Rocks, Landscapes & Water — Print · Earth & Space Science, Grade 9

Minerals & Their Properties

Learning goals

• I can list the five traits that define a mineral.
• I can identify a mineral using color, streak, luster, and hardness.
• I can use the Mohs hardness scale to predict which minerals can scratch each other.

Readings for this unit

• What happens to rain after it reaches the ground?

Not every hard, shiny rock you pick up is a mineral — to earn that name, a substance has to pass a strict five-part test. It must be (1) naturally occurring, (2) inorganic (not made by living things), (3) a solid, (4) of a definite chemical composition, and (5) built from atoms locked in an orderly, repeating pattern called a crystal structure. That last requirement is the secret behind almost everything else about a mineral: because the atoms line up the same way all through it, they give each mineral its characteristic hardness, its shape, and the way it breaks. Quartz and feldspar are minerals; granite is *not*, because it's a mixture of several minerals — quartz, feldspar, and mica — cemented together, more like a fruitcake than a single ingredient.

Because two different minerals can look almost identical at a glance, geologists don't trust their eyes alone — they run quick physical tests. Color is actually the least reliable clue of all, because tiny impurities can paint the very same mineral wildly different shades: pure quartz is clear, but a trace of iron can make it purple (the gem amethyst), and other impurities turn it pink or smoky, all still quartz. So color can start a guess, but it rarely settles one.

A far more honest test is streak — the color of a mineral's powder, which you reveal by scraping it across a rough, unglazed porcelain tile. Even when a mineral's outer surface is stained, tarnished, or weathered, its powdered streak stays true, which is why two shiny silver-gray minerals can be told apart the instant one leaves a reddish-brown streak and the other a gray-black one. Luster describes the way a mineral's surface reflects light — is it metallic, gleaming like a polished spoon, or non-metallic, looking glassy, pearly, waxy, dull, or earthy? And watch how a mineral breaks: some split along smooth, flat cleavage planes that follow the neat rows of their crystal structure, while others shatter along rough, uneven fracture surfaces — yet another clue to the hidden order inside.

One of the most useful tests of all is hardness — how well a mineral resists being scratched — measured on the Mohs hardness scale, which runs from 1 (softest) to 10 (hardest). Ten reference minerals anchor it: talc (1), gypsum (2), calcite (3), fluorite (4), apatite (5), feldspar (6), quartz (7), topaz (8), corundum (9), and diamond (10), the hardest natural material on Earth. The rule behind the scale is beautifully simple: a mineral will scratch anything softer than itself and be scratched by anything harder.

You don't need a fancy kit to use it, either — the objects in your pocket work fine as testing tools. Your fingernail has a hardness of about 2.5, a copper penny about 3.5, a piece of glass or a steel knife blade about 5.5, and a steel file about 6.5. If a mystery mineral scratches glass but not a steel file, you've already pinned its hardness between 5.5 and 6.5 without any lab at all. That's the real power of these properties: with nothing more than a porcelain tile, a nail, and a coin, you can identify minerals right there in the field — the same detective work professional geologists rely on every day.

A mineral is identified using several properties, such as hardness, streak, cleavage and crystal form. A rock is an aggregate of one or more minerals or other materials. Color alone can vary with impurities and is usually a weak identification test.

The Mohs scale ranks scratch resistance. Its numbered steps are not equal physical intervals, so hardness 8 is not twice hardness 4. Test which sample scratches which using a specified safe procedure rather than averaging the scale as if it were linear.

Record uncertainty when a property is ambiguous. A weathered surface may hide cleavage, and mixtures can show several streaks or grains. A defensible identification names the observed evidence and a next test instead of guessing from one familiar color.

Worked examples

Avoid these mistakes

• Mohs hardness is an ordered scale, not a linear measurement of strength.

Words to know

mineral

A naturally occurring, inorganic solid with a definite chemical composition and an orderly, repeating crystal structure.

luster

How a mineral's surface reflects light — described as metallic (shiny like metal) or non-metallic (glassy, pearly, dull, or earthy).

streak

The color of a mineral's powder, revealed by rubbing it across an unglazed porcelain plate; more reliable than color for identification.

hardness

A mineral's resistance to being scratched, ranked on the 1–10 Mohs scale.

Same atoms, totally different mineral: the polymorph surprise

Graphite (the 'lead' in your pencil) and diamond (the hardest natural material on Earth) are made of exactly the same element — pure carbon — and nothing else. The dramatic difference comes entirely from how the carbon atoms are arranged. In graphite, carbon atoms bond into flat, weakly-stacked sheets that slide past each other easily, which is why it feels soft and smudges onto paper. In diamond, each carbon atom bonds tightly to four neighbors in a rigid three-dimensional lattice, making it extraordinarily hard. Minerals like this, with identical chemistry but different crystal structures, are called polymorphs.

Cleavage is really about weak spots in the atomic lattice

A mineral doesn't cleave along smooth planes by accident — it's breaking along the weakest bonds in its atomic structure, the same way a piece of paper tears more easily along its grain. Mica is the classic example: its atoms are arranged in strong, tightly bonded sheets with much weaker bonds between the sheets, so mica peels apart into thin, flat, almost transparent layers with almost no effort, a property called perfect cleavage in one direction. Other minerals cleave in two, three, or more directions depending on how many weak planes their crystal structure contains — which is exactly why cleavage is such a reliable identification clue.

