Your turn
Each lab is one part of the video. Tap, slide and drag to see what happens.
Each lab isolates one variable. The crystal sizes and timings are illustrative teaching sketches, not measurements.
Tap the buttons and slide the sliders.
Drop each stone in the water. Which one floats, and why?
Granite, obsidian and pumice can share a composition. Which one is less dense than water?
Drop the stones in the water. Which one floats?
Tap a place to melt some rock and watch the magma rise.
Decompression at ridges and hot spots, flux melting at subduction zones. Tap each setting.
Slide the cooling speed from slow to instant. What happens to the crystals?
Slow cooling: few nuclei, coarse grains. Fast: many nuclei, fine grains. Quenched: glass.
Slide from slow to fast. Big crystals or tiny ones?
Drag the magma up (or use the buttons) and choose where it stops.
Depth of emplacement sets the cooling rate. Drag the magma body and compare textures.
Move the hot rock up. Does it stay inside, or come out?
Pick where it cools and how much gas, then drop your rock in the water.
Choose the setting and gas content, predict the rock, then test whether it floats.
Pick a place and some bubbles. Does your rock float?
Slide to wear the land away and uncover granite that cooled deep underground.
Plutons are exposed by uplift and erosion over millions of years. Slide to erode.
Slide to wash the rock away. What is hiding underneath?
Some rocks were once so hot that they were runny, like melted chocolate. When the hot, runny rock cools down, it turns hard. We call these rocks igneous rocks. The name means "fire rocks".
When the runny rock cools slowly, deep inside the Earth, big crystals grow. You can see them as speckles, like on a granite kitchen counter. When it cools fast, the crystals stay tiny. When it cools super fast, there are no crystals at all. It turns into shiny black glass, called obsidian.
Hot, runny rock inside the Earth is called magma. Most of it stays inside and cools very slowly. Some comes out of a volcano. Then it is called lava, and it cools fast.
Some lava is full of gas, like a fizzy drink. The bubbles get stuck as it cools. That rock, pumice, is so full of holes that it floats on water!
Short answer: igneous rocks form when melted rock cools and hardens. Underground, melted rock is called magma. If it cools slowly deep inside the Earth, big crystals have time to grow, making intrusive rocks like granite. If it erupts as lava and cools fast at the surface, the crystals stay tiny or don't form at all, making extrusive rocks like basalt, obsidian and pumice.123
The name comes from ignis, the Latin word for fire, so igneous rocks are "fire-born" rocks.1 The key idea to remember is simple: the size of the crystals in an igneous rock tells you how fast it cooled.
An igneous rock is any rock that formed from molten (melted) rock that cooled and turned solid.1 It is one of the three main rock families, along with sedimentary rocks, which build up from layers of sediment, and metamorphic rocks, which are changed by heat and pressure without melting.213
Almost all igneous rocks are made mostly of silicon and oxygen, bonded together into minerals such as quartz, feldspar, mica, pyroxene and olivine.1 As the melt cools, these minerals grow as crystals that lock tightly into each other, a bit like the pieces of a 3D jigsaw. That interlocking texture is one of the easiest ways to recognise an igneous rock.2
It's easy to imagine a sea of liquid rock just under our feet, but that isn't true. The mantle below the crust is very hot, but it is almost all solid rock.5 Magma forms only in a few special places, and even there usually only about a tenth of the rock melts. Geologists call this partial melting.58
Surprisingly, most rock doesn't melt because it gets hotter. It melts because something else changes:5
Magma is hotter and lighter than the solid rock around it, so it slowly rises, a bit like oil rising through salad dressing.212 Magma is roughly 800 to 1,300 °C, depending on what it's made of.4 Lava from Kīlauea in Hawaii erupts at about 1,170 °C.22 You can learn how plates move in what causes tectonic plates to move.
As magma cools, its atoms slow down and start to lock together into a regular pattern: a crystal. The Geological Society of London compares it to crystals growing in a solution as it cools, like sugar crystals.3
Magma of the same make-up can turn into completely different-looking rocks. The same silica-rich melt becomes granite if it cools slowly, or rhyolite if it cools quickly.13 Some rocks even have two sizes of crystal: big ones that grew slowly underground, set in tiny ones that formed after an eruption. Geologists call this a porphyritic texture, and it shows a two-step cooling story.6
About the explainer: the crystal-size slider and the erosion slider are simplified sketches, not measurements. They show the direction of the effect, not exact sizes or times.
The difference is where the magma cools, and that decides how fast it cools.12
| Intrusive (plutonic) | Extrusive (volcanic) | |
|---|---|---|
| Where it cools | Underground, from hundreds of metres to tens of kilometres deep4 | At the surface, as lava, ash or pumice1 |
| How long it takes | Centuries to millions of years4 | Seconds to years4 |
| Crystals | Large, easy to see1 | Tiny, or none at all (glass)1 |
| Examples | Granite, diorite, gabbro, pegmatite | Basalt, andesite, rhyolite, obsidian, pumice, scoria1 |
| Name comes from | Pluto, Roman god of the underworld | Vulcan, Roman god of fire1 |
Magma that stays underground is wrapped in solid rock, which works like a thick blanket and keeps the heat in.212 That's why intrusive rocks cool so slowly. In fact, most magma never reaches the surface. It cools in the crust as a body of rock called a pluton.7 Huge plutons, with more than 100 square kilometres showing at the surface, are called batholiths. Thin sheets of magma that squeeze between rock layers are sills, and ones that cut across them are dykes (also spelled dikes).721
Lava, on the other hand, meets cold air or water, and it cools quickly.2 Runny, hot basalt lava can flow a long way and builds broad, gently sloping shield volcanoes like the ones in Hawaii.24 When lava erupts under the sea, it forms rounded blobs called pillow lava.26 To see how eruptions happen, read how volcanoes erupt.
