This is part of the third installation of my personal project to help people see sand differently, and communicate the science of one of earth's most ubiquitous and fascinating materials. This week is all about what sand does to influence life on earth. You can read the first part about sediment-bound nutrients here, or check out the posts from week 1 and week 2 at these links or on my instagram.
In this photo, the spiral shell of a small squid-like creature (an incredible animal called a Spirula, which is worthy of its own entire post) rests on the black sand of a beach in New Zealand. What is invisible in this photo is the deeply intertwined backstory that these two substances share. The role that sediments play in the existence of shells and other biogenic minerals is the subject of today’s entry in the Story of Sand: Carbonates.
To understand how sediments are linked to seashells, we need some background on how water interacts with gas, specifically carbon dioxide.
Gasses dissolve in water, which you know if you’ve ever opened a carbonated drink and watched gas that had been dissolved suddenly emerging as bubbles. When CO2 dissolves in water, it does something unique: it reacts with the H2O molecules to create a new set of compounds, which are carbonic acid (H2CO3, the one that drives chemical weathering) and two charged ions called bicarbonate (HCO3-) and carbonate (CO3 2-), formed when hydrogen atoms fall away from carbonic acid.
These molecular residents inhabit water nearly from the start; as a raindrop falls through the air it is already absorbing its fill of atmospheric CO2 and accumulating carbonic acid, bicarbonate, and carbonate. By the time rainwater hits the ground, it is primed to react with the rocks of the earth’s crust, dissolving them and balancing its negative carbonate and bicarbonate ions with positive mineral ions such as Calcium (Ca2+).
This mineral water, now loaded up with calcium from rock and carbon from the atmosphere, coalesces into rivers and drains to the ocean. In the sea these minerals accumulate and get concentrated into an alkaline brine: the ocean’s saltiness that is the chemical legacy of earth’s eroded continents.
At last, the stage is set for one of the most wondrous and consequential reactions that life on earth performs: biomineralization.
Biomineralization is the process by which organisms construct their own hard materials by shepherding minerals from their environment into new complex crystals, like an organic imitation of rock. This is the process behind most protective shells, supportive skeletons, or durable teeth.
In the ocean, the most common product of biomineralization reflects the most abundant building materials: in a world saturated with calcium from rock and bicarbonate from the air, nearly every shell, reef, and exoskeleton you can imagine is made from the combination of the two- a compound called calcium carbonate (CaCO3).
Calcium carbonate is a hard white solid that is familiar to you in many forms, from limestone caves and chalky cliffs to colorful seashells. At their core these materials are all essentially the same, and they are all made up of the two building blocks delivered to oceans by the chemical weathering of rock.
So it is that the two materials in this picture- the delicate shell of the spirula and the weathered remnants of broken rock- actually share a common origin story. Both are ultimately products of the creation, transportation, and transformation of sand.
The next time you look at a seashell, I hope you can imagine its orderly crystals made up of the calcium stripped from sediments, bound up with carbon that once floated through the air as CO2.
Although the connection between sand and shells is fascinating, it is a tiny part of a much larger and more important picture. The carbonate system actually exerts an immense influence on life as we know it by driving the temperature of earth’s climate on a timescale of tens of millions of years. That story is coming in the next installment of the Story of Sand
This is the third installation of my personal project to help people see sand differently and communicate the science of one of earth's most ubiquitous and fascinating materials. You can read the posts from week 1 and week 2 at these links or on my instagram.
Part 1: Sediment-Bound Nutrients.
In this photo of the Amazon river, an invisible payload goes unseen among those billowing plumes of sediment: these milky waters also carry a precious cargo that was stripped from rocks of the Andes Mountains and is now fertilizing the Amazon Rainforest.
What these life-giving nutrients are, and how their creation is tied up in the creation of sediments, is the story I want to tell with this week's installation of Sand Month.
In the past posts we have talked mostly about clastic sediments, the fragments that result from the crushing and cracking of larger rocks. These are pretty straightforward: they are essentially miniature versions of their parent rock, and the processes that yield them generally occur on a visible scale. But, they are only one part of the picture.
Alongside and behind the scenes of that physical weathering is chemical weathering: the reactions caused by the acid that CO2 forms in water. This acid eats away at rock and pulls atoms from their crystal lattice, helping weaken the rock and creating its own suite of products along the way.
The atoms that are stripped away behave like table salt, dissolving in the water to become ions in solution. Thus water accumulates a trove of rock in liquid form, aqueous minerals carted along invisibly alongside their still-solid clastic cousins.
(side note: the parent rocks are transformed as well, as atoms are plucked from their minerals they leave an altered compound in their wake: once-rigid crystals rearrange into sheets of sticky plate-like structures that combine with water to form a slick, squishy earth. We call this new compound “clay.”)