The Mohs scale's hidden trap: it isn't a ruler

When Austrian mineralogist Friedrich Mohs created his hardness scale in 1812, he ranked ten minerals in order of which could scratch which — but the scale is ordinal, not evenly spaced like a ruler. The actual jump in absolute hardness between each step gets bigger near the top: diamond (10) is roughly four times harder in absolute terms than corundum (9), even though they're only one step apart on the scale. So while the Mohs scale is a fantastic quick field test, it would be a mistake to assume a mineral rated '8' is exactly 'twice as hard' as one rated '4.'

New York's own claims to mineral fame

New York doesn't just teach about minerals — it produces some famous ones. The Adirondack Mountains are home to some of the largest garnet mines on Earth, including operations near North Creek that have supplied industrial garnet abrasive for over a century (garnet is New York's official state gemstone). Farther south, quarries near Herkimer, NY produce 'Herkimer diamonds' — not true diamonds at all, but unusually clear, double-terminated quartz crystals that form naturally with points on both ends, prized by collectors for their exceptional sparkle and clarity.

In the real world

Geologists and jewelers use the same streak, hardness, and luster tests to tell a genuine diamond from cheap glass or to identify valuable ore minerals in rock samples brought up from a mine.

Exit check

A specimen scratches calcite (3) but is scratched by apatite (5). What can you conclude about Mohs hardness?

The Rock Cycle

Learning goals

• I can name the three rock types and the processes that transform one into another.
• I can trace a path through the rock cycle starting from any rock type.
• I can explain why the rock cycle has no true beginning or end.

Readings for this unit

• What happens to rain after it reaches the ground?

Rock might seem like the most permanent, unchanging stuff on Earth, but on a long enough timescale it is constantly being remade. The rock cycle is the name for that endless recycling — the set of processes that shuffle the same Earth material back and forth among the three great rock families: igneous, sedimentary, and metamorphic. Unlike a story, the cycle has no first page and no last page. You can jump in at any rock type and follow the arrows around forever, and no rock is ever truly 'finished' — it's only paused between transformations.

Every arrow in the cycle is a real, physical process. Magma — molten rock, called lava once it erupts — cools and hardens into igneous rock. Any rock left exposed at the surface gets broken down by weathering and hauled away by erosion, ending up as loose sediment. Bury that sediment deep enough and the weight of everything above squeezes it together while mineral-rich water glues the grains into stone, a two-step process called lithification that produces sedimentary rock. Push a rock deep enough to bake under intense heat and pressure without quite melting, and metamorphism reshapes it into metamorphic rock. And if any rock at all — igneous, sedimentary, or metamorphic — gets hot enough to melt completely, it becomes magma again, and the whole loop can restart.

What powers all this motion? Two energy sources working from opposite directions. From above, the Sun and gravity drive the surface half of the cycle — sunlight powers the weather that wears rock down, and gravity pulls the loosened sediment downhill to be buried. From below, Earth's own internal heat drives the deep half — it melts rock into magma and cooks rock into metamorphic form. The rock cycle is really the place where those two great engines, one solar and one geologic, meet and hand material back and forth.

Notice that there is no fixed route through the cycle. A sedimentary rock doesn't have to become metamorphic next; depending on what happens to it, it could be melted into magma, or weathered back into sediment, or left untouched for a hundred million years. Because any rock type can, in principle, turn into any other, geologists draw the rock cycle as a wheel with arrows crossing every which way, not a straight assembly line.

Through all of this, the atoms themselves are never used up — they're just endlessly rearranged, a neat everyday example of the conservation of matter. A single atom of silicon riding in a grain of beach sand today might have been part of a volcano's glowing lava a million years ago, and might be locked inside the metamorphic heart of a mountain a million years from now. Nothing is created or destroyed; the same material simply keeps taking new forms, which is why geologists say the rock cycle has been running nonstop since Earth's crust first cooled.

The rock cycle is a network of possible paths rather than one required loop. A sedimentary rock can be weathered into sediment again, transformed by heat and pressure, or melted. The conditions determine the next process.

Rock material and energy have different roles. Mineral grains are matter that may be rearranged or chemically changed; energy from Earth's interior and the Sun helps drive processes. A rock-cycle arrow should name a mechanism, not simply point to the next rock type.

Rates vary greatly. A sediment deposit can form quickly during a flood, while burial, uplift and erosion may take much longer. A diagram simplifies those timescales. Trace one possible history and identify evidence, such as texture or mineral changes, that would support it.

Worked examples

Avoid these mistakes

• A rock does not have to visit every rock type in a fixed order.

Words to know

rock cycle

The continuous, looping set of processes — cooling, weathering and erosion, lithification, metamorphism, and melting — that recycle Earth's materials among igneous, sedimentary, and metamorphic rock.

lithification

The combined process of compaction (squeezing sediment together) and cementation (gluing sediment grains with natural mineral 'glue') that turns loose sediment into solid sedimentary rock.

metamorphism

The process by which heat and/or pressure change an existing rock into metamorphic rock without melting it.

magma

Molten (melted) rock beneath Earth's surface; the same material is called lava once it erupts above the surface.

