Rocks, Weathering & the Rock Cycle · Grade 6 Science
Chapter: Earth Systems, Plates & Earth MaterialsRocks, Weathering & the Rock Cycle
Learning goals
- I can name the three rock groups and how each one forms.
- I can read cooling rate from the crystal size in an igneous rock.
- I can distinguish weathering from erosion and give an example of each.
Readings for this unit
Before this unit
Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.
Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.
Igneous rock forms when molten rock cools and solidifies, either underground as magma or at the surface as lava. Sedimentary rock forms when fragments and dissolved material settle, are buried, compacted and cemented together. Metamorphic rock forms when existing rock is changed by heat and pressure without complete melting, which is the condition that keeps it distinct from igneous rock.
That last point is worth pausing on. Heat a limestone enough and it recrystallises into marble, with the calcite grains growing larger and interlocking, while the rock stays solid throughout. Melt it completely and it would cool as an igneous rock instead, and the original structure would be gone.
The three groups are connected by processes that run in any direction. There is no first rock and no last one: granite can become sand, sand can become sandstone, sandstone can become quartzite, and quartzite can melt and cool as granite again.
Build your understanding
Where this fits
The last lesson identified individual minerals. This one is about how they get assembled into rock and taken apart again, and it closes the chapter on Earth materials. The weathering and erosion in the third section are the processes that feed the water cycle and the sediment load in chapter 6.
Use three flexible 40-45-minute sessions: the three rock groups and how each forms; crystal size as a record of cooling; weathering, erosion and the cycle, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.
Part 1 · Three routes to a rock
Igneous rock begins as melt at temperatures somewhere between about 700 and 1300 degrees Celsius and solidifies as it cools. Sedimentary rock begins as loose material: sand carried by a river, mud settling on a sea floor, or the shells of marine animals piling up over millions of years to become limestone. Burial under later layers squeezes the water out and presses the grains together, and dissolved minerals crystallise in the gaps to cement them, which is why sandstone is gritty and crumbly at the edges while granite is not.
Metamorphic rock is the group students find hardest, and the defining condition does the work. Existing rock is subjected to heat, pressure or both, deep in the crust or alongside an intrusion of magma, and its minerals recrystallise and often line up into bands, all without complete melting. Shale becomes slate, which splits into flat sheets because its flaky minerals have been rotated to lie at right angles to the squeezing. Limestone becomes marble. Granite becomes gneiss, with its light and dark minerals segregated into stripes. In each case the rock stayed solid, which is exactly why the original material can still be identified afterwards. The conditions needed are extreme by everyday standards and ordinary deep in the crust. Metamorphism typically begins around 200 degrees Celsius and continues up to about 800 degrees Celsius, under pressures found several kilometres down, which is why metamorphic rock at the surface has always been brought up by uplift and erosion after the event.
Pause and explain
Why does the definition of metamorphic rock insist that the change happens without complete melting?
Check your reasoning after answering
Because melting completely would erase the distinction. A rock that melts entirely and then cools solidifies as igneous rock, with no trace of its earlier structure. Metamorphism changes minerals and textures while the rock stays solid, which is why slate can still be recognised as altered shale and marble as altered limestone.
Part 2 · Crystal size as a clock
Granite and basalt can have almost the same chemistry and look completely different, and the reason is cooling rate. Crystals need time to grow, so magma cooling slowly deep underground over thousands or millions of years produces large interlocking crystals, often 1 to 10 mm across and sometimes far larger, which is why you can pick out the individual quartz, feldspar and mica grains in granite with the naked eye. Lava erupted at the surface cools in days or hours, giving crystals no time to grow, so basalt is uniformly dark with crystals too small to see without a microscope.
This turns a hand specimen into a record of where it formed. Large visible crystals mean the rock solidified at depth; crystals too small to see mean it solidified at or near the surface. Volcanic glass such as obsidian goes further: it cooled so fast that no crystals formed at all, which is why it has no ordered structure and fractures in smooth curved shells exactly as window glass does. Some rocks show two sizes at once, large crystals set in a fine background, which records magma that cooled slowly underground and then erupted and finished cooling quickly. The effect is easy to demonstrate outside geology. Cool a sugar solution slowly and you grow large single crystals of rock candy; cool the same solution fast and you get a mass of grains too small to pick out. Nothing about the sugar has changed, only the time available for crystals to form, and the rule transfers directly to molten rock.
Pause and explain
Two rocks have nearly the same chemistry, but one has crystals several millimetres across and the other has none you can see. What does this tell you?
Check your reasoning after answering
That they cooled at very different rates and therefore in different places. Large crystals need time to grow, so that rock solidified slowly deep underground. Crystals too small to see mean cooling took hours or days, which happens at or very near the surface after an eruption. The chemistry is the same; the history is not.