The dissolved minerals being carted away will be familiar to you-- things like potassium, calcium, sodium, phosphorous, iron, and zinc. This list reads like a nutrition label, because it is: these are essential nutrients that sustain life on earth. When your parents extol vitamins and minerals, it’s these that they mean-- their liberation from rock via chemical weathering is the leap they must make before being taken up by the food web, and eventually, you.
The upshot of this is that the process of creating sediment is also the process of fertilizing life on earth. Indeed, the sediment itself is the most productive source for these nutrients- its immense surface area and long exposure to water gives ample opportunity for water's acids to do their work.
That is why this photo of the milky sediment-rich waters of the Amazon contains so much more than meets the eye- these sediments brought down from the Andes provide the vast majority of mineral nutrients that the Amazon floodplain receives and enable the biological spectacle that exists there today.
This process is on spectacular display in the Amazon, but it occurs everywhere on earth. The richness of life on this planet would simply not be possible without the creation of sand.
If you want to learn more about the impact of sediments and their nutrients on the Amazon basin, I wrote a whole article about it for Massive Science last year. You can check it out here!
Also, this week's posts aren't quite as linear of a story as last week's on The Journey, so I'll be releasing them one at a time over the course of this week. The next post is about one of my favorite subjects of all time, the carbonate cycle.
I'll also be experimenting with some super brief video summaries of these subjects via TikTok/Instagram Reels, so you can check those out on my instagram profile if you're interested!
This is the second installment of my little personal project to try out some of my ideas about science storytelling, specifically the idea that everything in the natural world has both an ”endo-story” (the little world contained within them) and an “exo-story” (the role they play in the world around them).
Last week I alluded to the idea that sand can be interpreted and read- its history gleaned from traits both external and internal, individual and aggregate. This week I'm hoping to impart a sliver of what can be read from those and how to read it. To do so, we need to understand a few things about the journey sand takes from bedrock to beaches.
As always, I'd love to hear your thoughts and questions in the comments!
Part 1: The Highlands.
Tectonics push up mountains, and gravity tears them down. The rock that is carried thousands of feet up contains immense potential energy, a yearning to fall towards the sea.
On these mountainsides, physical weathering dominates: this is the name for the suite of processes that crack, pry, and wear at rock, splitting off fragments to be carried away by gravity. Here the rock is exposed to the elements. Air that rises up the mountain flanks grows colder and drops its precipitation, introducing the most potent and recurring character in the story of sediments: water.
Water sets to work even before it lands- absorbing carbon dioxide from the air that will form an acid to eat away at rocks below (more on that later). On the ground, it pries apart fissures as it freezes and expands, adds weight and lubrication to coax rockslides, and carts away the carnage in its nascent streams.
Mountainsides are steep; as fragments break from their mother rock they can fall fast and hard, carried on the raging currents of a mountain river or caught in a violent debris flow of sliding, grinding rock. In these early stages the fragments are large, but not for long. The energy of each collision increases exponentially with size, and breakage is common wherever bigger clasts collide. Corners take these hits particularly hard, leading angular rocks to become rounded as they move.
In this way, parent bedrock rock is broken down into sediments. These fragments carry the same internal composition as their source, but are now being shaped externally by the forces of transportation- rounding and splitting, first into boulders, then cobbles, then gravel, then sand. These multiplying fragments leave the mountains mostly on the currents of rivers, bound toward the sea...
Part 2: Transportation
The San Juan River, one of the most sediment-laden in the U.S., transports sand and silt from the highlands in the Rocky Mountains. These sediments once joined the Colorado river and emptied into the Gulf of Mexico, but they now settle to the bottom of Lake Powell, restrained behind the Glen Canyon Dam.
The rivers that carry sediments grow slower as they leave mountainous highlands and enter flatter terrain. The larger grains drop out as the current slows, but fine silts and clays stay aloft in even the laziest rivers, and sands are dragged, rolled, and bounced along the river bottom. The sediment carrying capacity of a river varies directly with its speed, so where the river accelerates in floods and rapids it moves coarse sands and large rocks, and where it slows the coarse sediments settle to the river bed. Any stops are simply layovers, though; eventually everything will continue its journey seaward when the right flood comes along or the landscape changes.
The sediments that rivers carry also give them their cutting power: an entrained abrasive that scours rock and carves canyons. Sand begets more sand as it excavates the landscapes it traverses.
The sands themselves change in transit too- As they depart the violent physical weathering of the highlands, they enter a phase dominated by chemical weathering instead.
When water falls as rain, it absorbs carbon dioxide from the air. This carbon reacts with H2O to form carbonic acid- which makes unadulterated rainwater nearly 1000 times more acidic than your average tap water. On the ground, water dissolves minerals from rock, stripping out ions like calcium to neutralize the acid. Though most people think of water as neutral, it is actually the interactions with earth’s minerals that make it so (thus the term “mineral water”).
Time, temperature, and surface area all accelerate these reactions with. In the transport phase, the lower, slower, warmer water has the opportunity to act upon the ample surface area generated by physical weathering above.
