Marcia Bjornerud is the Walter Schober Professor of Environmental Studies at Lawrence University and a fellow of the Geological Society of America. Her most recent book is “Turning to Stone: Discovering the Subtle Wisdom of Rocks.”
I didn’t expect to meet any garnets in Suffolk.
As a geologist, I feel that I haven’t truly arrived in a place without understanding its rocky substrate. So I’d looked into the geology of the area along the River Deben and learned that there aren’t really any proper rocks there at all — just glacial gravels and soft marine clays under wind-rippled grasslands and welly-sucking salt marshes. The boundaries between land and water are diffuse. Where the river meets the sea and forms a wide estuary, its main channel and capillary-like tributaries pulse in and out twice a day with the tides.
My kids and I had come to this liminal landscape from London for their school’s week-long late-summer retreat in an old manor house by the sea. By the time we left the outer London suburbs and the clutter of the city had given way to villages and open meadows, I was anticipating an adventure. The children would be kept busy with activities during the day, and I would have a week to ramble around the Suffolk countryside.
The first day, I walked as far as I could along the muddy coast and was thrilled to come across a pebble beach. I love beach stones and the fragmented stories they tell. Perhaps these wave-rounded cobbles could share insights into the deeper geologic origins of this land. And they did, obligingly, but every one of them said the same thing: They were nodules of chert, also called flint, weathered out of Cretaceous chalk further inland.
Interesting, thank you, I said to them, but then muttered to myself: rather monotonous. Shingle beaches with chert cobbles are common along the eastern and southern coasts of England. Chert can be knapped to a sharp edge and was prized by early human communities for making knife blades, hand axes, arrowheads and spearpoints. Archaeologists speculate that working chert into tools was essential to the evolutionary development of the human brain. There is an archaeological site in Suffolk that provides evidence of humans making hand tools from chert almost 700,000 years ago. And since medieval times, resourceful English masons who lacked better building stones have used the literally flinty, resistant cherts as cladding, typically cracking them in half and setting the fractured side outward. The exterior of Southwark Cathedral in London is a famous example.
“I admit to a predilection for rocks that speak vividly of magmatic drama and tectonic transformation, rocks that are portals into the heart of a living planet.”
I mused about these connections between human and natural history but couldn’t help thinking a bit wistfully about the spectacular variety of igneous and metamorphic rocks on Lake Superior beaches back home in Wisconsin. Those beaches, where Pleistocene glaciers left rocky souvenirs gathered from vast stretches of the ancient Canadian Shield, have memories spanning more than two billion years, recounting the epic saga of how an entire continent was constructed. If the cobbles on a Lake Superior beach are akin to an aged volume of significant literature, a collection of Cretaceous chert is like shreds of a local newspaper.
Two confessions. First, yes, I talk to rocks, privately and unabashedly. And second, I have favorites — but I do feel bad about that.
To be clear, I respect all rock types; each has a role to play in the governance of the world. But I’m a structural geologist — I study how mountains form. I admit to a predilection for rocks that speak vividly of magmatic drama and tectonic transformation, rocks that are portals into the heart of a living planet.
Aldo Leopold, the pioneering ecologist, confessed to loving pine trees above all others; he admired their ability to subsist in sandy soils and outlive many deciduous varieties. But he acknowledged that other species with different attributes are essential to natural ecosystems, and so it is too with minerals. All have virtues, and some perform crucial planetary duties.
For example, calcite [CaCO3], the main mineral in limestone, serves as a long-term repository for carbon dioxide exhaled by volcanoes. I do not love calcite, but I hold it in high esteem for preventing the Earth (so far) from becoming a runaway greenhouse planet. Or consider apatite [Ca5(PO4)3(OH)], a minor constituent of granite and the ultimate supplier of the element phosphorus, which is essential for all living things, a key ingredient in DNA, RNA and the teeth and bones of vertebrates, and in ATP (adenosine triphosphate), which transports energy within cells. Feldspars, a large family of silicate minerals that are the primary components of igneous rocks, allow themselves to be partly disassembled by rainwater and transformed into clay minerals, the basis for soil and thus critical to life on land.
