Answer key · Unit 24

Rocks and the surface.

Each lesson’s practice questions, their correct answers and the mistakes they spot. Answers read off the Scene use the same setting the learner sees.

Misconception catalogue

Answer key 3

Lesson 1 of 3 · About 25 minutes

The rock cycle

Follow one rock round the cycle: magma that cools into granite, sand pressed and cemented into sandstone, and burial that heats it by 25 °C a kilometre, until it is transformed and finally melts.

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The 6 steps and their answers

Step 1 of 6
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Three families
Practice · Choose
Question
Geologists sort all rocks into three families: igneous, sedimentary and metamorphic. What decides a rock’s family?

Correct answer
- Correct How it formed
Setup, choices and feedback
The Scene at this step
Magma cooled at
7 km deep
Sandstone buried to
34 km deep
Round the cycle
Igneous
Choices
Shown in a random order.
- Correct How it formed
- Wrong Its colour and hardness
  Misconception: Thinks rocks are sorted into families by their colour, hardness or look.
  Feedback from the catalogue
  Geologists sort rocks by how they formed, not by colour: an igneous rock set from molten material, a sedimentary rock was built from layers of grains or remains, and a metamorphic rock was changed by heat and pressure without melting. Pink granite and black basalt are both igneous, and white marble is a metamorphic rock that was once limestone.
- Wrong Its age: the oldest rocks are igneous
  Feedback
  There are very young igneous rocks, like lava that cooled yesterday in Hawaii, and very old sedimentary ones. Age does not decide the family.
Feedback for a right answer
How it formed: from melt, from layers, or from change by heat and pressure.

Worked solution
1. An igneous rock set from magma or lava as it cooled.
2. A sedimentary rock was built from grains, remains or chemical deposits that gathered in layers and were cemented.
3. A metamorphic rock is a rock of any family changed by heat and pressure, without melting.
Any rock can become a rock of any other family, and that is the rock cycle.
Step 2 of 6
Open this step
Big crystals and small
Practice · Predict, then look
Question
The rock in the Scene is lava that cooled at the surface: rhyolite, with tiny crystals. Predict: what will happen to the crystals if the same magma cools 7 km down?

Correct answer
- Correct They will be much bigger, because at depth the magma cools slowly
Setup, choices and feedback
The Scene at this step
Magma cooled at
0 km deep
Sandstone buried to
34 km deep
Round the cycle
Igneous
Choices
- Correct They will be much bigger, because at depth the magma cools slowly
- Wrong They will be even smaller, because the pressure at depth squeezes them
  Misconception: Thinks magma deep underground forms smaller crystals, squeezed by the pressure.
  Feedback from the catalogue
  Crystal size is set mainly by cooling time. At depth the surrounding rock is hot and insulating, and the magma cools over tens or hundreds of thousands of years: the atoms have time to join a few large crystals. Lava at the surface cools within days, and forms many tiny crystals, or glass.
- Wrong They will be the same size: it is the same magma, with the same minerals
  Feedback
  The minerals are indeed the same, quartz, feldspar and mica, but the size of the crystals depends on the time they had to grow.
Once the prediction is locked in, the Scene changes
Magma cooled at
7 km deep
The Scene then shows: 5.341 mm · Typical crystal size of the igneous rock/nodes/rock/outputs/crystalSize
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
About a hundred times bigger: granite. Slow cooling, big crystals.

Worked solution
1. Lava at the surface loses its heat to air and water within days to months: the crystals only have time to grow to about 0.05 mm.
2. At 7 km the surrounding rock is hot and insulating, and the magma cools over about $180{,}000$ ⁠ years.
3. The atoms have a long time to join existing crystals instead of starting new ones, and the crystals grow to about 5 mm: granite, in which every crystal can be seen by eye.
So the size of an igneous rock’s crystals tells where it cooled.
Step 3 of 6
Open this step
Sand and sandstone
Practice · Choose
Question
The sand worn from the granite settled in layers and was buried. How does it become sandstone?

