Earth's Surface & Water — Weathering, Transport, and the Watershed
Start Here — What Module 2 Is About
Read this before anything else on the page. It says what this module is for and why it is worth your time.
Scholars, the same architecture that raises mountains also disassembles them; every ridge is a temporary balance between the forces that build rock and the water, ice, and gravity that quietly carry it back down. In this module you will trace a single grain from its parent rock through weathering, transport, and storage, and learn to read a landscape as a ledger of energy spent and sediment moved.
The Ideas
The load-bearing concepts of this module, in studio prose.
Two Ways Rock Comes Apart
Weathering is demolition that happens in place, and it runs on two distinct budgets. Mechanical weathering breaks rock without changing its chemistry: frost wedging pries joints apart as freezing water expands, unloading lets buried granite exfoliate in sheets once overburden erodes away, thermal cycling and abrasion chip exposed faces, and roots and burrowing organisms lever grains loose. Chemical weathering rewrites the minerals themselves — hydrolysis converts feldspar to clay, oxidation rusts iron-bearing minerals, and carbonation dissolves limestone as slightly acidic rainwater turns carbonate into soluble bicarbonate. The two budgets reinforce each other: mechanical fracturing multiplies surface area, and every new surface is fresh ground for chemical attack, while chemical rotting weakens rock so it fractures more easily. Climate sets the pace. Chemical weathering accelerates in warm, wet settings where reactions run fast and water is abundant, while mechanical weathering dominates cold or arid zones. Mineral stability matters too — the minerals that crystallize last and at lowest temperature are the most resistant at the surface, which is why quartz survives to become beach sand long after feldspar has become clay.
Soil: The Skin Where Four Spheres Meet
Soil is not merely crushed rock; it is the working interface where geosphere, atmosphere, hydrosphere, and biosphere overlap, and it develops a vertical order over time. A mature profile stacks horizons: an organic O layer and dark, humus-rich A layer on top, a leached E zone where percolating water strips soluble ions and fine clays downward, an accumulation B horizon where those clays and oxides collect, a partially weathered C horizon, and unweathered R bedrock below. Five factors govern how a soil turns out — climate, organisms, relief, parent material, and time — a set worth memorizing because it explains why identical bedrock yields different soils on a hot floodplain versus a cold ridge. Scholars should also separate residual soils, which form in place atop their parent bedrock, from transported soils, whose mineral grains were delivered by rivers, wind, or ice from somewhere else. That distinction carries a warning: the rock directly beneath a transported soil may have nothing to do with the sediment above it, so soil chemistry is not a reliable readout of local bedrock.
Gravity's Ledger and the Angle of Repose
Every slope keeps a balance sheet. The driving force is the downslope component of gravity, which grows with slope angle; the resisting force is friction and cohesion holding grains together. Divide resisting by driving and you have the Factor of Safety — above one the slope holds, below one it fails. For dry, loose material the balance point is the angle of repose, typically near thirty to thirty-five degrees, the steepest angle a pile of sand or gravel will sustain. Water is the usual trigger for failure: infiltrating rain fills pore spaces, and rising pore-water pressure carries part of the load, reducing the effective force pressing grains together and with it the frictional resistance — which is why landslides cluster after storms. Mass movements span a spectrum of speed. Creep is imperceptibly slow, tilting fence posts over years; slumps rotate a block along a curved surface; debris flows and mudflows race downhill as saturated slurries; rockfalls drop in seconds. Vegetation is the cheapest stabilizer — roots reinforce the regolith and canopies intercept rain — so clearing a hillside quietly lowers its Factor of Safety.
Sculptors of Wind and Ice
Where water is scarce or frozen, wind and glaciers take over the transport work. Wind is a selective agent because moving air carries little mass: it lifts fine silt and clay high as suspended dust while nudging sand grains along the ground in short hops called saltation, and those bouncing grains sandblast obstacles through abrasion. Deflation strips loose fines until only a coarse lag remains, and the exported silt can blanket downwind regions as thick, fertile loess. Sand that saltates piles into dunes that migrate downwind. Glaciers move far more material and with far more force. They erode by plucking — freezing onto bedrock and quarrying blocks as they advance — and by abrasion, grinding striations into the rock beneath their sediment-studded base. Alpine glaciers carve steep-walled U-shaped troughs, cirques, and horns, while continental ice sheets scour entire regions — stripping soil, gouging basins that later fill as lakes, and smoothing bedrock over thousands of square kilometers, as the lake-pocked Canadian Shield records. Their debris sorts into two families: unsorted till dumped directly by ice into moraines, and layered, sorted outwash deposited by the meltwater streaming from the glacier's snout.