The most notable attribute of chert, which consists of pure silica (SiO2), is simply exceptional durability against physical abrasion and chemical weathering. The sand on most beaches is made of quartz (also pure silica) not because quartz is the most common mineral but because it is the toughest among the common minerals and survives in the greatest volume. Unfortunately, despite their longevity, chert and quartz don’t have good memories. The chemical simplicity that makes them tough also prevents them from recording their biographies in detail.
“Garnets are the true fruit of the underworld.”
By the third day in Suffolk, I had explored much of area around the manor house. I crossed the wide mouth of the Deben in a rowboat ferry, which prospective passengers flag down from the dock on the opposite side by waving a large white paddle. It was a charmingly low-tech mode of telecommunication compared to the history of this site. British technologists had covertly developed modern radar at the manor in the early days of World War II, and a transmitter built on the banks of the river in 1937 was crucial to the success of the Battle of Britain. To explain the exceptional performance of British pilots who had the advantage of the secret new technology, the intelligence service spread a rumor that eating carrots gave them especially sharp eyesight — a bit of quasi-misinformation that survives, like a chert cobble, as a durable truism.
On my fourth morning, I set out further afield toward Sutton Hoo, the renowned site of a 7th-century Anglo-Saxon ship burial excavated in 1939. It was only 10 miles away, so I started walking in that direction with a vague hope that someone might offer me a lift. After about an hour, I passed a house with an assortment of old bicycles arrayed on the grass next to a hand-printed “For Sale” sign. No one seemed to be around. All the bikes were rusted and rickety, but I spotted a green three-speed that had good tires and one working brake. I knocked on the door but got no answer. I hesitated for a moment, then wedged a 20-pound note into the frame of the front door, hoping it was a reasonable price for a broken-down bike, and pedaled off.
I quickly discovered that the bike had only one functional gear. It was hard work against a headwind that swept unimpeded across the flat landscape, but an hour later, I reached Sutton Hoo with wild, wind-whipped hair. Entering the well-groomed National Trust property with some embarrassment, I paid the admission fee to a startled attendant and entered into the presence of objects with vivid memories of another version of this place, when Suffolk was the kingdom of East Anglia.
The wooden ship and whoever was buried in it had long ago crumbled to dust, but the more durable metal and mineral artifacts sent on to the next world survived, exempt from mortality. There were elegant silver spoons, elaborate gold buckles, an exquisite bronze bowl, an antler comb embellished with iron — and of course the iconic helmet with copper and tin ornamentation. The original had been whisked away to the British Museum, but that did not diminish the helmet’s power. It is a thing of mystery and beauty. Dark eye openings compel one’s attention. Their mesmerizing effect emanates from the arched eyebrows, which, in a brilliant visual double entendre, are also the outstretched wings of a dragon flying toward the crest of the head.
Peering closer, I saw that the inner crescents of the imposing brows — the lower edges of the dragon’s wings — were rimmed with fiery crimson garnets, blood-red crystals that were key to the helmet’s penetrating gaze. In this low-lying salt-marsh terrain, the presence of garnet — a metamorphic mineral forged in the interior of mountain belts — matched the improbability of finding a ship buried in a grassy plain.
Those crystals had come from far away. I riffled through my knowledge of European geology; the nearest occurrences of garnet would be in Scotland, Norway and the Alps, any of which would imply long-distance trade networks. But the sign next to the display case indicated that there is compelling evidence that the garnets came from Sri Lanka, thousands of miles away. This windswept corner of England must have been connected to a vast network of international commerce and the Silk Road. How extraordinary it would be if those garnets could speak of the chain of events that carried them from the medieval kingdom of Anuradhapura in the Indian Ocean to East Anglia on the shores of the North Sea. Who were the people who collected and traded them? How many times had they changed hands? What was their route to this place?