Correct answer
- Correct The weight of the layers compacts it, and minerals from the water between the grains cement them
Setup, choices and feedback
The Scene at this step
Magma cooled at
7 km deep
Sandstone buried to
34 km deep
Round the cycle
Sedimentary
Choices
Shown in a random order.
- Correct The weight of the layers compacts it, and minerals from the water between the grains cement them
- Wrong The heat at depth melts the grains, and they fuse together
  Misconception: Thinks sand becomes sandstone by melting and fusing together.
  Feedback from the catalogue
  Sandstone forms at low temperatures, far below melting. When the sand is buried, the weight of the layers above compacts it, and minerals dissolved in the water between the grains, such as silica or calcium carbonate, come out between them and cement them, like mortar. Sand that melted would no longer become a sedimentary rock, but an igneous one.
- Wrong The sand dries out completely, and dry grains stick to each other
  Feedback
  Dry sand falls apart, as on a beach. What sticks the grains together is minerals the water left between them.
Feedback for a right answer
Compaction and cementation, with no melting at all.

Worked solution
1. Weathering and wear: at the surface the feldspar in the granite rots to clay, and the hard quartz is left as sand.
2. Deposition: rivers carry the sand and lay it down in layers, in a lake or the sea.
3. Compaction: new layers pile up above, and their weight presses the grains together.
4. Cementation: minerals dissolved in the water between the grains come out between them and bind them.
All this happens a few kilometres down, at a few tens of degrees. That is why fossils survive in sedimentary rocks, where heat would destroy them.
Step 4 of 6
Open this step
Heat at depth
Practice · Calculate
Question
Sandstone buried to is locked at 16 km, in the root of a mountain belt. At the surface the temperature is 15 °C, and in continental crust it rises by about 25 °C every kilometre. What is the sandstone’s temperature at that depth?

Correct answer
415 °C ± 5 °C
Setup, choices and feedback
The Scene at this step
Magma cooled at
7 km deep
Sandstone buried to
16 km deepfixed
Round the cycle
Metamorphic
Accepted within
± 5 °C
Computed from the Scene at this step’s setting
Temperature at the deepest burial/nodes/rock/outputs/peakTemperature
Any unit of the same quantity is accepted; it is converted before checking.
Feedback for a right answer
About 415 °C: hot enough to build new minerals, far from melting.

Hint
Start from the surface temperature, and add 25 °C for every kilometre.

Worked solution
1. The heating at depth: $25 \cdot 16 = 400$ ⁠ degrees.
2. Plus the surface temperature: $T = 15 + 400 = 415\ ^\circ\mathrm{C}$ ⁠.
At that temperature, and at a pressure of about $4300$ ⁠ atmospheres, the sandstone no longer stays as it was: the clay in it turns to mica, and the rock becomes schist.
Step 5 of 6
Open this step
Metamorphism
Practice · Reach the target
Question
The sandstone is now buried only 5 km, and stays sandstone. Change Sandstone buried to until it becomes schist, and press Check.

Goal
- 2 ± 0.1
  At the start of the step: 0
Setup, choices and feedback
The Scene at this step
Magma cooled at
7 km deep
Sandstone buried to
5 km deep
Round the cycle
Metamorphic
Read from the Scene at
0 unchanged sandstone, 1 metasandstone, 2 schist, 3 gneiss, 4 melting/nodes/rock/outputs/grade
One setting that reaches it
Magma cooled at
0 km deep
Sandstone buried to
13.5 km deep
Feedback for a right answer
Schist: a metamorphic rock, changed without melting.

Feedback for a wrong answer
Not schist yet. Too shallow stays sandstone or metasandstone; too deep makes gneiss.

Hint
Schist forms between about 350 °C and about 550 °C. At what depth is the temperature in that range?

Worked solution
1. $T = 15 + 25\,h$ ⁠, so $h = \dfrac{T - 15}{25}$ ⁠.
2. 350 °C at $\dfrac{335}{25} \approx 13.4$ ⁠ kilometres, and 550 °C at $\dfrac{535}{25} \approx 21.4$ ⁠ kilometres.
3. Any depth between about 13.5 km and about 21 km gives schist: new mica grows in flakes, all lined up across the squeeze.
Shallower, between 200 °C and 350 °C, the grains only grow into one another; deeper, the light and dark minerals separate into bands: gneiss.
Step 6 of 6
Open this step
Melting
Practice · Predict, then look
Question
At 24 km the sandstone has become gneiss, at about 615 °C. Predict: what will happen to it if it is buried to 34 km?

