the world. So are The Conversation’s authors and editors. You can get our highlights each weekend.]

Throughout a geologic time interval generally known as the Paleocene, India caught up with the volcanic island chain and collided with it, scraping up the rocks we ultimately sampled onto the northern fringe of India. India then continued northward earlier than ramming into Eurasia round 40 to 45 million years in the past – 10 to 15 million years later than was usually thought.

This last continental collision raised the volcanic islands from sea degree up over 4,000 meters to their present-day location, the place they kind jagged outcrops alongside a spectacular Himalayan mountain go.

Craig Robert Martin receives funding from the Nationwide Science Basis (NSF).