“Pyrope garnet is arguably the most important mineral on Earth — it is the motor of the planet’s signature tectonic habit of subduction, which is the secret to Earth’s long-term habitability.”
On the ride back, exhilarated at the sensation of having traveled in time, and propelled now by a tailwind, it occurred to me that the extraordinary journey of those garnets was simply a continuation of their earlier geological itinerations — which, paradoxically, could be reconstructed in far more detail.
Garnet is one of the mineral kingdom’s most meticulous record keepers. It has an appetite for a wide range of elements. The formula for garnet is often written as X3Y2(SiO4)3, where X may be calcium, magnesium, iron, manganese — in any combination of varying ratios — and Y is aluminum, iron and/or chromium. This makes for a huge spectrum of possible compositions, and the specific mix of ingredients bears clues to the origins of any particular crystal. And because all garnets share the same crystal form — a 12- or 24-sided shape akin to a faceted soccer ball — one compositional type can even grow as a rim on another if the chemical or physical environment changes.
This compact crystal form — a marvel of spatial economy, with atoms nested together for maximum efficiency — makes garnets dense and durable. Not merely pretty ornaments, they perform hard work in sandpaper, industrial abrasives, skid-resistant paints and water-jet cutting. During slow tectonic deformation, when other minerals break or flow, garnets rarely fail. Shattered garnets are the record of high transient stresses in ancient earthquakes.
Most garnets are deep red; in fact, the name garnet is derived from the Latin word for pomegranate (granatus), whose ruby seeds look like gemstones. In the Greek myth explaining the seasons, Persephone, goddess of spring and daughter of Zeus and Demeter, is abducted by Hades and taken into his subterranean realm, where she is tricked into eating the food of the underworld: a handful of pomegranate seeds. Her punishment for being kidnapped and accepting meager nourishment (in the misogynistic logic of antiquity) was to be forced to return underground for the three months of winter each year.
Garnets, however, are the true fruit of the underworld. They occur in a variety of colors, not just red, and all of them have colorful properties and histories. Calcium-aluminum garnet is called grossular, from the Latin name for gooseberry, a reference to its translucent pink to pale green hue. Calcium-chromium garnet is a rare grass-green variety named uvarovite, after Count Sergey Uvarov (1786-1855), who, when not busy with his duties as a statesman under Russian Emperor Nicholas I, spent his time collecting unusual minerals. These calcium-rich varieties of garnet are most commonly found in marbles and “skarns” — rocks formed by the metamorphism of limestones interbedded with shale and sandstone.
Iron-aluminum garnets, whose red shades toward purple, are called almandine, a reference to Alabanda, an ancient city in Turkey. (I prefer almandine to an older term, “carbuncle,” used by the Roman natural philosopher Pliny the Elder, whose geologic curiosity led to his death in the eruption of Vesuvius in 79.) Almandine garnets are the most common type in schists — the metamorphic equivalent of shales, clay-rich sedimentary rocks. If such sediments find themselves crumpled inside a growing mountain range, under unaccustomed heat and pressure, the aluminum, iron and silica atoms from the original clay and quartz will reorganize themselves into new crystal forms. Often, however, a garnet growing in a clay-rich sediment will not need all the “nutrients” from the original rock. Bits of those minerals will survive as vestigial “inclusion trails” locked into the garnet in their original positions, even if the rest of the rock experiences subsequent alteration, like a city’s street plan persisting even as buildings come and go.
“Garnet is omnivorous and adventurous, a diarist and archivist bearing chronicles of bygone worlds and subterranean realms. It keeps this ancient planet churning and gives us humans a glimpse of immortality.”