Correct answer
- Correct Part of it will melt, and the magma will rise and cool into new igneous rock
Setup, choices and feedback
The Scene at this step
Magma cooled at
7 km deep
Sandstone buried to
24 km deep
Round the cycle
Metamorphic
Choices
- Correct Part of it will melt, and the magma will rise and cool into new igneous rock
- Wrong It will become an even harder metamorphic rock, but will never melt
  Misconception: Thinks a metamorphic rock is one that melted and set again.
  Feedback from the catalogue
  A metamorphic rock changes while solid. With heat and pressure the atoms wander through the rock and build new minerals, such as mica, and grains grow into one another, but the rock does not melt. Once it starts to melt, about 680 °C for a wet sandstone, it is on its way to becoming magma, and what sets from that will be an igneous rock.
- Wrong All of it will melt at once, the moment it reaches its melting temperature
  Feedback
  A rock is made of many minerals, each with its own melting point. So it melts gradually, over hundreds of degrees: the light minerals first, the dark ones last.
Once the prediction is locked in, the Scene changes
Sandstone buried to
34 km deep
Round the cycle
Melting
The Scene then shows: 0.5781 · Share of the rock melted at its deepest, 0..1/nodes/rock/outputs/meltShare
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Part melts into magma: the cycle comes back round to igneous rock.

Worked solution
1. At 34 km the temperature is $15 + 25 \cdot 34 = 865\ ^\circ\mathrm{C}$ ⁠.
2. A rock like this, with a little water, starts to melt at about 680 °C, from about 27 km. A little melt gathers in pale veins: migmatite.
3. At 865 °C almost sixty per cent of the rock has melted. The magma is lighter than the rock around it and rises.
4. When it cools it will become new igneous rock, and the cycle closes.
Many of the granite bodies in the roots of mountain belts formed this way: from sedimentary rocks buried deep and melted.

Lesson 2 of 3 · About 25 minutes

Erosion and weathering

Rain and rivers wear down a whole upland over two million years. Tell weathering from erosion, work out how much faster land wears in a monsoon, compare granite with sandstone, and follow the worn rock to the sea.

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The 6 steps and their answers

Step 1 of 6
Open this step
Weathering and erosion
Practice · Choose
Question
Water gets into a crack in a rock, freezes, expands and splits it, and the pieces stay where they are. Later rain washes them into a stream, which carries them off. Which part is weathering and which erosion?

Correct answer
- Correct Splitting the rock is weathering, and carrying it in the stream is erosion
Setup, choices and feedback
The Scene at this step
Rainfall
800 mm/yr
Rock strength
60 MPa
Years
0.65 (Young upland → 2 million years)
Choices
Shown in a random order.
- Correct Splitting the rock is weathering, and carrying it in the stream is erosion
- Wrong Splitting the rock is erosion, and carrying it in the stream is weathering
  Feedback
  The other way round. Weathering is breaking down in place, and erosion is carrying away.
- Wrong Both are erosion: they are two words for the same thing
  Misconception: Uses weathering and erosion as if they meant the same thing.
  Feedback from the catalogue
  Weathering is the breaking down of rock in place: water freezing in cracks, roots, and chemical reactions that turn feldspar to clay. Erosion is the carrying away of what broke down: water, wind and ice take it elsewhere. Weathering without erosion leaves a heap of fragments; erosion without weathering finds almost nothing to take.
Feedback for a right answer
Weathering in place, erosion on the move.

Worked solution
1. Weathering breaks the rock down in place. Physical weathering cracks it, like ice in cracks; chemical weathering changes its minerals, like feldspar turning to clay.
2. Erosion carries away what broke down: water, wind, ice and gravity.
3. The two work together: weathering prepares the material, and erosion clears it away and bares fresh rock for the next round of weathering.
Step 2 of 6
Open this step
The rivers cut down
Practice · Choose
Question
At this stage the rivers are cutting valleys into the upland. Where does a river cut fastest?

