Photo Essay: High Desert, Deep Earth

In the Arabian peninsula nation of Oman, geologists are studying the Hajar mountains–a range containing rocks that have been thrust up from the deep earth. Accessible to humans in only a few places on earth, these kinds of rocks offer clues to the planet’s deep history–and possible ways that natural processes may be harnessed to combat modern climate change. READ THE FULL SCIENTIFIC STORY

Peter Kelemen, a geologist at Columbia University’s Lamont-Doherty Earth Observatory, has worked in Oman for years, along with colleagues, gathering samples, mapping formations and performing experiments back in the lab.

The mountains, once part of a seabed, were heaped onto land when normal tectonic processes went awry. In them, below layers of marine sediment and volcanic crust lies peridotite–the stuff of earth’s mantle, which undergirds seafloors and continents, miles below the surface.

One outstanding property of peridotite is its powerful chemical drive to react with carbon dioxide–not available in its normal deep-earth habitat, but abundant in air or near-surface groundwater. The reaction forms carbonate–a mineral that locks the carbon into solid form. This chunk of it was hammered out seconds before the photo was taken.

Kelemen thinks that with some engineering, natural carbonate-forming reactions could be corralled and speeded up a million times over. That could allow people to draw massive amounts of CO2 from our overburdened atmosphere and store it underground. This pump at an Omani gas station uses air holding nearly 400 parts per million carbon dioxide–a huge leap over preindustrial time, and a driver of climate change.

Carbonates form naturally in multiple ways. Here, a spring in a valley called Wadi Sudar gushes water that has passed through peridotite and undergone a series of chemical reactions to become rich in carbon minerals.

As water evaporates, crusts of carbonate form, and eventually the precipitated-out minerals solidify. In many places, a pebble thrown into a pool will be covered with a layer of carbonate within days.

Lamont geochemist Amelia Paukert photographs a wall of peridotite containing whitish veins of solid carbonate that formed underground, when water ran through cracks in the rock. The stream flowing at her feet later cut down through the rock, exposing the cracks.

Team members appear to be walking into a massive waterfall–but it is a frozen one. These giant stalagtites, estimated to be some 10,000 years old, formed as carbonate-rich water dripped out of the cliff, and solidified in thin sheets year after year; the process is still underway.

The mountains contain other geological wonders, many of which shed light on normally invisible processes of the seafloor and mantle. Here, Lamont postdoc Kristoffer Szilas confronts an exquisitely preserved outcrop of pillow lavas–toothpaste-like volcanic eruptions that squirted onto the seafloor tens of millions of years ago.

At one time, seawater migrated downward and circulated through metal-rich rocks fed by the mantle, then emerged back on the seafloor to precipitate out the metals–a process somewhat akin to how carbonates form. As a result, Oman has major deposits of copper and chrome; this massive open-pit mine is mostly played out, but others operate nearby.

It seems that from early times, humans have picked up on geology for both practical and mystical purposes. Here, Bronze Age tombs made of stone sit on a ridge below the crags of Jebel Misht, an iconic promontory near the village of Al Ayn.

Until very recently, life in rural Oman had remained little changed for many centuries. But in just the last decade, wealth from newly exploited oil reserves has instantly transformed the sparsely populated country, bringing roads, electric power and industry to all but the remotest settlements.

In a supermarket, some of the produce of exported oil returns in the form of imported packaged snacks and numerous other items available on the global market. The fossil fuels that drive this economy continually add more carbon dioxide to the atmosphere.

Researchers take every chance to inspect rocks that will help them understand how natural carbonation works. Here, Lamont marine geologist Suzanne Carbotte checks out a shaft dug through peridotite to an irrigation tunnel far below. Other convenient human-made features used for study include road cuts and quarries.

In places, the carbonation process has run its course and occupied nearly every possible pore space of rock. Kelemen admires a weathered peridotite outcrop heavily laced with veins and crowned with a solid mass of carbon minerals.

The upper half of this entire mountain has been converted to basically 100 percent carbonate–a process that probably took place not on land, but as the seabed was being squeezed upward and saturated with CO2-rich seawater. Software engineer Peter Mullen prepares to make a photo mosaic that will enable the team to map the mountain in great detail.

A swim in a hidden canyon pool serves a double purpose: cooling off, and examination of the carbonate flowstone making up the walls.

One key study site is Wadi Fins, a precipitous canyon that begins at the seashore and runs straight into the mountains, cutting through many layers of rock.

Along the wadi’s bottom, carbonate-laced peridotite lies exposed. Kelemen hopes to mimic natural processes by drilling down through such formations and forcing down CO2-rich water.

Before planning engineering experiments, scientists must first map rock formations. As a start, Kelemen and daughter Sarah use a hand compass to roughly map the dip and strike of the peridotite in Wadi Fins, which will help them understand the extent of unseen rock in three dimensions.

At the wadi’s lip, Kelemen and Mullen prepare to descend with a device that will map the canyon walls and floor using the satellite signals of a ground-positioning system.

One possible obstacle to the carbon-storage scheme: water necessary for dissolving CO2 is extremely scarce. Over centuries and perhaps millennia, farmers have built elaborate irrigation systems that bring water from deep wadi springs to small plots where they grow dates and other crops.

Kelemen aims to show exactly where and at what depth mantle rocks exposed in Wadi Fins extend under the nearby Gulf of Oman, which shares its waters with neighboring Iran. Drilling along the shallow coast to pump in CO2-rich seawater may be the best solution.

It might take many years for any project to come to fruition. In the meantime, human routines march on. Teens in a small village pass under newly installed power lines as they return from afternoon prayers at the local mosque.