soil moves through space and time
Thanks for making space for me (and soil) in your inbox! Today's essay is about how soil particles move across the Earth's surface by erosion. It's been raining in California recently, so erosion is on my mind. This natural process leads to interesting soil features that we can observe while exploring outside. I hope you enjoy!
If you're wondering why this email looks different than previous newsletters, I explain my decision to move to a new newsletter platform at the end of this email.
Soil moves through space and time
one thing about soil
Erosion happens in three steps: detachment, transport, and deposition
The surface of the Earth is a dynamic place. Even though soils form over hundreds to thousands of years, this doesn't mean that individual soil particles always stay put. The natural movement of soil materials leads to the unique mixtures of soils and rocks that we find out in nature.
Erosion is the process by which sediment and soil particles are moved over space and time. There are three components of erosion:
- detachment: when particles are lifted from a body of rock or soil
- transport: when particles are moved from one place to another by the aid of wind, water, gravity, and organisms
- deposition: when particles settle in a new place at lower elevation
Let's imagine you are out picking wild blueberries to make a pie. Pulling the berries off the vine would be the process of detachment. Carrying the berries from the field and into your kitchen in whatever cute little basket you happen to have - that's transport. Dropping the berries into the pie crust, baking, and then enjoying a slice... deposition.
Maybe you decided to pick some blackberries along your walk and end up making a mixed berry pie. This happens with sediments and soils, too! Once detached, loose particles are mixed together as they are transported across the surface of the Earth. Consequently, soils are often forming from mixtures of particles with different sizes, shapes, properties, and origins.
Erosion leaves behind features we can observe
We can observe the consequences of soil erosion all around us. Water is an important agent of erosion. Scientists categorize soil erosional features that are the result of moving water in three groups based on the size and intensity of the process.
Sheet erosion occurs when water moves slowly and more or less uniformly across the surface of soil. Along the way, water gently picks up and moves particles. Have you ever seen a sheet of water flowing across a concrete sidewalk? It's kind of like that, only slower. Sheet erosion can leave behind pedestals like those in the photo below. Small rocks that were too heavy for the slow-paced water to move stabilize the soil particles underneath. Meanwhile, water is gently washing away the particles around each rock. Over time, we are left with little columns of soil topped with rocks. Look closely and you'll start seeing them everywhere.
Once a groove in a soil surface forms, water will preferentially flow into the groove and pick up more soil particles as it goes. Eventually, this process will create a channel that grows over time. Rills are the erosional features that form from this process. We can differentiate rills from sheet erosion because they are larger in size and have a clear channel of flowing water.
Here's an example of parallel rills in a new neighborhood development. Construction crews were trying to stabilize the slopes that have been exposed during the groundworks process by seeding grasses. In this case, it wasn't enough to prevent rill erosion during a series of heavy rainstorms.
Over time, rills can grow in size from successive erosive events (heavy rainstorms). This can result in the formation of a gully, like the channel seen in the photo below. A gully is much larger than a rill. You can walk over a rill, but not a gully. Once formed, gullies are difficult to reverse because water continues to cut back at the surface during each rain event, leading to more soil losses into the waterway. Notice how the darker soil surface horizons have eroded more quickly than the lighter colored subsurface. This happens because there's more clay in the subsoil that holds the soil particles together and resists detachment from the profile.
I took this photo during a particularly intense period of rain. The soil was saturated and you can see water ponding at the surface. Any water that doesn't infiltrate into deeper soil layers will instead flow across the surface, following the pull of gravity and (in this case) further eroding away the soil along this gully.
Erosion is a natural process, but humans are accelerating it
Erosion is a natural process that shapes Earth's surface over time. Mountains, hills, and valleys are carved by erosion. Floodplains, alluvial fans, and deltas form when sediments are deposited by water. Landscapes form through these combined processes. If it wasn't for erosion, we wouldn't have the marvelous variety of landscapes we experience today.
Even so, accelerated erosion caused by human activities is one of the biggest threats to soils. We accelerate erosion by disturbing vegetation, tilling up hillsides, and overgrazing fields. We move soils around as we build infrastructure, leaving soil surfaces bare and exposed. It's estimated that the rate of human accelerated erosion can be anywhere from 100-1000 times higher than natural processes. Given that soils form over centuries, any soil lost from a site by erosion is soil we can't get back in our lifetime.
Keeping soils covered with vegetation helps slow down water as it travels across the soil surface and allows more water to make its way deeper into the soil horizons. Vegetation also helps stabilize soil against strong winds that might otherwise pick up and move soil particles. While the process of erosion is natural and cannot (and should not) be completely stopped, we need to conserve and protect soils from excess erosion.
Take home message
Erosion is the natural process that describes the detachment, transport, and deposition of sediment and soil particles across the surface of the Earth. Together, these processes help form the landscapes we admire and the variety of soils that exist in nature. We can find evidence of soil erosion if we bring our attention to the edges of soils - the places where soil, water, wind, and gravity meet. When we stop to notice erosion of the soils around us, we begin to see the impact of our activities as humans. Soil conservation begins with noticing these patterns.
There's certainly more to say about erosion. If you have questions you'd like me to answer in future essays, reply to this email with your ask.
why I switched to buttondown.email
and what it means for you
You may have noticed this email looks a little bit different today. I decided to migrate my newsletter from substack to buttondown.email. This change won't affect you much as a reader. You will continue to receive my newsletter and essays directly to your inbox, it'll just look a little different.
For those who may be curious, I switched to buttondown.email for three reasons:
- This is the simplest, most frictionless newsletter platform that I could find. Since it accepts Markdown format, I can integrate better with my workflow in Obsidian. The fewer steps between writing and sharing, the better.
- Buttondown prioritizes privacy for subscribers, a philosophy I appreciate. I decided to keep the tracking feature off so I won't even know whether you opened my email (unless you reply)! Beyond privacy, I think this will also deemphasive analytical metrics (opens, clicks, likes, etc) as a measure of success and rather allow me to focus on writing things I think you'll enjoy!
- Substack has been starting to feel more like social media lately. With the introduction of short-form content, I found myself scrolling more often than reading. I want to write on a platform I want to be on, so while substack helped get me started (and find many of you!), I felt like it was time to make a change.
I'll lose the ability to include voiceovers for now, but I am working on finding a solution to that. Thanks for sticking with me while I figure this out!
more ways to wonder of soil
a few more links for your curiosity
- Last week, I shared a guide to Soil Features of Death Valley National Park.
- You can find all my previous essays about soil on my website.
- I recently gave a talk about urban and anthropogenic soils. You can read my speaker notes to learn more.
Thanks for making the time to care about soil today. If you enjoyed this email, please consider sending it to someone in your life. You know that friend who loves walking around in the rain? Maybe they are looking for erosion in action. Forward this to them :)
Have questions or thoughts? Reply directly to this email and I'll get back to you soon. I would love to hear from you.
Take care and stay curious,
Yamina