Correct answer
- Correct Where a lot of water flows in it and its bed is steep
Setup, choices and feedback
The Scene at this step
Rainfall
800 mm/yr
Rock strength
60 MPa
Years
0.1 (Young upland → 2 million years)
Choices
Shown in a random order.
- Correct Where a lot of water flows in it and its bed is steep
- Wrong Near its source, where the water is clean and so stronger
  Feedback
  Near the source little water flows. And it is the sand and gravel the river carries that are the tools it grinds its bed with.
- Wrong At its mouth, where the river is widest
  Feedback
  At the mouth there is a lot of water, but the bed is almost flat, and the river slows and drops its load instead of cutting.
Feedback for a right answer
Lots of water and a steep slope: that is where the power is greatest.

Worked solution
1. A river’s cutting power depends on how much water flows in it: the larger the area it drains, and the more rain falls on it, the more water it carries.
2. It also depends on the slope: water on a steep bed flows faster.
3. So rivers cut fastest partway down, where there is already a lot of water and the bed is still steep, and side streams eat back into the upland, towards their sources.
Step 3 of 6
Open this step
Rain and pace
Practice · Calculate
Question
Rainfall is locked at 2500 mm a year, a monsoon, and Rock strength at 60 MPa, sandstone. In the Scene the pace at which the land wears grows with the square root of the rain, and falls in inverse proportion to the rock’s strength. How many times faster does the land wear here than with 800 mm a year on the same rock?

Correct answer
1.768 ± 3%
Setup, choices and feedback
The Scene at this step
Rainfall
2500 mm/yrfixed
Rock strength
60 MPafixed
Years
0.65 (Young upland → 2 million years)
Accepted within
± 3%
Computed from the Scene at this step’s setting
How fast this rain and rock wear the land, as a multiple of 800 mm a year on 60 MPa rock/nodes/land/outputs/pace
Wrong answers it recognises
- 3.125 ± 3%
  Misconception: Takes the pace of wearing as proportional to the rain, not to its square root.
  Feedback from the catalogue
  In the Scene the pace grows with the square root of the rain: a river cuts according to the power of its flow, and doubling the rain does not double its cutting. So $3.1$ ⁠ times the rain, from 800 mm to 2500 mm, gives only $\sqrt{3.1} \approx 1.8$ ⁠ times the pace.
Feedback for a right answer
About $1.8$ ⁠ times: the square root of three times the rain.

Hint
The ratio of the rains is $\dfrac{2500}{800}$ ⁠. The rock has not changed.

Worked solution
1. The ratio of the rains: $\dfrac{2500}{800} = 3.125$ ⁠.
2. The pace grows with the square root: $\sqrt{3.125} \approx 1.77$ ⁠.
3. The rock is the same, so that is the whole difference: the land wears about $1.8$ ⁠ times faster.
Three times the rain gives less than twice the pace. Multiplying the pace by the ratio of the rains itself gives $3.1$ ⁠.
Step 4 of 6
Open this step
Hard rock
Practice · Predict, then look
Question
After $1.3$ ⁠ million years of temperate rain on sandstone, the upland has worn into broad valleys. Predict: how will the same upland look after the same time if Rock strength is 180 MPa, granite?

Correct answer
- Correct The rivers will have cut only narrow valleys, and most of the upland will still stand
Setup, choices and feedback
The Scene at this step
Rainfall
800 mm/yr
Rock strength
60 MPa
Years
0.65 (Young upland → 2 million years)
Choices
- Correct The rivers will have cut only narrow valleys, and most of the upland will still stand
- Wrong The same: after a million years any rock is worn down
  Feedback
  The time is the same, but the pace is not: in rock three times as strong the land wears three times as slowly.
- Wrong The upland will be worn flat, because hard rock cracks and crumbles faster
  Feedback
  Strong rock cracks and weathers more slowly, so it survives: many mountains and cliffs are exactly the hard layers that were left.
Once the prediction is locked in, the Scene changes
Rock strength
180 MPa
The Scene then shows: 10.76 m · The land’s average lowering/nodes/land/outputs/lowered
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Narrow valleys, and an upland still standing: strong rock wears slowly.