The same cycle runs on wildly different clocks

Not every path through the rock cycle takes the same amount of time. Lava erupting into cold seawater can cool into solid volcanic glass within seconds to minutes. A layer of mud might take just a few thousand years to become shale. But squeezing a rock through deep metamorphism, or slowly crystallizing a large body of magma into coarse-grained granite far underground, can take many hundreds of thousands to millions of years. The rock cycle isn't one single speed — it's an enormous range of processes that can each take a wildly different amount of time to complete.

The 18th-century farmer who discovered 'deep time'

Before the 1780s, most Europeans assumed Earth was only a few thousand years old. Scottish geologist James Hutton changed that after studying rock outcrops like Siccar Point, where he saw layers of clearly different ages and tilts stacked on top of each other — physical proof that enormously long spans of time must separate them. Hutton's conclusion that Earth showed 'no vestige of a beginning, no prospect of an end' launched the idea of deep time and the principle of uniformitarianism — that the slow processes shaping Earth today (erosion, sedimentation, uplift) are the same ones that shaped it in the ancient past.

Plate tectonics is the rock cycle's real engine

The rock cycle doesn't run on its own — it's powered by plate tectonics. Subduction constantly destroys old oceanic crust by dragging it back into the mantle to melt, which is why the oceanic crust on Earth today is never older than about 200 million years — geologically young. Continental crust, though, is rarely subducted because it's too buoyant, so pieces of it can survive for billions of years; the oldest known mineral grains on Earth, tiny zircon crystals found in Australia's Jack Hills, have been dated to about 4.4 billion years old — almost as old as Earth itself.

Why the rock cycle is a web, not a circle

Textbook diagrams sometimes make the rock cycle look like a simple loop — igneous to sedimentary to metamorphic and back again, in order. In reality, any rock type can transform directly into any other, skipping steps entirely: an igneous rock can be metamorphosed directly without ever becoming sedimentary first, and a metamorphic rock can be weathered into sediment without ever melting. A single mineral grain might get recycled through dozens of different paths over Earth's history. The 'cycle' is really a tangled web of possible routes, which is exactly why geologists draw arrows crossing every which way instead of one tidy loop.

In the real world

The sand on a beach, the marble in a monument, and the granite in a kitchen countertop are all snapshots of the same rock cycle, caught at different points in a process that never really stops.

Exit check

Describe a path from sedimentary rock to igneous rock, naming the processes.

The Three Rock Types

Learning goals

• I can describe how igneous, sedimentary, and metamorphic rocks each form.
• I can classify a rock as igneous, sedimentary, or metamorphic using clues like crystal size, layering, or foliation.
• I can name at least two example rocks from each rock type.

Readings for this unit

• What happens to rain after it reaches the ground?

Igneous rock forms when magma or lava cools. Slow cooling often permits larger crystals; rapid cooling commonly produces fine grains or glass. Identify minerals and texture together rather than deciding from crystal size alone.

Sedimentary rocks form through accumulation and lithification of sediment or chemical and biological processes. Layering, grain relationships, and fossils can provide evidence. Fossils are most common in sedimentary rocks, but their presence and preservation depend on the organism and later history.

Metamorphic rock changes through heat, pressure, fluids, and deformation without bulk melting. Foliation is a planar fabric, such as aligned minerals or cleavage, and does not always mean visible bands. Slate has fine cleavage; gneiss commonly has compositional bands.

Marble and quartzite commonly lack foliation even though their crystals formed or changed during metamorphism. Some sedimentary rocks also have crystalline textures. No single clue identifies every rock: compare multiple observations with a reference chart and state uncertainties.

Igneous texture records cooling conditions: slow cooling can allow large crystals, while rapid cooling can produce small crystals or glass. Texture and mineral composition should be combined; dark color alone does not determine every rock's identity.

Sedimentary rocks can contain fragments, chemical precipitates or remains of organisms. Bedding and fossils can support a sedimentary origin, but later deformation may change the appearance. Metamorphic rocks form by changes in solid rock under heat, pressure and fluids, without complete melting.

Identification is an evidence task. Compare grain size, mineral arrangement and reaction data with a reference chart. If only one property is provided, state the likely class and what additional observations are needed rather than claiming an exact rock name.

Build your understanding

Plan three teaching sessions: core explanation and Parts 1–2; Parts 3–4 with diagrams and reasoning checks; then the whole-unit application, worked examples, independent practice, and exit check. Read one part at a time, explain it without notes, and revisit any gap. Allow about 40–50 minutes per session; adjust after checking understanding.

Part 1 · Read igneous texture as cooling evidence

As minerals crystallize from a melt, atoms attach to growing crystals. More time for growth commonly permits larger grains, whereas rapid cooling can limit grain size or preserve glass. Intrusive settings cool beneath the surface, while extrusive settings involve erupted material; these categories help frame the inference but do not make texture a perfect location label. Gas bubbles can leave vesicles, or cavities. A named igneous rock identification combines texture with mineral composition, rather than treating grain size as a complete chemical analysis.