Geologists sort igneous rocks two ways: by texture (crystal size, which tells you how it cooled) and by composition (what it's made of, which often shows in its colour).1 Dark rocks rich in iron and magnesium are called mafic. Light rocks rich in silica are called felsic or silicic.116
| Dark (mafic) | In between | Light (felsic) | |
|---|---|---|---|
| Slow, underground | Gabbro | Diorite | Granite |
| Fast, at the surface | Basalt | Andesite | Rhyolite |
Table based on the US National Park Service classification.1
Granite is the best-known intrusive rock. It's speckled, or "salt-and-pepper", because you can see its separate crystals: pale quartz and feldspar, and dark mica or hornblende.12 Some granite in Yosemite National Park has feldspar crystals up to 3 inches (about 7.5 cm) long.12 A related rock, pegmatite, can have crystals bigger than your hand.13
Basalt is a hard, dark volcanic rock. Its crystals (mostly olivine, pyroxene and plagioclase) are too small to see without a magnifier.10 Basalt lava erupts at about 1,100 to 1,250 °C and is runny, so it can flow more than 20 km from a vent.10 When a thick basalt flow cools and shrinks, it can crack into tall six-sided columns.20 If the same basalt magma cools slowly underground instead, it becomes gabbro.6
Obsidian is natural volcanic glass. It forms when sticky, silica-rich lava cools so fast that crystals can't form at all.317 It is usually black and shiny, and it breaks with smooth, curved surfaces and very sharp edges, so people across history, from ancient Greece to the Aztecs, made tools from it.17 (Never try to break obsidian yourself: the edges can cut.)
Magma holds dissolved gas, like a closed bottle of fizzy drink. As it rises, the pressure drops and the gas comes out as bubbles.19 If lava froths up and hardens with the bubbles still inside, you get a rock full of holes, called vesicles.1 Light-coloured pumice is so frothy that it usually floats on water.9 Dark scoria, which usually forms from basalt, is also full of holes, but it's usually heavier, darker and more crystal-filled than pumice.125
Everywhere, once you know where to look. The ocean floor, which covers about 55% of Earth's surface, is made of basalt about 6 km thick.1423 The continents are much thicker (20 to 70 km) and contain lots of granite.14
But if granite cools deep underground, how can we see it? Over millions of years the land is pushed up, and the rock above the granite is slowly worn away by weathering and erosion, until the old pluton is uncovered at the surface.2 Yosemite National Park is almost entirely granite, part of a huge buried body of rock called the Sierra Nevada Batholith.112 Devils Tower in Wyoming is another famous igneous landmark.13 Volcanic rocks are easiest to spot near volcanoes, such as the basalt of Hawaii or the rocks around Yellowstone, which sits over a hot spot on land.18 (Curious about Yellowstone? See what would happen if Yellowstone erupted.)
Igneous rocks are one stop on the rock cycle, the slow loop that turns one kind of rock into another.4
The rock cycle is still running on Earth because our planet's inside is still hot enough to keep the mantle moving, and we have air and liquid water at the surface. On the Moon, it has almost stopped.4
You can see the "cooling clock" with sugar, but only with an adult, because it uses very hot syrup. Ask an adult to dissolve sugar in hot water to make a thick syrup, then split it in two. Let one jar cool slowly on the counter with a string hanging in it, and over several days big sugar crystals grow (rock candy). Put a spoonful of the other on a cold plate in the fridge, and it sets fast with tiny crystals or none at all. Slow and fast cooling make a difference in the kitchen, just as they do deep inside the Earth.3 Safety: hot syrup can burn badly. An adult should do all the heating and pouring.
"All igneous rocks come from volcanoes." Most magma never erupts. It cools underground as plutons, forming intrusive rocks like granite.7
"There's an ocean of melted rock under the crust." The mantle is hot but almost entirely solid. Magma forms only where a little of the rock partly melts.5
"Rock melts because it gets hotter." Usually it melts because the pressure drops or water is added.5
"Obsidian is a crystal." It's glass. It cooled too fast for any crystals to form.3
"No rock can float." Pumice is so full of gas bubbles that it usually floats.9
Igneous rocks form when melted rock cools and hardens. Magma that cools slowly underground grows big crystals (intrusive rocks like granite). Lava that cools quickly at the surface makes tiny crystals or glass (extrusive rocks like basalt and obsidian).
Intrusive (plutonic) rocks cool slowly underground, over centuries to millions of years, so their crystals are big. Extrusive (volcanic) rocks cool quickly at the surface, in seconds to years, so their crystals are tiny or missing.
They are sometimes called fire rocks, from the Latin word ignis, meaning fire. Intrusive igneous rocks are also called plutonic rocks, and extrusive ones are called volcanic rocks.
Granite, gabbro and diorite (intrusive), and basalt, obsidian, pumice, andesite and rhyolite (extrusive).
Look for tightly interlocking crystals with no layers or fossils, or for a glassy surface or gas-bubble holes. The bigger the crystals, the more slowly the rock cooled.
If a metamorphic rock is buried deep enough and gets hot enough, part of it can melt into magma. When that magma cools and hardens, it becomes a new igneous rock. This is one path in the rock cycle.
Pumice forms when gassy lava froths up and hardens with the bubbles still inside. It is so full of holes that it is usually lighter than the same volume of water, so it floats.