Worked solution
1. The pace falls in inverse proportion to the strength: granite, 180 MPa, is three times as strong as the sandstone, so it wears three times as slowly.
2. After $1.3$ ⁠ million years the rivers have cut only narrow gorges, and the upland between the valleys still stands.
3. The land’s average lowering drops from about 40 m to about 11 m.
So in a landscape with strong and weak layers, the weak ones wear into valleys and the strong ones are left as ridges and cliffs.
Step 5 of 6
Open this step
Wear the upland down
Practice · Reach the target
Question
In a desert, on granite, the upland has hardly worn at all. Change Rainfall and Rock strength until the upland is worn down to low, rounded hills, fast wearing, and press Check.

Goal
- 32 ± 0.1
  At the start of the step: 30
Setup, choices and feedback
The Scene at this step
Rainfall
150 mm/yr
Rock strength
180 MPa
Years
0.65 (Young upland → 2 million years)
Read from the Scene at
Stage · 10 + pace class (0 slow, 1 between, 2 fast), for the Callouts/nodes/land/outputs/beat
One setting that reaches it
Rainfall
450 mm/yr
Rock strength
20 MPa
Feedback for a right answer
Rounded hills: plenty of rain and soft rock.

Feedback for a wrong answer
Not yet. The rock is still too strong or the rain too little.

Hint
Fast wearing means a pace more than $2.2$ ⁠ times that of 800 mm on sandstone. What raises the pace, and what raises it more?

Worked solution
1. The pace grows with the square root of the rain, and falls in inverse proportion to the rock’s strength.
2. Rain alone is not enough: even 3000 mm on sandstone gives only $\sqrt{3.75} \approx 1.9$ ⁠.
3. Weak rock has the bigger effect: mudstone, 20 MPa, with 800 mm of rain, already gives $3$ ⁠.
Any mix of plenty of rain and weak rock giving a pace above $2.2$ ⁠ works. That is how rainy regions of soft rock look: rounded hills, with no cliffs.
Step 6 of 6
Open this step
To the sea
Practice · Choose
Question
After two million years about 5.8 km³ of rock has been worn from the upland. What became of most of it?

Correct answer
- Correct Most was carried out to sea as fine mud, and only about a sixth built the delta
Setup, choices and feedback
The Scene at this step
Rainfall
800 mm/yr
Rock strength
60 MPa
Years
2 million years
Choices
Shown in a random order.
- Correct Most was carried out to sea as fine mud, and only about a sixth built the delta
- Wrong All of it piled up in the delta at the river’s mouth
  Feedback
  The delta is built only of the coarse part, sand and gravel, which settles as soon as the river slows. The fine mud stays suspended and is carried far out to sea.
- Wrong It crumbled and dissolved until it was gone
  Misconception: Thinks rock worn from the land is used up or vanishes.
  Feedback from the catalogue
  Erosion destroys no material, it only moves it. Every cubic metre of rock worn from the upland ends up somewhere: sand and gravel settle at the river’s mouth and build a delta, and fine mud is carried out to sea and settles on the floor. There, once buried, it will become new sedimentary rock, and so the cycle closes.
Feedback for a right answer
Most is mud in the sea. The delta is only the coarse part.

Worked solution
1. In the Scene, of the 5.8 km³ worn away, about 0.9 km³ of sand and gravel settled in the delta, and about 4.9 km³ of mud was carried out to sea.
2. The mud is about five sixths of the whole: most of what weathering leaves is fine clay, not sand.
3. The material does not vanish. The mud settles on the sea floor, will be buried, and will become mudstone and shale.
The Nile, for example, built its delta from the coarse part, and the current along the coast carries its sediment as far as Israel: most of the sand on Israel’s beaches came from the Nile.