Not all rocks are equally susceptible to this corrosion. Quartz, one of the most common constituents of earth’s crust, is uniquely invulnerable to it. As a result, chemical weathering of sediments gradually refines them into pure quartz.
Sands that are mostly quartz are said to be “mature;” their homogeneity is a telltale sign of a gauntlet of chemical weathering, perhaps millennia spent traversing a warm, wet floodplain (like that of the southeastern US, which thoroughly weathers the remnants of the Appalachian mountains into the white quartz beaches of northern Florida)
Immature sands indicate the opposite: their diversity indicates a quick trip from a nearby source- a common feature on coastlines near still-growing mountains like those of Northern California.
Regardless of how they travel and what stops they make along the way, the ultimate destination of almost all sediments is the ocean, which is the subject of...
Part 3: Deposition
When sands and sediments reach the ocean, they have generally reached their final resting place. Rivers that empty into the sea no longer have the pull of gravity to drive their currents, and the sediments they carried settle out for a final time. Larger grains settle close to shore, while finer silts drift far out to sea.
The ocean is basic, so their arrival marks the end of their chemical weathering. Except for the tireless wave action at the margins, the ocean is also still, so most physical weathering ends as well. For this reason, arrival in the ocean acts like a preservative for many of the features that hint at sediments’ endostory.
In the deep sea, sediments drift to the seafloor to form layers that eventually fuse into rock through the pressure of burial and the cementing action of microbes and precipitating minerals. Near the shore, wave action dredges the seafloor and scours the shoreline to stock beaches: melting pots that reflect all the sources and all the stories that contribute to that place in time.
A handful of beach sand tells those stories with every aspect of its character-
The mineral composition of grains reflects their parent rocks, which might be traceable to a specific formation or geologic event. The quartz content (maturity) reflects the journey they took and the climates they experienced- whether they endured extensive weathering in a hot and humid region or made a quick trip with many minerals intact. The texture hints at transport mode- smooth from endless collisions or jagged from recent breakdown. All of these stories, and much more, in a handful.
A note about the included photo and a teaser for things to come:
The ocean is relatively stable on a human time scale, but not so on a geologic time scale: sea levels rise and fall by hundreds of meters when ice ages gather earth’s water into glaciers, seas appear and disappear as the continents move on their tectonic plates, areas that were once seafloors can be uplifted to become mountaintops. Because of this, sand can make its journey from highlands to deposition many times. Sand on this beach likely combines re-erosion from this ancient cliff side and newer sediments brought down from the Sierra Nevada mountains and countless other highlands along the west coast! Some sands may run through this cycle many times over multi-million year lifetimes. But eventually, almost all will meet their final end...
Today I’m kicking off a little personal project: Sand Month
Every Sunday for the next month I’m going to be posting a little deep dive into a facet of a subject I think is one of the most fascinating and low-key profound things in the world: sand. I’m starting this project as a way to try out some of my ideas about science storytelling, specifically the idea that everything has both an “endo-story” (the little world contained within them) and an “exo-story” (the role they play in the world around them).
Without further ado-
Chapter 1: the galaxy in your hand
I have always loved playing in the sand- the infinite forms it can take, the sensation of it running through fingers or enveloping feet. When I was younger I mostly loved it physically, as I grew I started to love it conceptually-- slowly gaining an understanding, first visceral, then academic, of the unfathomable intricacy that existed where we seldom thought to look.
Some of my earliest experiences of awe at the natural world came from the unexpected jolt of realizing a humble handful of sand became something dazzling when you looked at it up close-- a handful of sand is a galaxy of little worlds, each grain with its own story of formation that may stretch back months or millennia. Each grain’s history is recorded in its form and composition: what they look like on the outside and inside can be read like a short story, and the accumulated stories in a handful come together like the whorls of a fingerprint-- an emergent snapshot of that exact place and time in earth’s geological history.
Sand, generally speaking, is just tiny rocks: the shattered and worn fragments leftover from the breakdown of the volcanic bedrock that makes up earth’s landmasses. This source material differs somewhat from place to place, but is nowhere near as diverse as the kinds of sand it spawns. What accounts for the leap in diversity is what happens to these rocks along the way: grains are defined by the scars of their formation.
These scars can be pondered, and sometimes deciphered: Is it polished in a swash zone or pockmarked by the stinging airborne impacts in the wind? Is it sorted by size in moving water and laid to rest in layered beds? Has it been gnawed at by acids until only invulnerable quartz remains, or pierced by tunneling fungi and colonized by critters?
Each grain of sand is the vessel for a tiny world, but it also exists as a tiny character within a much larger world: a stage for one cast of characters, and the tiniest actor on a much larger stage.
Over the next month I’m going to try and conjure up some of the stories that exist within those grains, as well as the larger role they play within our world. My hope is that by the end, you’ll be able to see beneath the surface of photos like these and of sand wherever you encounter it. My goal is that you’ll be able to see the galaxy in your hand, too.