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Heat and pressure rebuild a rock without melting it, and every metamorphic rock keeps a trace of the parent it was made from.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
By the end of this lesson you will be able to name a metamorphic rock's parent from the trace it keeps, say what the heat and pressure did to it — grew crystals, lined up flat minerals, or sorted bands — and point at where on a cross-section the rock was changed by heat without melting. You will also be able to say why a rock that melts has stopped being metamorphic.
You can tell a metamorphic rock by its bands or its flat sheets, and you know the other two histories: a melt that cooled, and grains that were cemented. This lesson is about the third history, and the one thing to hold onto from the start is that it happens to a rock that already exists, and happens without melting it.
Metamorphism is change in a rock by heat and pressure, in the solid state. The parent rock is what it was before. Recrystallisation is old grains growing into new, larger, interlocking crystals. Foliation is minerals lined up in flat sheets or bands, at right angles to the squeeze. Contact metamorphism happens in the rim round a body of magma; regional metamorphism happens over whole mountain ranges, deep under them.
Bury any rock deep enough and two things reach it: heat, from the depth and from any magma nearby, and pressure, from the weight above and from the sideways squeeze where mountains are being built. Short of melting, the rock does not turn to liquid — but its minerals are not fixed either. Atoms move, slowly, and the rock rebuilds itself in the solid state.
Three things can happen, and each leaves a texture:
Every metamorphic rock has a parent, and it keeps a trace of it: marble still fizzes like limestone; quartzite is still sand-coloured; gneiss still has granite's minerals. Reading the trace is how you name the parent.
There are two places this happens. In the narrow rim round a body of magma, the heat does most of the work — contact metamorphism, a zone a few metres to a few hundred metres wide. Deep under a mountain range, heat and squeeze work together over enormous areas — regional metamorphism, which is where slate and gneiss come from.
Another way: picture
A cross-section with a large red body of magma deep down, wrapped in a pink halo where the surrounding rock has been baked. Far from the body the flat layers run on unchanged. A second panel shows a mountain range with the same layers, folded and squeezed sideways deep beneath it, shaded to show the change spreading across the whole width.
Another way: steps
To read a metamorphic rock:
| Parent | Product | What changed | The trace it keeps |
|---|---|---|---|
| mudstone | slate | flat minerals grew, all lined up | fine grain; splits into sheets |
| limestone | marble | grains grew into interlocking crystals | fizzes with vinegar |
| sandstone | quartzite | sand grains fused into one mass | sand colour; made of quartz |
| granite | gneiss | minerals sorted into bands | granite's pink, white and black |
The table reads in both directions. Given a product, the last column names the parent; given a parent, the third column says what will happen to it. Slate is the one to remember for its sheets: they are the new minerals, and they usually cut across the old mud layers rather than following them, which is how you tell slate from a mudstone that merely splits.
Metamorphism is taken to include melting. It does not. The moment a rock melts it has left the metamorphic story and joined the igneous one; when the melt sets it is an igneous rock. Metamorphism is everything that happens short of melting.
Slate's sheets are taken for mud layers. They are new flat minerals, grown at right angles to the squeeze, and they usually cut across the old bedding. Bands in gneiss are the same kind of thing: sorted minerals, not settled beds.
Metamorphism is taken to mean only hardening. A metamorphic rock is usually harder than its parent, but that is a side effect. The minerals are new, or newly arranged, and that is why it has a new name.
A quarry shows a body of granite, and the rock beside it for fifty metres is harder and more crystalline than the same layer further away.
The change fades with distance from the granite.
That is contact metamorphism: the granite was a body of magma once, and its heat baked the rock it pushed into.
Heat from the body, in a rim round it.
Fifty metres out, the layer is ordinary sandstone; against the granite it is quartzite, the same sand fused into one mass. Same parent, different distance from the heat.
The tiles are thin, flat, hard and fine-grained, and they were split from a block rather than cut.
Splitting into sheets is the trace.
Fine grain and flat sheets mean slate: mudstone whose grains grew into lined-up flat minerals under a mountain range.
Regional metamorphism.
So every tile on the roof was once mud on a sea floor, and the sheets it splits along are not the sea-floor layers but the minerals that grew across them.
Sand-coloured and made of quartz points to sandstone as the parent…
Find the trace first.
…but the grains do not rub loose, so they have fused into one interlocking mass, which is what heat and pressure do to sand…
…so it is quartzite: sandstone recrystallised, with nothing melted.
Each metamorphic rock on the left was once one of the rocks on the right. Match each one to its parent.
| mudstone | limestone | sandstone | granite | |
|---|---|---|---|---|
| slate — splits into thin flat sheets | ||||
| marble — sugary crystals of one mineral, fizzes with vinegar | ||||
| quartzite — sand-coloured, with the grains fused so it no longer crumbles | ||||
| gneiss — pink, white and black minerals in wavy bands |
Two metamorphic rocks: gneiss, which the same pink, white and black minerals as granite, but in wavy bands; and marble, which made of sugary crystals, and fizzes with vinegar. For each, name the rock it was before, and say what the heat and pressure did to it.
| the rock it was before | what the heat and pressure did | |
|---|---|---|
| gneiss | ||
| marble |
The picture is a cross-section through the ground where molten rock once pushed up into layered rock. Mark the rock most changed by heat without having melted.
This task has no paper form; do it on a device.
A thick layer of mudstone is buried under a growing mountain range, where it is heated and squeezed hard from the sides for a very long time, but never hot enough to melt. What does it become?
Each metamorphic rock on the left was once one of the rocks on the right. Match each one to its parent.
| mudstone | limestone | sandstone | granite | |
|---|---|---|---|---|
| slate — splits into thin flat sheets | ||||
| marble — sugary crystals of one mineral, fizzes with vinegar | ||||
| quartzite — sand-coloured, with the grains fused so it no longer crumbles | ||||
| gneiss — pink, white and black minerals in wavy bands |
A statue in a town square is carved from a white stone made of sugary interlocking crystals, all of one mineral, with no layers. A drop of vinegar on a chipped corner fizzes. The stone came from a quarry in a mountain range. What was it before the mountains were built?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
Two metamorphic rocks: slate, which splits into thin flat sheets; and marble, which made of sugary crystals, and fizzes with vinegar. For each, name the rock it was before, and say what the heat and pressure did to it.
| the rock it was before | what the heat and pressure did | |
|---|---|---|
| slate | ||
| marble |
You can name a metamorphic rock's parent and say what changed. Say out loud what slate was before, what its sheets are made of, and why they are not the old mud layers. Next: the three histories joined into one cycle, and why that cycle has no start.
9. Your turn: a hard, sand-coloured rock whose grains do not rub loose and which does not fizz., step 3