Lesson 3 of 3 · About 25 minutes

Volcanoes

Magma rises, the pressure drops and the gas comes out as bubbles. Work out at what depth they appear, and see how silica and gas decide whether the lava flows, piles into a dome, throws up fountains or explodes into ash.

Open the lesson

The 6 steps and their answers

Step 1 of 6
Open this step
The magma chamber
Practice · Choose
Question
A chamber of magma sits 6 km down. Where did it come from, and why does it rise?

Correct answer
- Correct It formed from rock that melted in one particular place, and it is lighter than the solid rock around it
Setup, choices and feedback
The Scene at this step
Silica in the magma
60 %
Gas in the magma
3 %
Erupt
Magma at rest
Choices
Shown in a random order.
- Correct It formed from rock that melted in one particular place, and it is lighter than the solid rock around it
- Wrong Under the crust there is a layer of molten rock, and the volcano is a hole down to it
  Misconception: Thinks a volcano is a hole down to a layer of molten rock under the whole crust.
  Feedback from the catalogue
  There is no molten layer under the crust: S waves pass through the mantle, so it is solid. Magma forms only in certain places, when hot rock loses pressure, at ridges and hot spots, or when it gains water, above a sinking plate. It is lighter than the rock around it, rises, and gathers in a magma chamber a few kilometres down.
- Wrong It rises straight from the Earth’s core
  Feedback
  The core lies about 2900 km down and is made of iron and nickel. Magma is made of mantle and crust rock, and forms in the top few tens of kilometres.
Feedback for a right answer
Local melting, and magma lighter than the rock around it.

Worked solution
1. The mantle is solid, but hot. It partly melts only in certain places: where the pressure on it falls, at ridges and hot spots, or where water from a sinking plate lowers its melting point.
2. The melt is less dense than the solid rock, so it rises, like oil in water.
3. It gathers in a magma chamber a few kilometres down, until its pressure is enough to force a way to the surface.
That is why volcanoes lie mostly at plate boundaries and hot spots, and not everywhere.
Step 2 of 6
Open this step
Bubbles
Practice · Calculate
Question
Gas in the magma is locked at $3\%$ ⁠ dissolved water. A magma can hold $w = 0.41\sqrt{P}$ ⁠ per cent of water dissolved, with $P$ ⁠ the pressure in megapascals. The magma’s density is 2400 kg/m³, so its pressure rises by about 23.5 MPa every kilometre of depth. At what depth, as the magma rises, do bubbles start to form in it? (The air’s pressure, 0.1 MPa, can be neglected.)

Correct answer
2.27 km ± 3%
Setup, choices and feedback
The Scene at this step
Silica in the magma
60 %
Gas in the magma
3 %fixed
Erupt
0.3 (Magma at rest → Full eruption)
Accepted within
± 3%
Computed from the Scene at this step’s setting
Depth below the vent where bubbles start to form/nodes/volcano/outputs/bubbleDepth
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 0.31 km ± 5%
  Misconception: Takes the water a magma can dissolve as proportional to the pressure, not to its square root.
  Feedback from the catalogue
  The water dissolved in a magma grows with the square root of the pressure: $w = 0.41\sqrt{P}$ ⁠. So to hold twice the water takes four times the pressure. The common slip is $P = \dfrac{w}{0.41}$ ⁠, without squaring, which gives too little pressure and puts the bubbles too close to the surface.
Feedback for a right answer
About 2.3 km: below it the gas is dissolved, and above it, it fizzes.

Hint
First find the pressure at which $3\%$ ⁠ is exactly what the magma can hold: $P = \left(\dfrac{w}{0.41}\right)^2$ ⁠.

Worked solution
1. Bubbles appear when the magma can no longer hold all its water: $3 = 0.41\sqrt{P}$ ⁠.
2. $\sqrt{P} = \dfrac{3}{0.41} \approx 7.32$ ⁠, so $P \approx 53.5\ \mathrm{MPa}$ ⁠.
3. $\rho g = 2400 \cdot 9.81 \approx 23{,}500\ \mathrm{Pa/m} = 23.5\ \mathrm{MPa/km}$ ⁠, so $h = \dfrac{53.5}{23.5} \approx 2.3\ \mathrm{km}$ ⁠.
From there up the pressure keeps falling, and the gas keeps coming out, as in a fizzy drink being opened. Forgetting to square gives 7.3 MPa and only about 0.3 km.
Step 3 of 6
Open this step
Runny or sticky
Practice · Predict, then look
Question
The magma is now basalt, with $50\%$ ⁠ silica and little gas, and the lava runs far down the slopes. Predict: what will happen if Silica in the magma rises to $73\%$ ⁠, rhyolite?