Part 3 · Breaking it and carrying it away
Weathering is the breakdown of rock where it stands, and it comes in two kinds. Physical weathering breaks rock into smaller pieces without changing it chemically: water freezing in a crack expands by about 9 per cent and levers it wider, and repeated heating and cooling makes surfaces flake away. Chemical weathering changes the minerals themselves, as when slightly acidic rainwater dissolves calcite in limestone, or feldspar in granite reacts with water to become clay. Neither of these moves anything anywhere; the fragments stay where they fell.
Erosion is the transport. Rivers, glaciers, wind and waves pick up the loosened material and carry it, sometimes thousands of kilometres, before dropping it where the energy falls away. The distinction matters because the two can be separated in time and place: a cliff can be weathered for centuries while the debris piles at its base, and then a single storm can erode the whole pile into the sea. The Mississippi carries roughly 200 million tonnes of eroded material to the Gulf each year, and every grain of it was loosened by weathering somewhere upstream first. Living things do a surprising share of the weathering. Tree roots grow into cracks and widen them by force, lichens release acids that etch into bare rock, and burrowing animals expose fresh surfaces to water and air. Over long periods this biological weathering is comparable to the physical and chemical kinds, which is another case of the biosphere acting on the geosphere as the first lesson of this chapter described.
Pause and explain
A cliff face crumbles and the fragments pile at its base for years before a storm washes them into the sea. Which part is weathering and which is erosion?
Check your reasoning after answering
The crumbling is weathering, since the rock is broken down in place without being moved anywhere. The storm washing the pile into the sea is erosion, since that is transport of already loosened material. The two can be separated by years, which is why they are treated as different processes rather than one.
Part 4 · A cycle with no beginning
Draw the rock cycle as a circle and it is easy to assume there is an order to follow. There is not. Any rock can become any other given the right conditions, and the arrows cut across the middle as well as round the edge. Sedimentary rock can be buried and metamorphosed, or lifted and weathered straight back to sediment without ever becoming metamorphic. Igneous rock can be metamorphosed without ever being eroded. Metamorphic rock can melt and become igneous again.
The timescales involved are the reason this is hard to picture. Sediment can become rock in a few thousand years; the metamorphism that makes gneiss takes millions; the oldest rocks on Earth are about four billion years old and have been through several of these transformations already. Meanwhile the ocean floor is recycled completely in under 200 million years by the subduction from the last lesson, which is why nothing older than that exists anywhere on the sea bed. A rock in your hand is a snapshot of one stage, and the only reason it seems permanent is that you are looking at it for such a short time. Measured rates make the point concrete. Continental crust erodes on average by a few hundredths of a millimetre a year, which sounds negligible until you multiply: 0.05 mm a year removes 50 m in a million years and 5 km in a hundred million. Mountains are not permanent features; they exist only where uplift is currently outpacing erosion, and they disappear within a few hundred million years once it stops.
Pause and explain
Why is it misleading to teach the rock cycle as a fixed sequence of stages?
Check your reasoning after answering
Because any rock can become any other and the paths cut across the circle. Sedimentary rock may be uplifted and weathered straight back to sediment without passing through metamorphism, and igneous rock may be metamorphosed without being eroded first. A fixed order would predict transitions that do not have to happen and rule out ones that commonly do.
Apply the whole unit · Read the history
Rock A is dark, uniform and so fine-grained that no crystals are visible. Rock B has visible pink, grey and black grains 2 to 5 mm across. Rock C splits easily into flat sheets and its minerals all lie in the same direction. Rock D is made of rounded sand grains cemented together and crumbles at the edges.
Your task
Name the group each rock belongs to and describe where and how each one formed.
What your response should show
- Assigns all four rocks to the correct group
- Uses crystal size rather than colour to separate the two igneous rocks
- Identifies aligned minerals as the metamorphic evidence
- Reads rounded grains as evidence of transport before deposition
Response guidance — after your attempt
Assign each rock to the igneous, sedimentary or metamorphic group.
For each, state the evidence in the description that decides it.
For rocks A and B, say where they solidified and how you know.
Rock A is igneous. The evidence is that it is crystalline but with crystals too small to see, which means it solidified from melt at or very near the surface, cooling in hours or days after an eruption; this is basalt. Rock B is also igneous, and the visible 2 to 5 mm crystals mean the opposite history: it cooled slowly deep underground over a very long period, giving crystals time to grow; this is granite. The chemistry may differ too, but the crystal size alone settles where each one solidified. Rock C is metamorphic. The decisive evidence is that its minerals all lie in one direction and it splits along that direction, which happens when existing rock is squeezed hard enough to rotate its flaky minerals while staying solid; this is slate, formed from shale. Rock D is sedimentary. The rounded grains show transport by water or wind that wore off their corners, and the cement between them shows burial and compaction afterwards; this is sandstone.