The transformation of lowly gray mudstones into shiny mica schists studded with raspberry-like almandines seems to me a miraculous kind of geological alchemy. Equally astonishing is their subsequent return to the surface over the long arc of a tectonic season, in an echo of Persephone’s trip to and from the underworld. Perhaps most remarkable of all is their capacity to recount the journey there and back again.
The most intensely red garnet is composed of magnesium and aluminum and is known as pyrope. The highest-pressure variety of garnet, pyrope forms in the stygian depths of the Earth’s mantle. Although it is rare at the surface, pyrope is arguably the most important mineral on Earth — it is the motor of the planet’s signature tectonic habit of subduction. Subduction, the process by which ocean crust is recycled into the mantle, is the secret to Earth’s long-term habitability. Among other important services, it rejuvenates landscapes by putting continents on collision courses, and it recharges the mantle with water, where it is stored for future release by volcanoes.
The subduction of basaltic ocean crust keeps the interior and exterior of Earth in communication. Ocean basalt is formed by mantle-melting and volcanism at submarine mid-ocean ridges (which peek above sea level in only a few places, like Iceland). After its fiery birth, the basaltic seafloor is steadily pushed away from its natal ridge as new generations of magma erupt, then slowly cools and contracts until, at an age of about 150 million years, it is dense enough to start to founder into the mantle from which it came. But cold basalt is not dense enough to sink into the deeper parts of the mantle. Deep subduction of ocean crust is possible only because ocean basalt undergoes a metamorphic transformation, at depths of about 30 miles, into a far denser rock called eclogite. And eclogite owes its high density in large part to pyrope garnet. The name pyrope means, evocatively, “fire eye.”
Back in the manor house after my adventure, I recalled that Sri Lanka, like the garnets that found their way to Sutton Hoo, has itself made improbable pilgrimages. The island was once adjacent to Madagascar and Antarctica but broke away, together with India, about 120 million years ago. It docked with Asia 70 million years later. The bedrock of Sri Lanka represents the core of the ancient southern supercontinent called Gondwanaland. The rocks were originally marine sediments, abducted into the underworld in a tectonic collision when Gondwanaland was still being assembled 600 million years ago. In the heart of a long-vanished mountain belt, they experienced extreme conditions — depths of more than 15 miles and temperatures nearly 1,500 degrees Fahrenheit — according to almandine garnets that began to form under those conditions.
Around 500 million years later, erosion finally unearthed these far-traveled rocks, exposing them, at long last, to the open air. At the surface, they became vulnerable to Sri Lanka’s ruthless rivers, whose waters pried crystals from the rocks and swept them into the current. Lesser minerals broke into ever-smaller pieces as they tumbled downstream, but the durable garnets survived unscathed. As the rivers approached the coast, they lost momentum. Unable to carry garnets any further, they jettisoned them onto the shore. Over time, this process created some of the only garnet-sand beaches in the world, well known to medieval treasure hunters, who sent the garnets on new adventures along the Silk Road.
In learning this, I realize that the eyebrows in the Sutton Hoo helmet probably came from a shoreline just as “monotonous” as the chert beach I had snidely disparaged. I am chastened; the Suffolk cherts, like the Sri Lankan garnets — and the Sutton Hoo artefacts — are all that remain of worlds that have otherwise weathered away, stubborn survivors that have defied time and tide. I said a silent apology to chert.
And yet (hoping I am out of earshot of other minerals) I can’t completely set aside my special fondness for garnet. It invents itself from humble beginnings, putting to use whatever resources are at hand. Garnet is beautiful but not too vain to do hard labor, whether sandblasting or moving tectonic plates. It’s distinctive, but not so rare as to engender warfare; there are blood-red garnets but no “blood garnets.” It’s omnivorous and adventurous, a diarist and archivist bearing chronicles of bygone worlds and subterranean realms. Garnet keeps this ancient planet churning and gives us humans a glimpse of immortality, a feeling for what it might be like to gaze across time with fiery eyes, to remember when the kingdom of East Anglia shimmered in the distant future.