Correct answer
- Correct The lava will be sticky, pile up into a dome over the vent and barely flow
Setup, choices and feedback
The Scene at this step
Silica in the magma
50 %
Gas in the magma
1 %
Erupt
0.75 (Magma at rest → Full eruption)
Choices
- Correct The lava will be sticky, pile up into a dome over the vent and barely flow
- Wrong The lava will run even further: any lava is runny when hot enough
  Misconception: Thinks all lava is runny and flows like the lava of Hawaii.
  Feedback from the catalogue
  Only basaltic lava, poor in silica, is runny like Hawaii’s. Silica builds long chains of silicon and oxygen in the magma that tangle together, so the more silica, the stickier the magma: rhyolite lava is a billion times stickier than basalt. It barely flows, piles up into a dome, and traps its gas.
- Wrong The volcano will explode at once
  Feedback
  To explode, a sticky magma also needs plenty of gas. With little gas there is nothing to tear it apart, and it simply piles up.
Once the prediction is locked in, the Scene changes
Silica in the magma
73 %
The Scene then shows: 3 · Eruption style: 1 lava flows, 2 lava fountains, 3 lava dome, 4 explosive/nodes/volcano/outputs/regime
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
A sticky dome: more silica, lava a billion times stickier.

Worked solution
1. In silica, silicon and oxygen link into chains and networks. The more silica, the longer the networks and the more they tangle.
2. Rhyolite magma is also cooler, about 830 °C against about 1170 °C for basalt, which raises the viscosity further.
3. Together, the viscosity at the vent rises from about 150 Pa·s, like honey, to about $10^{11}$ ⁠ pascal-seconds: some billion times more. Lava like this hardly flows, and piles up into a dome.
That is the difference between Hawaii, where basaltic lava flows, and volcanoes like Mount St Helens, where domes of sticky lava grow.
Step 4 of 6
Open this step
An explosion
Practice · Reach the target
Question
Silica in the magma is locked at $60\%$ ⁠, andesite, and the lava piles up into a dome. Change Gas in the magma until the eruption is explosive, and press Check.

Goal
- 4 ± 0.1
  At the start of the step: 3
Setup, choices and feedback
The Scene at this step
Silica in the magma
60 %fixed
Gas in the magma
0.5 %
Erupt
0.75 (Magma at rest → Full eruption)
Read from the Scene at
Eruption style: 1 lava flows, 2 lava fountains, 3 lava dome, 4 explosive/nodes/volcano/outputs/regime
One setting that reaches it
Gas in the magma
2.6 %
Feedback for a right answer
An explosion: sticky magma and enough gas.

Feedback for a wrong answer
Not yet. There is still too little gas to swell the bubbles until the froth tears.

Hint
In a sticky magma the bubbles cannot escape. How much gas does it take for them to swell until the froth tears?

Worked solution
1. In a sticky magma the bubbles are trapped: they do not rise through the magma, but rise with it.
2. As the magma rises, the pressure falls, more gas comes out of solution, and the bubbles swell.
3. With enough gas, from about $2.6\%$ ⁠ in andesite, the bubbles fill about three quarters of the volume before the vent, and the froth tears into shreds: ash.
That is how Vesuvius erupted in 79 and buried Pompeii, and how Pinatubo erupted in 1991.
Step 5 of 6
Open this step
Gas in basalt
Practice · Predict, then look
Question
Basalt with $0.5\%$ ⁠ gas pours out lava quietly. Predict: what will happen if Gas in the magma rises to $5\%$ ⁠, more than in the andesite that exploded before?