Read the visual model

Compare grain sizes using the same depicted specimen area. Shapes are schematic grains rather than individually identified minerals.

What can coarse interlocking crystals suggest, and what else is needed to name a rock?

Visual reasoning — after your attempt

They can suggest time for crystal growth under slower cooling, but composition and other texture/field evidence are needed. Some other rock types can also be crystalline.

Crystals grow as minerals form from cooling melt. Slower cooling commonly permits larger crystals; rapid cooling can produce fine grains or glass. Gas can leave vesicles. Combine texture with composition because grain size alone cannot identify a specific rock or prove every detail of its setting.

Coarse crystals support time for crystal growth, while a glassy texture supports very rapid cooling. A mixed texture can indicate changing conditions rather than a single constant cooling rate.

Pause and explain

What can coarse texture suggest, and what can it not uniquely establish?

Check your reasoning after answering

It supports slower crystal growth/cooling but does not identify every mineral, exact location or age.

Part 2 · Read sedimentary texture as transport evidence

Sorting describes the range of grain sizes; rounding describes how sharp or smooth grain edges are. Transport can select sizes and abrade grains, but source material, travel time and later alteration also matter. A glacier, stream, beach or wind system can produce different combinations under varying conditions. Some sedimentary rocks are chemical or biological deposits rather than transported fragments. Record multiple texture properties and field evidence before assigning an environment; a single rounded grain does not uniquely establish a beach.

Read the visual model

Compare size variation, not the number of drawn grains. The circles isolate sorting and intentionally do not show detailed angularity.

Why would similar grain size alone be insufficient to conclude a beach origin?

Visual reasoning — after your attempt

Different processes can sort grains. Rounding, composition, structures and outcrop/environmental relations supply additional constraints and help test alternatives.

Clastic sedimentary rock contains fragments whose size, sorting and rounding can record transport and deposition. Other sedimentary rocks form by precipitation or biological accumulation. Grain properties interact with sediment supply and later change; interpret them as evidence with conditions rather than perfect environmental labels.

Well-rounded gravel suggests abrasion during transport, but sorting and grain size provide additional constraints. A shell-rich rock can preserve biological accumulation without requiring every grain to have traveled far.

Pause and explain

Why use sorting and rounding together rather than a single feature?

Check your reasoning after answering

They record different aspects of transport and deposition, and one property can have several causes.

Part 3 · Read metamorphic texture as solid-state change

Foliation is a planar alignment or compositional banding related to deformation and mineral responses. Directed stress can favor orientations of platy minerals or segregated bands. But mineral composition matters: some rocks recrystallize without developing obvious foliation, as often occurs in marble. Foliation is not the same as original sedimentary bedding, although later deformation can affect both. Identify grains, minerals, fabric and field relations instead of classifying every banded specimen by appearance alone. The key metamorphic condition is solid-state change, not a requirement for visible stripes.

Read the visual model

Compare the aligned texture with the grain mosaic. The pressure arrows mark one simplified stress geometry, not complete three-dimensional forces.

Can a rock without foliation still be metamorphic? Explain a suitable mechanism.

Visual reasoning — after your attempt

Yes. It can recrystallize in the solid state without a pronounced aligned fabric, depending on composition and conditions; marble is a common example.

Recrystallization and deformation can alter grain relationships without melting. Foliation commonly reflects alignment or banding under directed stress; some metamorphic rocks are nonfoliated. Parent composition and conditions matter, so absence of foliation does not establish that metamorphism never occurred.

Marble can be nonfoliated; a mica-rich rock can develop aligned minerals. Describe the actual structure before naming a parent or pressure history.

Pause and explain

Can a nonfoliated rock be metamorphic?

Check your reasoning after answering

Yes; texture depends on composition and conditions, and examples include marble.

Part 4 · Combine specimen and outcrop evidence

An outcrop is exposed rock whose contacts and structures can be observed in place. A contact where a body cuts through another unit suggests a different history from a bed deposited continuously above it. Grain size, mineral types and fabrics supply specimen-scale evidence, while contacts supply field-scale evidence. The two should support one account, or their mismatch should be investigated. A photograph can show texture and contacts but may lack scale, hardness or microscopic detail. Keep a description of what is visible separate from what formation process is inferred.

Read the visual model

The schematic intrusion cuts the existing layered units. That relationship adds a sequence constraint beyond the word crystalline.

Why might a specimen photograph alone leave a rock’s origin uncertain?

Visual reasoning — after your attempt

Several pathways can yield crystals or bands, and the photograph may omit scale, mineral tests and field relations. Combine texture/composition with contacts and structure.

Identify a rock by a pattern of minerals, texture and formation evidence. Distinguish sedimentary bedding from metamorphic foliation where possible, and use outcrop relationships to test the proposed history. A photograph can reveal texture but may lack scale, hardness information or mineral detail.

If a banded sample could be layered sedimentary rock or foliated metamorphic rock, inspect grain alignment, recrystallization and geological context instead of deciding from bands alone.

