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One of the biggest perceptual changes I’ve had in my journey to become a landscape scientist is seeing the surface of the earth as a process of continuous change. Some landscapes change quickly, and some move slowly—it depends on what they are made of and the forces exerted on them, like rain and wind (and plants and bulldozers).
I like to think of the earth surface as two dynamic layers between the earth’s interior and the atmosphere: bedrock and soil. This is a big linguistic simplification that would invoke ire by both soil scientists and geologists, probably, but let’s keep it straightforward. “Bedrock” is hard rock, part of the geologic cycle, determined usually by large-scale geologic processes like volcanoes and tectonics. Bedrock can be eroded into particles, which then move around and mingle with particles made by plants (and corals and stuff) and other little solids, and this mixture we can call soil.
In the video above, the “bedrock” would be the exhibit enclosure, and the “soil” is the sand that the demonstrators are pushing around with their hands. On earth, some places have little or no soil and let the bare bedrock be exposed—usually where the climate doesn’t lend itself to letting layers of particles accumulate. Here in Stockholm, there’s bedrock poking up all over the place across the islands, the result of geological uplift being more rapid than processes that erode bedrock, and a cold climate that does not invite a lot of soil production (aka vegetation growth and wear-and-tear). Other places on earth have more soil with many layers that have become deeply buried with time—hosting entire ecosystems within. A meter or two is common in more temperate flatlands. River basins can get really deep, as rivers both carve deeply and carry in sediment: this report suggests that the sediment deposits at the bottom of the Mississippi river, in the Gulf of Mexico, exceed 9000 meters. Yes, 9 kilometers deep of mud. (Granted, this is all wet and mostly underwater, but still. I think it was Ryan C. Clarke who first exposed me to this amazing fact.) In most normal places, deep sediments begin to undergo diagenesis under compression, essentially transforming back into hard rock, at depths shallower than that.
There’s not much we humans can to do bedrock (other than blast it apart), so most of our energy goes towards pushing around the softer layers on top. The most mobile stuff is dry and unconsolidated... so despite my specialty being mud, sand is a more fun place to start. Dunes and beaches made of sand are some of the most dynamic landforms on earth, rapidly responding to changes in wind speeds/direction and ocean waves, currents, and tides, when coastal. As I’ve mentioned previously, at Ocean Beach San Francisco, the windy season makes the dune field constantly smother the beachside roadway (now a park!!!). Loads of money are spent every year to bulldoze it away.
Image capturing the dynamism of sandy shoals, beach, and dune systems on the US East Coast.
But where is "away?" In much of coastal engineering, we typically consider sand "lost" once it drifts far enough and deep enough offshore to be difficult to retrieve in cost-effective ways. For various reasons like recreational value and coastal protection, coastal structures are used in many places in attempt to keep waterways open and sand in-place. Elena and I just today talked about the jetties and groins—rocky installations perpendicular to the beaches—of the Rockaways in New York, which try to interrupt the “river of sand” that is a beach. Yes, a beach, which may look like a pile of sand dancing around with the waves, actually refreshes itself, with sand coming from somewhere (usually a river or cliffs) and going elsewhere (often an underwater canyon). A beach is a sandbox on a conveyor belt.
Sand is increasingly a scarce resource worldwide. It gets mined for use as a material in construction and manufacturing, and it gets stuck behind dams upstream. Sand actually has many different flavors, based on its geochemical composition and the shapes of the particles, and the different flavors get used for different purposes—like silica sand for computer chips.
Even coastal sand isn’t interchangeable. Each beach has its own fingerprint — grain size, color, mineral composition — that affects how it compacts, holds shape, and resists wave energy.
That quote is from this article on Bring Back Our Beaches, a San Clemente (Southern California)-based nonprofit to bring attention to the issue of beach loss. ("Net negative littoral sediment budget" just doesn't have the same ring to it as "beach loss," of course.) I learned about this group via their partnership with Surfline, the de facto source for compiled surfing intel (and beautiful webcam pictures). They're saying many things that I'm just repeating here: that not all sand is created equal, and that there just isn't enough on many of the shorelines we care about.
From this Nature article:
The demand for that material is so intense that around the world, riverbeds and beaches are being stripped bare, and farmlands and forests torn up to get at the precious grains. And in a growing number of countries, criminal gangs have moved into the trade, spawning an often lethal black market in sand.
The details of the black markets and developing/developed world dualities of sand trade would be good for another post; heck, there's even a book about it. But the point I want to make here is to view these landscapes as snapshots in dynamic processes: they temporary repositories of sediment when there is enough sand around to accumulate. To this end, there is increasing scrutiny of sediment in highly engineered places, where there are dams in the rivers and jetties on the beaches, complicating access and use of this resource we need to navigate rapid sea level rise.
Next time you're on hard ground (or even better, a beach or dunefield), as yourself: how long has this soil (sand) been here? Where did it come from? Is it being replaced by anything? What can we do to make that process work again?
Trucks won't cut it. Strategic sediment nourishment work is great, but as Jon Warrick explains in his Good Sediment, Bad Sediment slide deck about California's 34 million ton sediment discharge per year:
What a waste of gasoline. To get at the scale of this issue, we will need to let nature do a lot of the work for us. And in ways that are fair and just to those impacted. We all deserve a chance to play in sandbox earth.
Undergoing cementation,
Lukas
P.S. Thanks to Elena for a number of ideas in this issue! And I compiled some of the visuals for it previously for a note on sand for Varyer media, published here.