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Crystal size is the record of cooling time: large crystals set slowly underground, tiny crystals or glass set fast at the surface.
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 look at an igneous rock's crystals — or a description of them — and say how fast the melt cooled and therefore where it set: slowly, deep underground, if the crystals are large enough to pick out; quickly, at the surface, if they are too small to see or the rock is glass. You will find the size of a crystal from a measured length, and explain why two rocks of the same minerals can look so different.
You can tell an igneous rock by its texture: crystals locked together with no gaps, because it cooled from a melt. You also know from everyday life that a thing cools faster in the open than wrapped up. This lesson puts the two together: the crystals record how fast the melt cooled, and how fast it cooled says where it was.
Magma is molten rock underground; lava is the same molten rock once it reaches the surface. A rock that set underground is intrusive; one that set at the surface is extrusive. Crystal size is how big the individual crystals are, and it is read with a ruler or a hand lens. A cooling rate is how quickly the melt lost its heat.
A crystal grows by adding atoms from the liquid around it, and it can only do that while there is liquid around it. So the size the crystals reach depends on one thing: how long the melt stayed liquid.
Deep underground, a body of magma is wrapped in kilometres of rock, and rock is a poor conductor of heat. The magma cools over thousands of years, and its crystals grow to a millimetre, a centimetre, sometimes more. The result is a coarse rock like granite or gabbro, made of crystals you can pick out one by one.
At the surface, lava meets air or water and loses its heat in hours or days. The crystals barely start, and the result is a fine rock like basalt, a dense mass whose crystals need a microscope. Cool it faster still — a thin flow, or lava meeting the sea — and no crystals form at all: the melt freezes as glass, which is obsidian, or as a froth of glass round gas bubbles, which is pumice.
The minerals can be the same in all of these. Basalt and gabbro are chemically much alike; what differs is where the melt set, and the crystals are the only record of it that survives.
Another way: picture
A cross-section through a volcano. Deep down, a large red body of magma sits in the rock, and beside it a magnified circle shows crystals as big as fingernails. A narrow neck runs up to the surface, where a dark flow lies on the ground, and its magnified circle shows a fine speckle with no crystals to be seen. Same melt, two magnifications, two histories.
Another way: steps
To read the history:
| Rock | Crystal size | How it cooled | Where it set |
|---|---|---|---|
| granite | about 8 mm | slowly | deep underground |
| gabbro | about 5 mm | slowly | deep underground |
| dolerite | about 2 mm | slowly, but nearer the surface | in a sheet or a neck underground |
| basalt | under 1 mm | quickly | a lava flow at the surface |
| obsidian | none — glass | very quickly | a lava flow, often meeting water |
| pumice | none — glassy froth | very quickly | a frothing eruption |
Read the table down the crystal-size column and the other two columns follow. Dolerite is worth a second look: it sets in cracks and sheets close to the surface, cooler than a deep chamber and warmer than the open air, and its crystals are between the two. The line at one millimetre is a rule of thumb rather than a law, and the table shows why.
Big crystals are taken to mean an old rock. Crystals grow only while the melt is liquid; once it has set, they stop, and a granite a billion years old has the crystals it had the day it set. Size records cooling time, not age.
Colour is taken for cooling rate. Dark basalt cooled fast and dark gabbro cooled slowly. Colour comes from the minerals; cooling rate comes from the crystals.
Different crystal sizes are taken to mean different minerals. Basalt and gabbro can be made of the same minerals. The one that set at the surface is fine; the one that set underground is coarse. One melt, two places.
The crystals are pink, white and black, and about 8 mm across; you can pick each one out.
Size first.
Crystals that large had thousands of years to grow, so the melt cooled slowly, wrapped in rock deep underground.
Size to time, time to place.
It is granite, and it is at the surface now only because everything that once lay over it has been worn away.
Which is a later lesson's subject.
It is a dense dark mass with no crystals to be seen, and a few round holes.
No visible crystals.
No visible crystals means the melt froze in days: a lava flow at the surface. The holes are gas bubbles caught as it set.
Bubbles form in a liquid, and only near the surface does the gas come out.
It is basalt. The cliff is a stack of old lava flows, one on another.
Crystals of 2 mm can be picked out one by one, so they had time to grow…
Size to time.
…so the melt cooled slowly enough to be underground, though not as slowly as a deep chamber, since the crystals are not the size of granite's…
…which fits a sheet of melt that pushed into a crack near the surface and set there: dolerite.
Three igneous specimens are described. For each one, say whether the melt cooled slowly or quickly, and where it became solid. Specimen A has crystals well under a millimetre, a dense even mass to the eye. Specimen B has dark crystals about 5 mm across, locked together. Specimen C has crystals about 8 mm across, easy to see one by one.
| the melt cooled | where it became solid | |
|---|---|---|
| specimen A | ||
| specimen B | ||
| specimen C |
Four igneous rocks, each with its crystal size. Put them in order from the one that cooled fastest to the one that cooled slowest.
Number the steps in order (write the number in the box):
Along a ruler laid across a cut face of an igneous rock, $20$ crystals lie end to end in $2$ cm. How big is one crystal, on average? Give the answer in millimetres.
Answer: unit: m / cm / km / mm
Two igneous rocks are made of exactly the same minerals in the same proportions. In one the crystals are several millimetres across; in the other they are too small to see. What is the difference between the two rocks?
Three igneous specimens are described. For each one, say whether the melt cooled slowly or quickly, and where it became solid. Specimen A has crystals about 2 mm across, just large enough to pick out. Specimen B has no crystals at all, a solid glass. Specimen C has no visible crystals, a frozen froth full of gas holes.
| the melt cooled | where it became solid | |
|---|---|---|
| specimen A | ||
| specimen B | ||
| specimen C |
A new road has been cut through a hillside. The cutting shows a dark rock with crystals too small to see lying on top of a pale rock with crystals a centimetre across. A sign says both are igneous. What does the cutting tell you about how each one formed?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
Along a ruler laid across a cut face of an igneous rock, $10$ crystals lie end to end in $1$ cm. How big is one crystal, on average? Give the answer in millimetres.
Answer: unit: m / cm / km / mm
You can read crystal size as cooling time and cooling time as place. Say out loud why granite and basalt can be made of the same minerals and still be two rocks, and what you would look for in a specimen to tell which is which. Next: the histories a sedimentary rock's grains and layers record.
9. Your turn: a rock with crystals about 2 mm across, found in a thin sheet cutting through layered rock., step 3