Pause and explain

Why is banded appearance alone insufficient for a rock classification?

Check your reasoning after answering

Bedding and foliation can both appear banded; mineral relationships and context help distinguish them.

Apply the whole unit · Classify and justify three samples

Illustrative samples: A has interlocking coarse crystals; B has rounded cemented sand grains; C has aligned mica and recrystallized minerals.

Your task

Propose a rock family for each, explain the formation mechanism, and identify one additional observation needed before assigning an exact rock name.

What your response should show

• Use the supplied evidence accurately and label units or locations.
• Explain a physical mechanism or evidence-to-claim relationship.
• Show calculations or a clearly labeled model when the task calls for one.
• Identify a meaningful assumption, uncertainty, or competing explanation.

Response guidance — after your attempt

A supports igneous crystallization, B clastic sedimentary lithification, C solid-state metamorphism. Exact names require composition and suitable reference evidence. Describe uncertainty and do not treat a single texture as a complete geological history.

Scientific references and adapted source material

• USGS — groundwater flow and the water cycle
• OpenSciEd 6.4 — public material examples

Parent / teacher teaching notes

Use a table of observation, inference, mechanism and missing information. Check that the learner explains sedimentary cementation and metamorphism without substituting vague heat and pressure labels. Ask the learner to explain each visual label and causal step before calculating. Preserve a first model, attempt the changed independent case, then use feedback to revise. Diagrams state whether values are illustrative or observed. Selected OpenSciEd excerpts are middle-school source material used for supported foundations; the independent high-school explanations and supplements are not the complete OpenSciEd teacher editions or an endorsement.

Average Time per Sediment Layer

time per layer = total accumulation time ÷ number of layers

estimates how long, on average, each layer in a sequence of sedimentary rock took to form.

Worked examples

Avoid these mistakes

• Metamorphism involves solid-state change; complete melting starts an igneous path.

Words to know

igneous rock

Rock that forms when molten magma or lava cools and hardens (crystallizes).

sedimentary rock

Rock that forms when weathered sediment is deposited in layers, then compacted and cemented together.

metamorphic rock

Rock that forms when heat and/or pressure change an existing rock's structure or minerals without melting it.

foliation

The parallel banding or wavy layering seen in some metamorphic rocks, created when pressure lines up minerals in the same direction.

Bowen's Reaction Series: the order minerals crystallize in

As magma cools, its minerals don't all crystallize at once — they solidify in a predictable order first worked out by geologist Norman L. Bowen in the early 1900s. Very high-temperature minerals like olivine crystallize first, followed by progressively lower-temperature minerals, with quartz forming dead last as the magma finishes cooling. This explains why certain minerals are reliably found together in the same igneous rock and others almost never are, and it separates igneous rocks into mafic (dark, iron- and magnesium-rich, like basalt) and felsic (light-colored, silica-rich, like granite) families.

Metamorphic 'index minerals' act like a thermometer

As heat and pressure increase during metamorphism, specific new minerals grow in a predictable sequence — chlorite at the lowest grade, then biotite, then garnet, then staurolite at progressively higher temperatures and pressures. Geologists call these index minerals, and finding one in a rock is like reading a built-in thermometer for how intensely that rock was metamorphosed. This also lets geologists distinguish regional metamorphism (huge areas transformed by mountain-building pressure, like the Appalachians) from contact metamorphism (a much smaller zone 'baked' next to a hot magma intrusion).

Sedimentary rocks are a fossilized weather report

Beyond fossils, sedimentary rock preserves physical structures that reveal the exact environment where they formed. Ripple marks record ancient shallow water or wind-blown dunes; mudcracks reveal a surface that dried out in the sun; cross-bedding (slanted internal layers) can reveal the direction an ancient river flowed or the shape of an ancient sand dune. Geologists can 'read' these clues the same way a detective reads a crime scene, reconstructing whether a rock now sitting in New York State once formed at the bottom of a tropical sea, a desert, or a river delta.

Why Manhattan can hold up skyscrapers

It's not a coincidence that New York City's tallest buildings cluster in Lower Manhattan and Midtown: both sit on outcrops of incredibly strong, ancient metamorphic bedrock — Manhattan schist and Fordham gneiss — that can support enormous weight close to the surface. In between, where that solid bedrock dips deeper underground (around Greenwich Village and SoHo), builders historically avoided the tallest towers because reaching a solid foundation would have required much deeper, more expensive excavation. The skyline's shape is, in part, a map of the metamorphic rock hiding beneath the streets.

In the real world

Builders choose granite countertops for their hardness and interlocking crystals, while sculptors prize marble for carving because its non-foliated, evenly sized crystals let a chisel cut smoothly in any direction.

Exit check

Why are visible crystals alone insufficient to identify a rock as igneous?

Weathering, Erosion & Deposition

Learning goals

• I can distinguish physical (mechanical) weathering from chemical weathering.
• I can explain how erosion transports sediment and what causes deposition.
• I can connect weathering, erosion, and deposition to landforms shaped by New York's glacial history.

Readings for this unit

• What happens to rain after it reaches the ground?