Correct answer
- Correct The gas will throw up fountains of lava, but there will be no explosion
Setup, choices and feedback
The Scene at this step
Silica in the magma
50 %
Gas in the magma
0.5 %
Erupt
0.75 (Magma at rest → Full eruption)
Choices
- Correct The gas will throw up fountains of lava, but there will be no explosion
- Wrong The volcano will explode, because plenty of gas always makes an explosion
  Misconception: Thinks plenty of gas is enough to make any eruption explode.
  Feedback from the catalogue
  Gas alone is not enough. In runny magma the bubbles rise and escape easily, as in a cola, and at most throw up fountains of lava. An explosion needs sticky magma that traps the bubbles: they swell on the way up until the gas fills about three quarters of the volume, and the froth tears apart into ash.
- Wrong Nothing will change: basalt has no room for gas
  Feedback
  Basalt can dissolve gas like any magma. The difference is that the gas escapes from it easily.
Once the prediction is locked in, the Scene changes
Gas in the magma
5 %
The Scene then shows: 2 · Eruption style: 1 lava flows, 2 lava fountains, 3 lava dome, 4 explosive/nodes/volcano/outputs/regime
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Fountains: the gas escapes from runny lava instead of being trapped.

Worked solution
1. In runny basalt the bubbles rise through the magma and escape from it, like bubbles in a cola.
2. Plenty of gas escaping fast throws lava up high: lava fountains, sometimes hundreds of metres.
3. But the bubbles are not trapped, so the froth does not tear: there is no ash column.
That is why the volcanoes of Hawaii and Iceland are spectacular but usually not explosive, and can be watched from fairly close.
Step 6 of 6
Open this step
Shields and cones
Practice · Choose
Question
Mauna Loa in Hawaii is broad and flat like a shield, and Fuji in Japan is steep and conical. What explains it?

Correct answer
- Correct Runny basaltic lava runs far and builds gentle slopes; sticky lava and ash pile up close to the vent
Setup, choices and feedback
The Scene at this step
Silica in the magma
50 %
Gas in the magma
1 %
Erupt
Full eruption
Choices
Shown in a random order.
- Correct Runny basaltic lava runs far and builds gentle slopes; sticky lava and ash pile up close to the vent
- Wrong Mauna Loa is older, and the rain has already worn it flat
  Feedback
  Mauna Loa is active, and its slopes are built of fresh lava flows. Its shape comes from the lava, not from wear.
- Wrong Fuji is smaller, and every small volcano is steep
  Feedback
  Small basaltic volcanoes are flat too. What sets the slope is how far the lava runs before it sets.
Feedback for a right answer
The lava sets the shape: runny builds a shield, sticky builds a cone.

Worked solution
1. Basaltic lava, poor in silica and hot, runs tens of kilometres before it sets. Layer on layer it builds a shield volcano, broad and flat.
2. Andesitic lava, richer in silica, is sticky and sets close to the vent, and between flows there are explosive eruptions that leave layers of ash. Together they build a stratovolcano, steep and conical.
3. In the Scene the basalt volcano rises about 1.6 km over a wide base, and the andesite one about 2.8 km on a narrower base.
Mauna Loa is the largest mountain on Earth by volume, yet its slopes are so gentle that it is hard to tell it is a mountain.

Across the unit · 11 misconceptions

Misconception catalogue

Each common mistake this unit recognises, in a few words. Open one for what the learner reads when an answer matches it, and where it is asked.

Index

The rock cycle

Three families

Question

Correct answer

The Scene at this step

Choices

Worked solution

Big crystals and small

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

Sand and sandstone

Question

Correct answer

The Scene at this step

Choices

Worked solution

Heat at depth

Question

Correct answer

The Scene at this step

Hint

Worked solution

Metamorphism

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

Melting

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

Erosion and weathering

Weathering and erosion

Question

Correct answer

The Scene at this step

Choices

Worked solution

The rivers cut down

Question

Correct answer

The Scene at this step

Choices

Worked solution

Rain and pace

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

Hard rock

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

Wear the upland down

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

To the sea

Question

Correct answer

The Scene at this step

Choices

Worked solution