Differential erosion is a simple concept with a beautiful impact. Variability in rock hardness changes how it is sculpted by water and wind. It's a bit of a tease that geoscientists look at any strange landscape and attribute it to differential erosion, but it's not far off.
Rock hardness depends on the chemical composition and formation conditions of a rock impacts its strength. The farther a rock is from its formation conditions, the less at equilibrium it is with its surrounding environment. Minerals that form under surface conditions of temperature, pressure, and moisture are most resistant to weathering at the surface, while minerals that form deep within the Earth are weakest at the surface. Rocks that are crystalline solids are more resistant to weathering, while those with cracks, joints, and fractures have more exposed surface area prone to weathering. When rocks with different weathering properties are adjacent to each other, the result can be beautiful. Here's a few examples:
Top Image: Buttes are formed when a resistant cap-rock protects underlaying softer rock. Everywhere not covered by the hard rock erodes away, leaving a towering hill exposed above the surrounding landscape. Image credit: Wolfgang Staudt.
Valley and Ridges in the Zagros Mountains
The Zagros Mountains in southwestern Iran are long ridges and valleys dictated by differential erosion. Massive tectonic stress from collision rumpled the landscape into folds. Erosion removed the softer mudstone and siltstone, leaving behind harder limestone and dolomite. Image credit & read more: Earth Observatory.
Sandstone Steps on Mars
Sandstone with variable resistance to erosion forms these stair-step structures near the Kimberly waypoint on Mars. The foreground has weaker sandstones eroding into flatter steps, while background has stronger sandstones eroded to steeper steps. Image credit & read more: NASA.
Turnip Rock is a sea stack in Port Austin, Michigan. The top of the stack has more resistant rock than the bottom, so that the base erodes more quickly. The result is a lopsided stack, although I see it more as a mushroom than a root vegetable. Image credit: canonfather.
The Red Cliffs of Iron Canyon
The oxidized red sandstones alternate with white lakebed sediments in Red Rock Canyon State Park, California. The sandstones are more resistant to weathering, holding their shape better than the soft lacustrine sediments which erode deeply. Explore the Gigapan. Image credit: Ron Schott.
The Goblins of Goblin Valley
The Goblins of Goblin Valley State Park, Utah are actually hoodoos from the weathering of Entrada Sandstone. Once upon a time, the Entrada was a tidal flat, with alternating layers of sandstone, siltstone, and shale depositing to build layers of variable strength. Weathering enhances joints and fractures, with the greater surface area increasing weathering further until the goblin is free-standing in the plain. Explore the Gigapan. Image credit: Ron Schott.
Red Cliffs on a Strong Base
The red cliffs of Scarborough are softer sediments sitting on top of a hard rock base. The brown rough texture along the base of the cliff is a harder rock, holding a steeper face and eroding slower. The bright red of oxidized, rusted sediments are softer, eroding back in a gentler slope. Image credit: John (shebalso).
Sea Stacks in Italy
Sea Stacks form in medium-hard rocks that can hold a steep face, yet are soft enough to erode as free-standing structures along fractures and caves. The stacks at Torre Sant'Andrea have a harder, more resistant base of darker that juts out as platforms fringing the stacks. Image credit: Freddyballo.
Variation in thickness of shale levels in a gorge near Hector Falls, New York results in a jagged cliff edge. Image credit: Laurie VanVleet, Ithaca City School District.
The Red Castle of Argentina
The Los Castillos rock formation in Quebrada de las Conchas reserve in Argentina has alternating layers of hard rock forming steep faces, and softer rock. The changes in colour also reflect the changing chemical makeup of the rocks, with different minerals weathering through different processes into different colours. (Did I say "different" frequently enough in that paragraph?) Image credit: Mariano Mantel.
Volcanic Eruptions in Nicaragua
A pair of large volcanic deposits from explosive eruptions in northwestern Nicaragua show different erosional patterns. The harder rock presents a smooth, steep face, while the soft ash layer erodes in deep gullies. As the ash is eroded, it is carried away, but not very far: you can see it deposited at the base of the cliff as a splaying fan. Image credit: Ian Saginor.
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Do you have any favourite examples of differential erosion? Please share them!