Weathering is the slow breaking-down of rock right where it sits — nothing is carried away yet, the rock is just being taken apart in place. It comes in two flavors, and the difference between them is whether the rock's actual chemistry changes. Physical (or mechanical) weathering changes only a rock's size and shape, not what it is made of.

The most powerful physical weathering agent in a place like New York is frost wedging: water trickles into a crack, freezes, and — because water expands by about 9 percent when it turns to ice — pushes outward with surprising force, prying the crack a little wider with every freeze. Repeat that through hundreds of winters and it can split a boulder clean apart. Plant roots wedge into cracks and widen them the same way as they grow (root wedging), and rock tumbling against rock in a river or a rockslide grinds down surfaces through abrasion. All of it makes rock smaller without changing the mineral it is made of.

Chemical weathering, by contrast, actually rearranges what the rock is made of. Rainwater is naturally a little acidic, and over long spans of time it slowly dissolves limestone — the very same process that hollows out caves underground. Oxygen in air and water attacks iron-bearing minerals and rusts them (oxidation), which is why some rocks and soils are stained a red-brown color, and water can dissolve certain minerals away entirely. Warm, wet climates speed chemical weathering up; cold or dry ones slow it down.

Once rock has been broken into loose sediment, erosion is the step that picks it up and carries it somewhere new. The movers are gravity (landslides and slow downhill creep), running water (rivers, rain, crashing waves), wind, and ice (glaciers grinding forward). It helps to think of erosion as the moving truck of the rock cycle: it doesn't make any new sediment, it just transports what weathering already loosened, usually downhill and eventually toward the sea.

Every moving agent has a limited budget of energy, and the moment it slows down and runs low, it can no longer hold onto its load — so it drops it. That dropping is deposition. A rushing river that suddenly widens into a calm lake loses its energy and dumps its sediment, often piling up a fan-shaped delta; slowing wind lets its sand settle into dunes; a melting glacier simply lowers its cargo of rock to the ground as the ice disappears. Together, this trio — weathering, erosion, and deposition — has been quietly sculpting the land around you for ages. The very hills, ridges, and lake-dotted valleys of Westchester County and the Hudson Valley were carved out and then piled up by glaciers that advanced and retreated across the region during the last Ice Age.

Weathering breaks or changes rock where it is; erosion transports the products; deposition occurs when transport capacity falls. One place can experience all three over time. Label the process based on what happens to the material, not just the landscape's appearance.

Water, wind, ice and gravity sort and move particles differently. A slowing stream commonly drops coarser sediment first, but particle density and shape also matter. Compare observations with stated flow conditions rather than assuming every deposit follows one exact size rule.

Vegetation, slope, rainfall and land use affect erosion. Roots can stabilize soil, while removing cover can increase runoff. Test a proposed intervention on equal slopes and soil amounts, collect repeated runoff measurements and report whether a model tray represents a full watershed.

Build your understanding

Plan three teaching sessions: core explanation and Parts 1–2; Parts 3–4 with diagrams and reasoning checks; then the whole-unit application, worked examples, independent practice, and exit check. Read one part at a time, explain it without notes, and revisit any gap. Allow about 40–50 minutes per session; adjust after checking understanding.

Part 1 · Distinguish changes in size from composition

Physical weathering changes size or shape without requiring a new mineral identity; chemical weathering changes minerals through reactions. Splitting a cube can expose surfaces previously inside it. A 2 cm cube has volume 8 cm³ and surface area 24 cm². Eight separated 1 cm cubes retain total volume 8 cm³ but expose 48 cm². Extra available area can increase contact with water or reactive gases, affecting reaction rate. This link depends on exposure and conditions: tightly packed fragments do not expose all modeled faces equally.

Read the visual model

Compare equal total volumes and count all six faces of each separated cube. The small cubes are not a different amount of material.

Why can physical breakdown accelerate chemical change without itself being a chemical reaction?

Visual reasoning — after your attempt

It can expose more reactive surface while retaining the mineral’s identity during the breakup. Subsequent reactions may then proceed over a larger available area.

Physical weathering changes size or shape; chemical weathering alters minerals through reactions. Both can operate together. Cracks increase exposed surface area, which can affect chemical reaction rates. Temperature, water, mineral type and exposure time affect rates; not every rock responds identically.

Breaking one cube into smaller cubes increases total exposed area without changing total material volume. The extra area can expose more mineral to water, connecting a physical change to a chemical process.

Pause and explain

Why can physical breakdown accelerate chemical weathering?

Check your reasoning after answering

It exposes more surface and pathways for reactions without itself requiring a composition change.

Part 2 · Explain transport through energy and material properties

Bed load moves along the bed by rolling, sliding or short hops; suspended load is carried within the water; dissolved load consists of chemical species in solution. A grain’s size, density and shape, the flow and interactions with other grains affect its transport and settling. Increasing flow can enable different transport, but a single universal speed threshold is not implied. Identify the pathway in the evidence: clear water can still carry dissolved material, and cloudy water need not have a large total transported mass if discharge is small.

Read the visual model

Compare markers near the bed with those within the water. The arrow indicates the selected flow direction, not a speed measurement.