The Watershed: Routing, Carrying, Storing
A watershed is every square meter that drains to a common outlet, bounded by divides that send rain one way or the other. Water arriving as precipitation is partitioned among evapotranspiration, runoff, and infiltration — over a year, the return of water to the atmosphere by evaporation and plant transpiration is often the largest of the three. The runoff concentrates into channels whose discharge follows continuity: discharge equals cross-sectional area times velocity, so for a given discharge a narrowing channel speeds up. A stream carries its load in three modes — dissolved ions, suspended fines, and bed load rolling along the bottom — and two limits govern how much: competence, the largest grain a current can move, rises steeply with velocity (the mass of the largest movable grain scales roughly with the sixth power of velocity, which is why a single flood can move boulders that decades of normal flow cannot), while capacity, the total quantity, rises with discharge. Rivers erode toward base level, the lowest elevation to which they can cut, approaching a graded profile that balances erosion and deposition. What does not run off or return to the sky soaks in, filling pore spaces below the water table in the saturated zone. Here porosity sets how much water rock can hold and permeability sets how fast it flows; a permeable, saturated unit is an aquifer. For Texas scholars this is not abstract — but the state's two famous aquifers pose different problems. The Ogallala stores largely ancient water and recharges only millimeters per year, so irrigation is drawing it down far faster than nature refills it; the karstic Edwards recharges rapidly through fractured limestone yet is acutely vulnerable to drought and contamination, which is why its pumping is tightly managed.
Mental Picture · Defuse the Traps
Where instinct misleads — and what the picture actually shows.
🧠 Defuse the trap: Fine clay washes away in the slowest current; coarse sand needs the fastest water to move.
Heavier grains obviously need more force, so erosion resistance should rise smoothly with grain size — big rocks hardest, dust easiest.
The Hjulström curve dips to a minimum around fine-to-medium sand, then turns back UP for silt and clay. Clay particles are so small that electrostatic cohesion binds them into a resistant surface, so eroding packed clay actually demands a HIGHER velocity than eroding loose sand. Sand, not clay, is entrained at the lowest velocity.
🧠 Defuse the trap: Weathering and erosion are two names for the same breakdown of rock.
Both words describe rock falling apart, so they must be interchangeable steps in one process.
Picture a granite boulder rotted soft by decades of chemical hydrolysis but still sitting exactly where it formed — that is intense weathering with zero erosion. Weathering is disassembly in place; erosion is the transport step that removes the debris. One can run without the other, and the two often occur in different places and times.
🧠 Defuse the trap: Groundwater flows through underground rivers and pools in vast hidden lakes.
We've all seen cave streams, so beneath our feet there must be a network of open channels and reservoirs of standing water.
In almost all aquifers, water occupies the microscopic pore spaces and fractures within rock and sediment, seeping slowly — often centimeters per day — through permeable material. The water table is a saturation surface, not the roof of an open cavern. Cave streams are the rare exception in soluble limestone, not the general model.
Standards in This Module
Selected planning targets from 19 TAC §112.49 Earth Systems Science. Slides support instruction; they do not by themselves prove full standards coverage or mastery.
- Direct planning targets: §112.49(c)(9)(B), (9)(C), (9)(D), and (12)(E).
- Instructional bridges: imagery, system models, hazard evidence, and resource evaluation connect the three publisher modules.
- Evidence boundary: watershed, erosion, groundwater, and landform investigations provide the evidence needed beyond slide review.
Lesson Slides
Open the Studio overview, publisher-module launch, or available lesson deck to review class work, catch up on a missed day, or pull a diagram into your notebook.
The Watershed Sandbox
An original studio instrument — explore it, then work the guided investigations below.
Want to See This in 3D?
Optional, and always second. The lab above is where you measure and calculate; this is the same phenomenon as an everyday object you can turn over and inspect. Open it when the lab already makes sense — or skip it entirely.
Practice Blueprint
The skills this module trains. Use it as your study checklist — the self-checking Practice Gym for this module follows underneath.
Draw elevation drop D from U[100,1200] m and channel length L from U[5,80] km. Ask for gradient in m/km = D/L, rounded to one decimal. Self-check: student value × L must reproduce D within rounding.
Randomly withhold one of {Q, width w∈U[4,40] m, depth d∈U[0.5,4] m, velocity v∈U[0.3,2.5] m/s}; A = w·d. Provide the other three and solve. Self-check: recomputing Q from w·d·v must match the stated discharge.
Well heads h1∈U[180,260] m and h2 = h1 − Δ where Δ∈U[3,40] m; horizontal separation x∈U[100,900] m. Gradient = (h1−h2)/x, express as decimal and percent. Self-check: gradient × x must equal the head difference.
Start with a cube of edge s∈U[4,12] cm; subdivide n∈{2,3,4} times per edge into n³ equal cubes. Original SA = 6s²; new total SA = 6s²·n. Ask for the multiplication factor and both areas. Self-check: factor must equal n. EXTENSION (solve-for-n): give a target factor k∈{5,8,10,20} and ask how finely the cube must be subdivided per edge to multiply surface area by at least k (answer: smallest integer n ≥ k), then connect to why fracturing accelerates chemical weathering.
Friction angle φ∈U[28,38]°, slope angle θ∈U[15,34]°. FS = tan(φ)/tan(θ). Ask for FS to two decimals and whether the slope is stable (FS>1) or fails (FS<1). Self-check: FS>1 exactly when φ>θ.
Rainfall R∈U[20,120] mm falls on catchment area Aᵣ∈U[2,50] hectares; infiltration fraction f∈U[0.2,0.7]. State the assumption explicitly in the stem: evapotranspiration is negligible over the storm event, so rainfall splits between runoff and infiltration only. Runoff depth = R·(1−f); runoff volume = runoff depth × area (convert mm·ha → m³ via ×10). Self-check: runoff volume + infiltration volume must equal R×Aᵣ×10.
Shaky on the quantitative footing? Visit Foundations. Ready for the course map? Back to the Earth Systems Science hub.