Rocks are not eternal. Even the tallest mountain will eventually dissolve and disintegrate. Geologists call this process "weathering." It sounds harmless enough, but weathering is one of the most destructive forces on the face of the planet.
The repeated growth of salt crystals from evaporating brine have carved this stoney coastline along the Great Ocean Road in Australia with distinctive honeycomb weathering. Loonie for scale. Image credit: Mika McKinnon
Weathering is a collection of physical and chemical processes that pulverize rocks over time. It can involve the physical disintegration and chemical decomposition of rocks into soil, loose clasts (rock shards), dissolved chemical components (ions), and solid chemical residues. In other words, weathering is so powerful that it can break rocks down into their fundamental molecular structures.
But usually, all this destruction takes place really slowly. The rate of weathering is driven by how the rock was formed, and what type of environmental conditions it is exposed to.
Rounded boulder in glacial moraine in Sólheimajökull, Iceland. Image credit: Dark Sapiens
Weathering and erosion go hand-in-hand, as wind, water, and gravity transport weathered fragments away from their original rock. When a rock weathers unevenly due to its composition and sensitivity to the weathering process, differential erosion occurs, producing unusual shapes and textures.
Differential erosion can highlight rock features by bringing the most resistant minerals into greater contrast, like the thin quartz veins in this outcrop on a windy beach near Jenner, California. Image credit: Christie Rowe
Weathering processes are broken down into two categories: physical weathering, and chemical weathering.
The Mechanical Breakdown Of Rocks By Physical Weathering
Prothero and Schwab accurately describe physical weathering is a "slow, unspectacular process" in their textbook Sedimentary Geology. Physical weathering is the actual, physical break-down of changing solid, large rock into smaller, movable unconsolidated debris. These physical processes are driven by changes in temperature, changes in pressure, and organic activity.
The Claron Formation in Bryce Canyon is limestone with a harder dolestone cap shaped into spires formed by frost wedging and freeze-thaw cycles. Image credit: Brian Castro
When the ambient temperature oscillates near 0°C, water will repeatedly freeze into ice, melt, and re-freeze. As ice has a larger volume than water, water in joints or fractures will expand as it freezes, wedging the fractures into larger cracks. During melting, water will fill the new, larger crack, forcing it even larger during the next freeze. This freeze-thaw weathering is most effective in rocks with lots of fractures, in moist climates hovering near 0°C. The timescale of the temperature variation (hours, days, weeks, or months) will impact the rate of oscillation between freeze and thaw, with faster oscillations producing faster weathering.
In hotter climates, salt crystallization of evaporating brine can produce similar effects, with salt crystals playing the role of ice crystals.
The ocean fills this pit on the coast near Melbourne, Australia with seawater each high tide. The water evaporates, leaving behind salt crystals that break the rock, expanding the pit and allowing it to fill with even more brine. Loonie for scale. Image credit: Mika McKinnon
When the ambient temperature shows significant daily variation (±20-30°C), the thermal contraction and expansion of the component minerals can break the rocks apart. Rocks subject to repeated hydration (thus swelling) and desiccation (thus contracting) will be subject to similar stresses. This insolation weathering produces identical weathering products to freeze-thaw weathering.
Stress has broken this outcrop in Tanzania apart, with rock shards precariously balanced. Image credit: Mika McKinnon
As a rock is unburied, the removal of overburden drops the confining pressure on the rock. This stress release results in expansion of the rock, producing expansion cracks or joints roughly parallel to the ground surface. Over time, these develop into curved, subparallel cracks between onion-like sheets on the rocks. As the curved slabs are spalled off, the exfoliation produces roughly spheroidal cores. Weathering is typically an even duller process to watch than paint drying, but exfoliation can be downright exciting with startling pops, ominous bangs, and spiderwebbing cracks marking a sudden release of stress.
Spheroidal weathering of a Quaternary glacial erratic near Sierra de Gredos, Ávila, Spain. Image credit: Nahum Mendez Chazarra
Finally, lifeforms that eke out a living on or in rocks can pry apart cracks, ingest mineral components, or burrow through material. This biological weathering is just one of the many ways in which geology and biology are more related than it seems like they should be.
Ants break compacted sediments into small fragments as they tunnel, carrying the clasts out of their nest in Tanzania. Image credit: Mika McKinnon
The Dissolution Of Rocks By Chemical Weathering
Chemical weathering is the process of dissolving rocks completely, or dissolving some components resulting in new, altered minerals.
An intact quartz-calcite vein in northern British Columbia (left), and an adjacent vein where the calcite dissolved, leaving behind a lacy framework of quartz (right). Moss grows in the pits, further breaking down the rocks with biological weathering. Image credits: Christie Rowe
Simple solution is when a solid mineral is exposed to water or an acid, dissolving some ions into solution. Halite (rock salt) exposed to water will dissolve completely into sodium and chlorine ions. Calcite exposed to acid will dissolve, resulting in surface pitting or even entire cave networks (karst terrain). Hydrolysis is when a mineral with mobile cations (positively-charged ions) is exposed to hydrogen ions (H+), resulted in a dissolved or altered rock. Calcite exposed to acid is a form of hydrolysis where no solid residue remains; other examples include feldspars altering to kaolinite clays or olivine dissolving into magnesium ions and dissolved silica.
Scientists think the floors of Noctis Labyrinthus on Mars are covered in a hydrated silica mineral like opal, formed by the weathering of basaltic laval flows or volcanic ash in the presence of water. Image credit: NASA/JPL/University of Arizona
Hydration of rocks occurs when a mineral is exposed to water and altered into a hydrated variant of the mineral. Dehydration is the inverse process, drying out hydrated minerals into dehydrated equivalents. Examples include gypsum dehydrating into anhydrite, the iron oxide hematite hydrating into the corrosive rusted limonite, or kaolinite clay hydrating into gibbsite clay and dissolved ions. Hydrated minerals are typically softer than their dehydrated counterparts.
