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Shaping of Landscape: A primer on weathering and erosion

Most of us love landscapes –and many of us find ourselves wondering how they came to look the way they do. In most cases, landscapes take their shape through the combined processes of weathering and erosion. While weathering and erosion constitute entire fields of study unto themselves, this primer outlines some of the basics—which pretty much underlie all the further details of how natural processes shape landscapes.

Incised meanders on the Green River, Utah

Aerial view of incised meanders of Green River, Utah.

Two definitions: weathering describes the in-place breakdown of rock material whereas erosion is the removal of that material. Basically, weathering turns solid rock into crud while erosion allows that crud to move away.

Weathering
Weathering processes fall into two categories: physical and chemical.  Physical weathering consists of the actual breakage of rock; any process that promotes breakage, be it enlargement of cracks, splitting, spalling, or fracturing, is a type of physical weathering.  Common examples include enlargement of cracks through freezing and thawing, enlargement of cracks during root growth, and splitting or spalling of rock from thermal expansion during fires.

Spalling of volcanic rock

Spalling of volcanic rock–likely from thermal expansion during a fire.

Chemical weathering alters the composition of the rock and is critical for soil development.  The most prevalent processes are oxidation of iron-rich minerals, dissolution of material, most notably of the calcium carbonate that makes up limestone or the cement of many sandstones, and hydrolysis, which turns feldspars and micas to clay.  As chemical processes all require water to proceed, they are most active in wet, warm climates, and least active in dry cold ones.

Strongly chemically weathered granitic rock above less weathered granitic rock.

Grus, disintegrated granitic rock, Montana

Disintegration of granitic rock through hydrolysis of feldspars

 

Physical and chemical weathering processes help each other degrade rocks. By breaking rock into smaller pieces, physical weathering processes greatly increase the surface area over which chemical processes can attack.  At the same time, chemical processes greatly weaken a rock’s strength and make it more susceptible to breakage.  Chemical weathering of individual mineral grains also increases their volume, which itself leads to fracture.

Groundwater staining along fractures in granite, Sierra Nevada,

Chemical weathering (oxidation and hydrolysis) concentrated along fractures in granitic rock.

 

Erosion
In contrast to breaking things down in-place, erosion removes material, typically through gravity, water, or wind.  The single most important influence is gravity, which drives processes such as rock falls, avalanches, and debris flows.  Most erosion, however, is a product of gravity and water acting together: water facilitates gravity-driven processes like debris flows, and gravity ultimately lies behind the power of water.   Wind can also be important, but is a relatively minor contributor overall because it can transport only the finer-grained particles.

Recent rock avalanche

Rock avalanche deposit, Utah

 

Weathering and Erosion together
Weathering and erosion work hand-in-hand in their creation of landscapes. Weathering processes break exposed bedrock into smaller and weaker fragments, which allows erosion to proceed.  By removing that material, erosion then exposes new bedrock to weathering processes.

Landscape and Bedrock

Besides telling us about Earth history, the bedrock of a particular place helps determine how it looks.  The bedrock provides the initial block of clay, so to speak, that is carved by weathering and erosion to create the landforms at the surface.  As a result, the shape of the landscape depends on the actual type of rock and its structure– that is, its orientation, relationship to other rock types, and presence or absence of fracture or fault zones.  For example, some rock types are easily weathered and removed by erosion while others are extremely resistant, and these may show through as topographic lows and highs respectively. 

Differential erosion: a hogback, Utah

Resistant sandstone forms a “hogback ridge” whereas less resistant shale and mudstone form gullies, Utah.

While a number of factors affect a rock’s resistance to weathering and erosion, the single most important one is its ability to repel water.  As a result, prevalence of bedding in sedimentary rock or foliation in metamorphic rock, both of which allow water penetration, decrease the resistance.  By the same reasoning, increased grain size tends to increase resistance because coarser grains offer less surface area for a given volume.  Similarly, stronger cementation in sedimentary rocks increases resistance, as does the crystalline nature of metamorphic rocks.  Carbonate rocks, such as limestone and dolomite, provide interesting exceptions to these rules.  In arid environments, they are resistant, but in wetter climates where they can dissolve, they are not resistant.

Differential erosion of sandstone vs shale, Utah.

Sandstone beds, being coarser grained and thicker bedded than the red-colored shale and siltstone, stand out in relief because they are more resistant to erosion. Utah.