Why should equal-size settling comparisons state particle density and shape?

Visual reasoning — after your attempt

Density contrast and resistance to motion depend on material and shape. Equal nominal size alone does not guarantee the same settling behavior or transport threshold.

Erosion involves removal and movement by water, wind, ice or gravity. Transport depends on fluid speed, particle size, density, shape and other conditions. A flowing stream can carry dissolved material, suspended particles and bed load simultaneously; transport evidence requires identifying the pathway.

When flow slows, its ability to transport some particles decreases. Larger or denser particles may settle sooner under the same conditions, but shape and turbulence can alter the simple size ordering.

Pause and explain

Why should a settling prediction state particle density and shape?

Check your reasoning after answering

They affect motion and settling along with size; size alone is not a complete model.

Part 3 · Link deposition to a changing environment

Deposition is accumulation of transported material, not necessarily a complete stop in fluid motion. Where a stream spreads into a basin, flow conditions change and some grains may settle while water continues through the system. Coarser or denser grains can behave differently from fine particles or dissolved material. Layering, sorting and structures can help test a depositional-environment interpretation, but sediment supply and later changes matter too. Explain what changed in transport capacity for the selected material rather than using deposition as a synonym for no motion.

Read the visual model

Trace water into the wider basin and identify the accumulating sediment wedge. Width and arrow lengths indicate a qualitative contrast only.

Why could some grains settle while fine material continues farther into the basin?

Visual reasoning — after your attempt

Different grain sizes, densities and shapes have different settling/transport behavior under the changed flow conditions. Depositing one fraction does not stop all water or all material transport.

Deposition occurs when transported material accumulates. Channel changes, entry into still water, reduced wind speed or melting ice can change transport conditions. Sorting and sediment structures can support an environmental inference but should be tested with multiple observations.

A river entering a lake can lose speed and deposit sediment. Coarser deposits near the entry and finer sediment farther out may support a gradient in transport, subject to flow and grain conditions.

Pause and explain

Does deposition mean that all water motion has stopped?

Check your reasoning after answering

No. Material can accumulate when transport conditions change even while water continues moving.

Part 4 · Use controls to test a landscape explanation

A runoff comparison should measure water volume as well as collected sediment mass. Here the bare tray yields 8 g in 80 mL, or 0.10 g/mL, while the covered tray yields 6 g in 40 mL, or 0.15 g/mL. The covered case exports less total mass but the collected water is more concentrated. These are distinct outcomes. Matched slope, rain input, material and collection time are needed to evaluate a cover effect. Real vegetation can alter infiltration, soil binding and storage, so a classroom tray models only selected processes.

Read the visual model

The upper bars measure volume and the lower bars mass. They are different units; do not compare their heights as one physical quantity.

Which tray exports less total sediment and which has the greater calculated concentration?

Visual reasoning — after your attempt

Covered exports less mass, 6 g rather than 8 g, but has greater concentration, 0.15 rather than 0.10 g/mL. State which outcome the design criterion concerns.

A classroom erosion comparison needs the same slope, applied water, material and collection method except for the variable being tested. Vegetation can affect runoff, infiltration and soil cohesion; a simplified tray represents only selected processes. Record both sediment and water so reduced runoff is not confused with reduced sediment concentration.

If one tray receives less water, a smaller sediment output cannot isolate the effect of plant cover. Compare outputs over equal rainfall input and explain remaining differences.

Pause and explain

Why measure runoff volume as well as sediment mass?

Check your reasoning after answering

Sediment mass and concentration answer different questions; volume helps distinguish transport amount from water amount.

Apply the whole unit · A controlled erosion comparison

Illustrative equal-rainfall trials: bare tray yields 2.0 L runoff and 12 g sediment; covered tray yields 1.0 L and 3 g.

Your task

Calculate sediment concentration for each tray, compare total loss and runoff, and explain what the result supports and what an actual experiment would need to control.

What your response should show

• Use the supplied evidence accurately and label units or locations.
• Explain a physical mechanism or evidence-to-claim relationship.
• Show calculations or a clearly labeled model when the task calls for one.
• Identify a meaningful assumption, uncertainty, or competing explanation.

Response guidance — after your attempt

Bare concentration = 6 g/L; covered = 3 g/L. The covered tray has less runoff, total sediment and concentration. Equal rain supports a comparison, but slope, soil, cover type and repeated trials are needed to isolate causes and generalize.

Scientific references and adapted source material

• USGS — groundwater flow and the water cycle
• OpenSciEd 6.4 — public material examples
• OpenSciEd middle-school material — CC BY 4.0 license

Parent / teacher teaching notes

Use supplied data or a parent-approved contained model. Mark units, equal-input logic and limits; do not accept less water proves less weathering without identifying the measured process. Ask the learner to explain each visual label and causal step before calculating. Preserve a first model, attempt the changed independent case, then use feedback to revise. Diagrams state whether values are illustrative or observed. Selected OpenSciEd excerpts are middle-school source material used for supported foundations; the independent high-school explanations and supplements are not the complete OpenSciEd teacher editions or an endorsement.

