Unit 7: Weathering, Erosion, and Soil
How rock breaks down where it sits, how the pieces get carried off and dropped somewhere new, and the soil that forms out of the whole process.
10 topics · 31 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Mechanical (Physical) Weathering
- How it worksHow does water expanding as it freezes wedge a rock's cracks a little wider with every freeze-thaw cycle?
- How it worksWalk me through how a plant root growing into a crack slowly pries a rock apart.
- Common mistakeWhy isn't mechanical weathering changing what a rock is chemically made of?
Your turn1 of 2
A boulder splits apart after many winters of water freezing and thawing in its cracks, with the ice never directly striking the rock's outer surface. Why can plain water freezing in a crack split apart a boulder that ice never even touches with force?Chemical Weathering
- How it worksHow does carbonic acid in rainwater dissolve limestone and other rocks over long periods of time?
- How it worksWalk me through iron in a rock rusting through oxidation until the whole surface crumbles.
- Common mistakeWhy isn't chemical weathering just mechanical weathering happening a little more slowly?
- Ties togetherHow does chemical weathering work faster in a warm, wet climate than it does in a cold, dry one?
Your turn1 of 2
A gravestone from centuries ago has an inscription so worn it can no longer be read, even though no one ever vandalized it. Why do gravestones from centuries ago sometimes have inscriptions you can no longer read at all?How Weathering Rate Depends on Climate
- How it worksHow do temperature and moisture together speed up both mechanical and chemical weathering?
- Worked exampleFind which climate weathers rock faster: one averaging 5 degrees C and dry, or one averaging 25 degrees C and humid.
- Going furtherWhat happens to a rock's weathering rate once it's broken into many smaller pieces with more surface exposed?
Your turn1 of 2
Chemical weathering's rate roughly doubles for every 10 degree Celsius rise in temperature. If a baseline weathering rate is 7 at 5 degrees Celsius, what is the approximate rate at 25 degrees Celsius, two 10 degree increases higher?Erosion by Water: Streams and Rivers
- How it worksHow does a stream's speed determine the size of sediment it's able to pick up and carry?
- How it worksWalk me through how a meandering river cuts a new curve into its bank on the outside of each bend.
- Common mistakeWhy doesn't a river erode its own bed evenly along the entire length of the channel?
Your turn1 of 2
A stream's competence, the maximum particle diameter it can move, increases with the sixth power of its velocity. If a stream's velocity increases by a factor of 2.45 during a flood, by what factor does its competence increase?Erosion by Wind
- How it worksHow does wind picking up sand grains sandblast a rock's surface unevenly over time?
- Going furtherWhat landform does wind build when it deposits sand instead of eroding rock?
- Ties togetherHow does wind erosion sort particles by size the same way water erosion does, only through the air?
Your turn1 of 2
A desert rock formation ends up top-heavy, with a wide cap balanced on a narrow, pinched base, after years of exposure to blowing wind and sand. Why do desert rock formations end up carved into strange, top-heavy shapes?Erosion by Ice: Glaciers
- How it worksHow does a glacier's own weight, plus rock embedded in its base, grind material off the valley floor as it flows?
- How it worksWalk me through a glacier picking up rock debris, carrying it downhill, and dropping it at its melting edge.
- Common mistakeWhy doesn't a glacier need to be moving fast to erode a huge amount of rock over time?
Your turn1 of 2
A valley carved by flowing water tends to keep a narrow, V-shaped cross section, while a valley carved by a glacier ends up broad and U-shaped. Why does a valley carved by a glacier end up U-shaped while a river valley stays V-shaped?Mass Wasting: Landslides and Slumps
- How it worksHow does added water or a steepened slope tip the balance between gravity and friction holding a slope together?
- How it worksWalk me through the curved scarp and rotated block of ground that forms during a slump.
- Going furtherWhat makes a slope more likely to fail right after a wildfire has burned off the vegetation holding it together?
Your turn1 of 2
Loose, dry soil or rock debris tends to stay stable on a slope up to its angle of repose, typically around 33 degrees, beyond which it fails and slides. A hillside is currently graded at 45 degrees. By how many degrees does it exceed the angle of repose?Deposition and the Landforms It Builds
- How it worksHow does a river losing speed at its mouth drop sediment and build a fan-shaped delta?
- Common mistakeWhy isn't all deposited sediment sorted by size the neat way a river delta's sediment is?
- Ties togetherHow does the same principle of losing energy explain both a river delta and a wind-built sand dune?
Your turn1 of 2
A river carrying sediment for hundreds of miles suddenly drops nearly all of it within a short distance of reaching the ocean, building a delta. Why does a river slow to a crawl and dump nearly all its sediment the moment it reaches the ocean?How Soil Forms From Parent Rock
- How it worksHow do weathering, organic matter, climate, and time combine to slowly build soil out of parent rock?
- Worked exampleFind roughly how long it takes to build 1 cm of soil if soil forms at about 1 cm per 200 to 400 years.
- Common mistakeWhy isn't soil just crushed-up rock with nothing at all living in it?
Your turn1 of 2
Soil forms at roughly 1 centimeter per 339 years under typical conditions. About how many years does it take to build 4 centimeters of soil at that rate?Soil Horizons
- How it worksHow do organic topsoil, mineral-rich subsoil, and weathered parent rock stack into the O, A, B, and C horizons?
- Going furtherWhat happens to a soil's horizon profile once its topsoil is stripped away by heavy erosion?
- Ties togetherHow does a soil's horizon profile reveal the climate and amount of time it took to form?
Your turn1 of 2
Digging a hole in undisturbed ground reveals a dark, organic-rich layer near the surface, a lighter mineral-rich layer beneath it, and partially weathered rock fragments deeper still. Why does digging a hole reveal distinct layers instead of the same dirt all the way down?