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Igneous, sedimentary and metamorphic: how to tell which from the texture of a specimen, and why its colour does not help.
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.
In this lesson you sort three described specimens before anything is named, and then find out what you were sorting by. By the end you will be able to say whether a rock is igneous, sedimentary or metamorphic from how its pieces are joined — grown together, stuck together, or lined up in bands — and explain why its colour tells you nothing.
You know that a solid can melt when it is hot enough and set again when it cools, that sand and mud settle to the bottom of still water, and that squeezing something changes its shape. Those three facts are the three ways a rock forms. What this lesson adds is how to tell, from a rock in front of you, which of the three happened to it.
A rock is a solid mass of one or more minerals — the natural crystals it is made of. Igneous rock cooled from molten rock. Sedimentary rock is grains of older rock, or shells, pressed together and cemented. Metamorphic rock was changed by heat and pressure without melting. The texture of a rock is how its pieces are joined, and it is the thing to look at.
Three specimens are described. Before reading on, decide which two are most alike and which is the odd one out, and write down what you looked at to decide.
Specimen A is pale pink and grey, made of crystals a few millimetres across — pink, white and black ones — locked together with no gaps and no layers.
Specimen B is yellow-brown and made of rounded grains you can rub loose with a thumbnail, stuck together and lying in flat layers.
Specimen C is banded pink, white and black, with the minerals sorted into wavy bands that run right through it.
Most people pair A with C, because the colours match. Now ask a different question of each one: how are the pieces joined? In A they grew into each other. In B they are stuck together. In C they have been lined up. That question has three answers, and they are the three types of rock.
Every rock was made by one of three processes, and each process leaves its own mark on how the pieces of the rock are joined.
Igneous rock was molten. As the melt cooled, crystals grew until they met their neighbours, so the texture is crystals locked together with no gaps and no layers. Granite and basalt are igneous.
Sedimentary rock was loose grains — sand, mud, pebbles, shells — that settled in layers under water and were pressed and cemented into stone. The texture is grains stuck together, usually rounded, usually in layers, sometimes with fossils. Sandstone, mudstone and limestone are sedimentary.
Metamorphic rock was already a rock when it was buried, heated and squeezed short of melting. New minerals grew in line with the squeeze, so the texture is bands of light and dark, or a rock that splits into flat sheets. Slate, marble and gneiss are metamorphic.
A rock's colour says almost nothing about how it formed. There are pale igneous rocks and dark ones, pale sedimentary rocks and dark ones. What settles the type is the texture — whether the rock is made of interlocking crystals, of grains stuck together, or of bands and sheets that were squeezed into line — because the texture is what the forming process left behind.
Another way: picture
Three close-up panels side by side. In the first, many-sided crystals fit together like a mosaic with no cement between them. In the second, round grains sit in rows like peas in a tray, with a thin line every few rows where one layer ends and the next begins. In the third, dark and light wavy bands run across the whole panel like the grain in a plank of wood.
Another way: steps
To sort a specimen:
Colour is read as type. Dark rocks are called igneous and pale ones sedimentary. Basalt is dark and igneous; obsidian is black and igneous; limestone is pale and sedimentary; gneiss is pink and metamorphic. Colour comes from which minerals are present, not from what happened to them.
Hardness is read as type. A hard rock is called metamorphic because it "must have been squeezed". Granite is hard and was never squeezed; slate is metamorphic and can be split with a knife.
Bands are read as layers. Layers in a sedimentary rock are beds that settled one on another, flat and separate. Bands in a metamorphic rock are minerals sorted and lined up, usually wavy, running through the whole rock. Both are stripes; only one was settled.
The specimen is pale grey, fine-grained, in thick layers, with the shapes of shells in it.
Colour noted and set aside.
Shells settle on a sea floor; nothing grows a shell inside molten rock or squeezes one into being. The layers are beds.
The shells and the layers are the texture.
So it is sedimentary: grains and shells cemented together. It is limestone.
The name comes last, from the type, not the other way round.
The specimen is dark grey, a dense mass of crystals too small to see one by one, with round holes in it.
Dark, which means nothing yet.
Crystals too small to see are still crystals locked together — there are no separate grains to rub loose. The round holes are gas bubbles frozen in place.
Bubbles form in a liquid, so this was a liquid.
So it is igneous: it cooled from molten rock, fast enough that the crystals stayed tiny. It is basalt.
How fast it cooled is the next lesson's question.
Three specimens are on the bench. Specimen A is smooth glass with no crystals at all, breaking with curved, sharp-edged surfaces. Specimen B is made of interlocking sugary crystals, all the same kind, with no layers, and it fizzes when a drop of vinegar is put on it. Specimen C is fine-grained, in thick layers, with the shapes of shells in it, and it fizzes when a drop of vinegar is put on it. For each one, give the type of rock and the way it formed.
| type of rock | how it formed | |
|---|---|---|
| specimen A | ||
| specimen B | ||
| specimen C |
Each description is of a rock's texture. Sort them into the three types. Two descriptions go to each type.
| igneous — cooled from molten rock | sedimentary — grains cemented together | metamorphic — changed by heat and pressure | |
|---|---|---|---|
| pink, white and black crystals locked together, no layers | |||
| a glassy black mass with no grains and no crystals | |||
| rounded sand grains stuck together in flat layers | |||
| fine grey layers with the shapes of shells in them | |||
| wavy bands of light and dark minerals right through the rock | |||
| a hard rock that splits into thin flat sheets across its old layers |
A specimen is grey-brown. Looked at closely, it is very fine-grained, soft enough to scratch with a fingernail, in thin flat layers that break into slabs. What type of rock is it?
The picture shows three rock textures, drawn close up. Mark the texture a beach of sand would have once it had turned to stone.
This task has no paper form; do it on a device.
Three specimens are on the bench. Specimen A is a dense mass of crystals too small to see one by one, with a few round gas holes, and no layers. Specimen B is fine-grained and hard, and it splits into thin flat sheets along one direction that cuts across the faint traces of old layers. Specimen C is very fine-grained, soft enough to scratch with a fingernail, in thin flat layers that break into slabs. For each one, give the type of rock and the way it formed.
| type of rock | how it formed | |
|---|---|---|
| specimen A | ||
| specimen B | ||
| specimen C |
In a kitchen showroom a polished worktop is on sale as "black marble". Up close it is made of black, white and grey crystals locked together with no gaps, no layers and no bands, and a drop of vinegar on it does nothing. What is it, and what settles the question?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
A specimen is banded pink, white and black. Looked at closely, it is made of coarse crystals sorted into wavy bands of light and dark that run right through the rock. What type of rock is it?
You can sort a rock by its texture and name the process that left that texture. Say out loud why a pink rock and a banded pink rock can be two different types, and what you looked at to tell them apart. Then find three stones outside and try it on them: that is the part no screen can check, and it is the part that matters. Next: what an igneous rock's crystals say about how fast it cooled.