Deposition (Accumulation) Rate

amount deposited = rate × elapsed time

rate is how fast sediment builds up (for example, meters per 100 years for a glacial moraine); multiply by elapsed time to find the total thickness or amount deposited.

Worked examples

Avoid these mistakes

• Weathering and erosion are different processes even when they occur together.

Words to know

weathering

The breakdown of rock into smaller pieces in place, without being moved, caused by physical forces or chemical reactions.

erosion

The transport of weathered sediment from one location to another by water, wind, ice, or gravity.

deposition

The settling and dropping of transported sediment when the agent carrying it — water, wind, or ice — loses energy and slows down.

sediment

Small, loose pieces of rock, mineral, or organic material produced by weathering.

Differential weathering carves dramatic landscapes

When a rock formation contains layers or sections of differing hardness, weathering doesn't wear them down evenly — softer rock erodes away faster, while harder, more resistant rock is left standing, a process called differential weathering. This is exactly how features like hoodoos (tall, thin rock spires), natural arches, and even waterfalls form: a resistant cap rock protects softer rock underneath until undercutting eventually causes collapse. Niagara Falls is a spectacular local example — a hard dolostone caprock protects softer shale beneath it, and as the softer rock erodes away, the waterfall's edge slowly retreats upstream.

Climate controls which kind of weathering wins

Physical and chemical weathering don't happen at equal rates everywhere — climate tips the balance. Chemical weathering reactions speed up dramatically with heat and moisture, which is why it dominates in hot, humid, tropical climates, sometimes breaking rock down to great depths. Physical weathering, especially frost wedging, dominates in cold climates with frequent freezing and thawing, like much of the Northeast United States in winter. This means a single rock type could weather completely differently depending on where on Earth it ends up — a direct link between this unit and the climate patterns covered elsewhere in this course.

The Hudson River: technically a fjord

By strict geologic definition, a fjord is a valley carved by a glacier that was later flooded by the sea — and the lower Hudson River, from roughly Troy down to New York Harbor, fits that definition. Ice age glaciers gouged the Hudson's valley far deeper than sea level at the time; when the glaciers melted and sea level rose, seawater flooded back into that deep, glacially-carved channel, making the lower Hudson a true, tidal, saltwater-influenced fjord — one of the few on the entire Eastern Seaboard, and part of why the river's flow near NYC is influenced by ocean tides all the way up to Troy.

Long Island: a giant pile of glacial leftovers

Long Island isn't bedrock at all — nearly the entire island is a depositional landform, built from debris that glaciers bulldozed forward and dumped as they advanced and then retreated at the end of the last ice age. Long Island's twin forks and its hilly spine are terminal and recessional moraines (ridges of unsorted rock and sediment dropped right at a glacier's edge), while the flatter land to the south is an outwash plain, built from sediment-choked meltwater spreading finer material out ahead of the ice. In other words, the entire island is a monument to deposition.

In the real world

The moraines, drumlins, and river valleys across Westchester County and the Hudson Valley are direct evidence of glacial erosion and deposition from the last Ice Age, shaping the hills and lakes we see in the region today.

Exit check

Separate weathering, erosion, and deposition in one river example.

Groundwater, Watersheds & Water Budgets

Learning goals

• I can distinguish porosity from permeability and trace water through a watershed.
• I can use a water budget to evaluate a change in storage.

Readings for this unit

• What happens to rain after it reaches the ground?

Rain can run over land, infiltrate soil, enter groundwater, or return to the atmosphere. A watershed drains toward a common outlet, while groundwater follows hydraulic gradients that may not match surface boundaries exactly. Water and dissolved substances connect apparently separate parts of a landscape.

Porosity is the fraction of a material’s volume occupied by pores. Permeability describes how readily connected pores transmit water. A clay can have substantial pore space yet transmit water slowly because its pores are tiny and poorly connected. An aquifer is a body of material that can store and transmit usable quantities of groundwater.

A water budget accounts for flows: change in storage = inputs − outputs over a specified time and boundary. Pumping more water than recharge can lower groundwater storage. “Renewable” does not mean withdrawals can increase without limit. Paving surfaces can change runoff and recharge, while contaminants may move along subsurface flow paths.

A watershed drains surface water toward a common outlet. Groundwater moves through connected pore spaces and fractures, not usually through a vast open underground river. Porosity describes space available; permeability describes how easily water can pass through connected spaces.

An aquifer can store and transmit useful groundwater. Pumping faster than recharge can lower the water table and change connected stream flows. Recharge depends on infiltration, land cover, precipitation and geology, so a single wet week does not guarantee long-term replacement.

Use a budget: change in storage=inputs−outputs over the same interval. Include precipitation, surface and underground flows, evaporation and withdrawals when relevant. If a term is missing, mark the budget incomplete and explain how measuring it would improve the conclusion.

Worked examples

Avoid these mistakes

• High porosity does not guarantee high permeability.

Words to know

porosity

The fraction of a material’s volume that is pore space.

permeability

How readily water passes through connected pores.

recharge

Water entering groundwater storage.

In the real world

Exit check

Why must the time interval and system boundary be stated in a water budget?