Sand Month, Week 2: The Journey

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. 
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As always, I'd love to hear your thoughts and questions in the comments!

Part 1: The Highlands. ​

The birth-place of sand: a mountain stream tumbling over the rapidly eroding highlands in the Southern Alps of New Zealand

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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...
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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.

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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

In this photo, water drains through the sand of a modern-day beach, at the foot of sedimentary sea cliffs deposited on an ancient seafloor.

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.
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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...​