As the single most important factor in weathering and erosion is water, it stands to reason that the presence of fractures or fault zones in rock, which tend to localize water flow, greatly influences the landscape. Additionally, movement along fault zones tends to crush some of the adjacent rock, which makes them even easier to erode.  As a result, fractures and faults frequently form canyons or valleys.  For example, at Arches and Canyonlands national park in Utah, vertical fractures erode into slot canyons to leave the intervening, non-fractured rock standing upright as narrow ridges called fins.  Some of these fins erode in from their sides to form arches.  At Pt. Reyes National Seashore in California, Tomales Bay protrudes inland as a long narrow bay eroded along the San Andreas fault. Click here to see a post about the San Andreas fault!

Vertical joints in sandstone, SE Utah.

Vertical fractures in sandstone eroding into fins, Arches National Park, Utah.

The vertical changes in a sequence of sedimentary rocks show the most predictable, yet dramatic, impacts on landscape, especially in arid landscapes. In flat-lying rocks, such as in many parts of the Colorado Plateau, the resistant rock units form cliffs whereas more easily erodable units form slopes.  The easily eroded slopes are typically littered with large blocks from the cliffs above. These blocks fall when erosion of the slopes undercuts the cliffs and gravity takes over.

Talus cones and Cedar Mesa, Utah

Edge of Cedar Mesa, Utah. The cliffs consist of resistant sandstone whereas the slopes are made of less resistant shale and siltstone.

Below is another example of how the resistant cliffs tend to erode by rock fall.

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Sandstone blocks, fallen from the cliffs above, onto slopes of less resistant shale and siltstone, Utah.

 

In places where the rocks are tilted, the more resistant rocks form ridges whereas the less resistant ones form valleys.  In places where the rock is folded, these ridges and valleys curve in the same way as the rocks.   Importantly, bedding takes on an important role in erosion of tilted rocks, as it encourages sliding of rock in the direction of tilt.  As a result, the ridges are typically asymmetric: they slope parallel to bedding on one side to create a “dip slope”, and form ledges and cliffs on the opposite side.

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Tilted resistant sandstone forms asymmetric ridge with dip-slope (on the left). Colorado.

The photo below shows numerous ridges held up by resistant sedimentary rock with intervening “strike” valleys eroded into less-resistant rock. The rocks dip steeply to the right (east).

Hogback Ridges near Mora, New Mexico

Aerial view of resistant hogback ridges and strike valleys near Mora, New Mexico.

 

Lava flows and metamorphic rocks exhibit layering that can influence topography in a similar way, although in these rocks, the extent of the layering tends to be more local and less predictable.  The widespread lava flows of late-Cenozoic age in the western US have probably the biggest effect on the landscape.  Many of these flows are relatively undeformed and so remain approximately flat-lying.  Similar to sedimentary sequences, they form large plateaus that in places are incised by deep canyons.  Examples of these flows include the basalt flows of the Snake River Plain in Idaho and the basalt flows of the Columbia Plateau in Oregon and Washington.

Lava Flows of Columbia River Basalt Group, Washington

Flat-lying Lava Flows of Columbia River Basalt Group, Washington

So landscapes are shaped by erosion, but the erosion depends on the weathering processes at hand and the rock type and structure. The weathering processes allow erosion to proceed and the rock type and structure guide both the weathering and erosion. The lead-off photo in this post shows canyons in the meandering Green River. It looks that way because the rocks are flat-lying. The stair-stepped topography down to the river, of cliffs and slopes, simply results from alternating resistant and less-resistant rock types. And the river? It’s entrenched in the canyons, probably because it continued to downcut as the region uplifted.

Here’s another example –another aerial from the Colorado Plateau –you can see how flat-lying rocks create the stair-stepped topography down to the river –and the uppermost resistant rock forms a nearly flat upland.

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Stair-stepped topography and entrenched meanders, northern Arizona.

Yay!

For more images of weathering and erosion, all freely downloadable, please check out my gallery of weathering and erosion photos –or go to the keyword search on my website and type in “weathering” or “erosion” –or both!

 

Oregon’s rocky headlands: geologic recycling through erosion and uplift and erosion…

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Crashing waves at Heceta Head, Oregon

You can’t avoid thinking about erosion while standing on one of Oregon’s rocky headlands. The waves keep coming, one after another, each crashing repeatedly against the same rock. Impossibly, the rock appears unmoved and unchanged. How can it not erode?