Reduction spotted this Permian desert sandstone on the Isle of Arran, Scotland with pale green splotches. A tiny black speck within the center spot is a trace of organic matter. Pen for scale. Image credit: Laura Hamilton
Finally, oxidation and reduction are the paired processes of oxygen losing and gaining electrons when a rock is exposed to the atmosphere. During oxidation, rusted materials are produced: pyroxene exposed to air and water results in limonite and dissolved silica, pyrite exposed to air produces hematite and dissolved sulfur. While oxidated minerals are rusty yellows, oranges, or reds, the inverse process of reduction in oxygen-poor environments produces minerals that are more sickly greens and grays.
Parts of this banded iron formation in the Red Lake District of Ontario are the original black-and-white, while patches of pyrite and magnetite have oxidized to rusty red. Lens cap for scale. Image credit: Christie Rowe
All these processes can be linked together into weathering chains. Shiny pyrite, more commonly known as fool's gold, oxidizes into vivid red hematite, which in turn hydrates into soft, yellowish limonite. Another chain is feldspar decomposing into kaolinite then later gibbsite clay with exposure to water and hydrogen ions. As feldspar is one of the most common minerals in granites, this weathering chain once led to me identifying old volcanic rocks in Australia by poking them and having the now-clay be so soft my finger sank in to the first knuckle.
Basalt cobbles cocooned in squishy clay mark an ancient volcanic flow on a beach near Melbourne, Australia. Geoscientist for scale. Image credit: Mika McKinnon
Chemical reactions are reversible in favourable conditions providing all the components are still present; it is only once the reaction products are transported away that the rock can be considered permanently altered.
How A Rock's Environment Influences Weathering
Weathering can happen anywhere, even on other planets. The environmental conditions will impact what types of weathering will dominate, with rocks breaking down fastest in conditions farthest from the conditions under which they formed.
The iron in this Triassic desert sandstone in Budleigh Salterton in Devon, United Kingdom has leached out, highlighting the contact between overlaying sandstone and underlaying pebble beds. Pencil for scale. Image credit: Laura Hamilton
Areas with high relief, high elevation, or at high latitudes are more likely to be subject to faster rates of physical weathering than chemical weathering, while areas of low relief, low elevation, and low latitudes are more likely to be subject to faster chemical weathering than physical weathering. Physical weathering will dominate in the arctic, in mountain environments, or in deserts, while chemical weathering will dominate in swamps, the tropics, and depositional basins.
Physical weathering is fastest areas of high topographic relief, in regions with temperature variation around the freeze-thaw temperature (0°C), or in regions with extreme daily temperature variation. Rocks with lots of existing fissures or joints weather more rapidly as various physical processes expand existing cracks, which in turn creates more surface area for those processes to weather the rock even further.
The Claron Formation in Bryce Canyon required freezing temperatures to shape its distinctive spires. Image credit: Brian Castro
Chemical weathering is fastest in warm, wet climates (like the tropics). Hot, dry climates lack the water necessary for hydrolysis, simple solution, or hydration, while water is frozen into ice in colder climates. The abundance of hydrogen ions (acids) increases hydrolysis: more acidic waters occur in waterlogged soils, bogs, mine water, and rain, while more alkaline waters occur in saline water (oceans), groundwater, or streams. Areas with good drainage remove reaction products faster, preventing re-reaction, while areas of high relief may remove debris through physical weathering before much chemical decomposition has occurred. Oxidation happens when minerals are exposed to air, while reduction happen when minerals are in a low-oxygen environment like a swamp. Minerals with finer-grained textures or lots of fissures have more grain surface area per unit volume, increasing the area for chemical reactions to take place.
Warm temperatures and high humidity are accelerating chemical weathering of this roadcut in Tanzania. Image credit: Mika McKinnon
The key rule of weathering is that the farther a rock is from its formation conditions, the more quickly it will be destroyed. Rocks are most stable at their formation conditions: if a rock remains at the same the temperature, pressure, and other environmental conditions (water and air) that were present at the time the rock crystallized or lithified (compacted and cemented), it is more suited it is to continue existing in those same conditions. Rocks that formed at the surface are at equilibrium with surface conditions, and are more resistant to weathering. Rocks that formed far underground are not at equilibrium with surface conditions, and are more prone to weathering.
A gossan, a large, thick splotch of oxidized rock, can be a surface indication of deeper ore, like this gossan marking nickel in northwestern Ontario. Geoscientists for scale. Image credit: Christie Rowe
While slow and predictable, weathering is also a disconcertingly destructive process. Weathering grinds mountains into plains, and rocks into soils. Without it, we'd live in a world of inhospitable and unending rock. Weathering processes are spectacular mostly in their complete dullness, yet result in striking landforms carved by differential erosion, and intriguing textures of naturally-occurring negative relief maps. The weathering rates of gravestones can help scientists map out a local history of climatic change and acid rain. A changing climate can halt entire processes of weathering over to whole new forms of destruction based on the new environment, writing a history of change in the rocks. Weathering is the ultimate slow-and-steady tortoise racing the hare, and if you underestimate it, it will take you by surprise as you discover that "rock solid" doesn't necessarily stay that way.
Plant roots help to weather lava flows into soil on Rangitoto Island, the volcano near Auckland, New Zealand. Image credit: Mika McKinnon
This explainer is based on an article that originally appeared on GeoMika. Thank you to all of those who gave me permission to use their photographs of weathered rocks! All image rights reserved to their respective photographers.