Scientific references for this unit
- US Geological Survey: the rock cycle and rock-forming processes
- National Park Service: igneous, sedimentary and metamorphic rocks in the parks
- US Geological Survey: weathering, erosion and sediment transport
- Smithsonian National Museum of Natural History: rock and ore collections
Parent / teacher teaching notes
Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.
The key idea
Rocks are not permanent; they are a stage in a cycle that has no fixed starting point and no required order. The size of the crystals in an igneous rock records how fast it cooled, and the difference between weathering and erosion is the difference between breaking and carrying.
Worked examples
Example 1 · Label a rock pathway
- Name cooling and the first rock category.
- Separate breakdown, transport, and accumulation.
- Identify cementation and a branching possibility.
Example 2 · Transfer to a different case
- Identify the distinction and conditions in this new case.
- Use the relevant mechanism or quantity; preserve units and the chosen boundary.
- Explain why the answer fits and what further evidence would be needed.
Example 3 · Distinguish weathering, erosion, and deposition · a focused example
- State the literal condition or observation in the example.
- Apply the distinction explained in this teaching part.
- Check whether the claim stays within the given evidence.
Avoid these mistakes
- Misleading claim: The rock cycle forces every rock through one fixed sequence.
- Misleading claim: Metamorphism requires complete melting of the rock.
- Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
- Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.
Check your understanding
What process makes them igneous in the stated model?
Discuss the answer
Cooling and solidification from molten material, not merely their color or hardness.
Which stage is transport?
Discuss the answer
Erosion. The earlier breakdown is weathering and the later settling is deposition.
Why is the melted-and-cooled product not classified only by pressure?
Discuss the answer
Its stated formation pathway includes solidification from melt, which is igneous; classification follows the relevant process.
Must every rock complete the same cycle in order?
Discuss the answer
No. The model has alternatives governed by conditions; time spent on each pathway also varies.
Sustained scientific explanation and revision
These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.
Activity 1 · Rocks, Weathering & the Rock Cycle · Model and explain
Read A Stone and Several Possible Histories and the linked unit teaching; use its complete supplied case and diagrams.
Your task
For Rocks, Weathering & the Rock Cycle, study Parts 3–4 and attempt the worked case: Identify the processes and rock categories, and explain one alternate route. Explain the steps before opening the model.
Success criteria
- An accurate scientific claim answering the actual question.
- Relevant evidence or a labeled model with quantities and units when applicable.
- Reasoning that connects evidence with the mechanism rather than only repeats it.
- A specific method, boundary, scale, or alternative-explanation limit.
Hint
Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.
Response guidance — discuss after your attempt
Cooling forms igneous rock; weathering supplies loose fragments; erosion transports them; deposition makes sediment layers; cementation can form sedimentary rock. The original igneous rock could instead undergo metamorphism while solid, or later remelt. The sequence does not require every rock to follow these events or specify their duration.
Activity 2 · Rocks, Weathering & the Rock Cycle · Practice, revise and exit
Read A Stone and Several Possible Histories and the linked unit teaching; use its complete supplied case and diagrams.
Your task
For Rocks, Weathering & the Rock Cycle, complete practice questions 1–6 and both choice checks, then answer independently: A record says a solid rock changed under heat and pressure without melting. Which major category fits? Save a correction explaining one changed claim or method.
Success criteria
- An accurate scientific claim answering the actual question.
- Relevant evidence or a labeled model with quantities and units when applicable.
- Reasoning that connects evidence with the mechanism rather than only repeats it.
- A specific method, boundary, scale, or alternative-explanation limit.
Hint
Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.
Response guidance — discuss after your attempt
Metamorphic rock under the stated process. If complete melting and later cooling were recorded instead, the formation account would follow an igneous pathway.
Words to know
- weathering
- Breakdown or alteration of material in place.
- erosion
- Transport of material by agents such as flowing water, wind, ice, or gravity.
- evidence
- A relevant observation, measurement, or record used to evaluate a claim.
- system boundary
- The chosen limit specifying what is included and which exchanges cross it.
- uncertainty
- A stated limit in measurement or knowledge, distinct from a careless mistake.
In the real world
Exit check
A record says a solid rock changed under heat and pressure without melting. Which major category fits?
Check your explanation after answering
Metamorphic rock under the stated process. If complete melting and later cooling were recorded instead, the formation account would follow an igneous pathway.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.