The answer, of course, is that headlands do erode, quickly, but on a geologic time scale. We just miss out because we live on the much shorter human time scale. And the erosion belongs to a cycle in which coastal uplift causes eroded and flattened headlands to rise and become headlands once again, all subject to more ongoing erosion and uplift.

Wave energy is most intense at headlands because the incoming wave typically feels the ocean bottom near the headland first, which causes the wave to refract. As shown in the aerial photo below, this refraction focuses the wave energy on the headland.

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Wave refraction causes wave energy to focus on the headland. Arrows are perpendicular to wave fronts.

As you can see in the next few images, headlands don’t erode evenly. They erode irregularly, as the waves exploit any kind of weakness in the rocks such as faults and fractures, or if they’re sedimentary, bedding surfaces. The products of this erosion are as beautiful as they are interesting: sea stacks, sea arches, sea caves… The list goes on and on.

Headland and lighthouse, Heceta Head, Oregon

Aerial view of Heceta Head, Oregon.

From the above photo, you can see that sea stacks are simply the leftover remains of a headland as it retreats from erosion. That’s a critical point, because some sea stacks, especially the one with the arch in the photo below, are a long way from today’s coastline.

Sea stacks and sea arch, southern Oregon

Sea stacks and sea arch, southern Oregon

Those rocks, 1/4 to a 1/2 mile away used to be a part of the coastline? The land used to be way out there? YES!!! For me, that’s one of the very coolest things about sea stacks –they so demonstrate the constant change taking place through erosion.

Taken to its extreme, erosion renders headlands into wave-cut platforms, such as the one below at Sunset Bay. Being in the intertidal zone, these platforms make great places for tide-pooling–and ironically, for people-watching too. Geologically, they form important markers because they’re both flat and form at sea level. When found at higher elevations, they indicate uplift.

Wave-cut bench, Sunset Bay, Oregon

Wave-cut bench at Sunset Bay, Oregon

In fact, looking carefully at the photo above, you can see a flat surface on the other side of the bay. It’s an uplifted wave-cut platform! Called a marine terrace, it’s covered by gravel and sand originally deposited in the intertidal zone. Those deposits rest on bedrock that, at an earlier time, was also flattened by the waves. The photo below shows a better view of this terrace from the other side.

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Breaking wave at Shore Acres State Park, Oregon. Tree-covered flat surface in the background is an uplifted marine terrace.

These uplifted marine terraces can be found up and down Oregon’s coastline. Researchers recognize several different levels, the oldest being those uplifted to highest elevations. The one in the photo above at Shore Acres State Park is called the Whiskey Run Terrace and formed about 80,000 years ago. You can see a similar-aged terrace below as the flat surface beneath the lighthouse at Cape Blanco, Oregon’s westernmost point. An older, higher terrace forms the grass-covered flat area on the right side of the photo.

Cape Blanco, Oregon

Cape Blanco, Oregon looking NE. The flat surface beneath the lighthouse is the ~80,000 year-old Cape Blanco Terrace, probably equivalent to the Whiskey Run Terrace at Shore Acres; the flat area on the right side of the photo is the higher Pioneer Terrace,  formed ~105,000 years ago.

Researchers take the approximate ages of the terraces and their elevations to calculate approximate rates of uplift. In this area, Kelsey (1990) estimated a rate of between 4-12 inches of uplift every 1000 years. That might seem slow, but over hundreds of thousands of years, it can accomplish a great deal.

And look! The uplifted terraces? They’re on headlands! Of course, because they’ve been uplifted! And the headlands are now eroding into sea stacks and then platforms –to be uplifted in the future and preserved as marine terraces that sit on top headlands. And on and on, as long as the coastline continues rising.

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Blowhole near Yachats, Oregon. Incoming wave funnels up a channel eroded along a fracture and explodes upwards on reaching the end.

Some links and references:
Kelsey, H.M., 1990, Late Quaternary deformation of marine terraces of the Cascadia Subduction Zone near Cape Blanco, Oregon: Tectonics, v. 9, p. 983-1014. (Detailed study of Cape Blanco, including uplift rates).

Miller, M., 2014, Roadside Geology of Oregon, Mountain Press, Missoula, 386p. (General reference which details the concepts and includes several of the photos used here).

Earth Science Photographs–free downloads for Instructors or anybody: my webpage!

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