Igneous Rocks

Igneous rocks are rocks that form when hot molten rock (magma underground, or lava at the surface) cools down and hardens. The name comes from the Latin ignis meaning "fire". Igneous rocks make up the vast majority of Earth's crust by volume, including most of the ocean floor and the cores of mountain ranges. They are also the original source of all other rocks: sedimentary rocks are made from weathered igneous (and other) rocks, and metamorphic rocks are squeezed and cooked versions of either of the others.

  • Latin nameIgnisMeans "fire"
  • % of crust by volumeApproximately 90%Most of Earth's solid crust
  • Two main typesIntrusive (deep), extrusive (surface)
  • Famous intrusiveGraniteSlow cooled deep underground
  • Famous extrusiveBasaltQuick cooled from lava
  • StrangestObsidianVolcanic glass with no crystals

Two main types: intrusive and extrusive

  • Intrusive (or plutonic) rocks: form when magma cools slowly deep underground, often over thousands or millions of years. Slow cooling lets large crystals grow, so intrusive rocks have visibly grainy textures. Granite is the most familiar example.
  • Extrusive (or volcanic) rocks: form when lava cools quickly at or near the surface, often in just hours or days. Fast cooling traps the minerals as tiny crystals (or no crystals at all). Basalt and obsidian are familiar examples.

Granite: the intrusive king

Granite is the most common intrusive igneous rock. It forms deep underground when magma cools over millions of years. The slow cooling allows large interlocking crystals to grow, giving granite its distinctive speckled appearance. The main minerals are quartz (grey or clear), feldspar (pink or white) and mica (dark flakes).

Granite is hard, durable and beautiful, which is why it has been used for buildings and monuments for thousands of years. The pyramids of Egypt contain granite. Many of the world's mountain ranges (including the Sierra Nevada, much of the Himalayas, and large parts of Scotland and Cornwall) are made of granite.

Basalt: the extrusive workhorse

Basalt is the most common extrusive igneous rock and is by far the most common rock on Earth's surface. Almost the entire ocean floor is made of basalt. It forms when lava from volcanoes cools quickly, producing tiny crystals that are barely visible to the naked eye. Basalt is usually dark grey or black.

The famous Giant's Causeway in Northern Ireland is made of approximately 40,000 hexagonal columns of basalt, formed when a thick lava flow cooled and cracked into regular shapes about 60 million years ago. Similar columns appear at Fingal's Cave in Scotland and Devil's Tower in Wyoming.

The strange ones: obsidian and pumice

  • Obsidian: volcanic glass. Forms when lava cools so fast that no crystals can form at all. Smooth, shiny, and breaks into sharp curved flakes that early humans used as cutting tools. The world's sharpest known surgical scalpels are made of obsidian.
  • Pumice: full of bubble holes, light enough to float on water. Forms from frothy lava blasted out of explosive volcanic eruptions. Used as a cleaning stone for centuries, and still found in many bathrooms today.
  • Scoria: a darker, denser cousin of pumice with smaller bubbles. Common around volcanoes.
Fact The largest single intrusive rock formation in the world is the Wyoming Batholith, a giant mass of granite covering approximately 50,000 square kilometres in the western United States. It formed deep underground 60 to 80 million years ago and has since been exposed by uplift and erosion. Half the famous Rocky Mountain peaks of Wyoming, Montana and Idaho are carved from it.

Where igneous rocks form

Igneous rocks form wherever magma exists.

  • Mid-ocean ridges: where new ocean floor is continuously being made from rising magma. Almost all of it is basalt.
  • Volcanic arcs: chains of volcanoes above subduction zones (like the Andes, Cascades, Japan). Produces andesite, basalt and various other lava types.
  • Hot spots: places where rising plumes of magma punch through the crust from deep below. Examples include Hawaii and Yellowstone.
  • Continental rifts: where continents are pulling apart, like the East African Rift.
  • Mountain roots: deep below colliding continents, where vast amounts of granite form intrusively.
Did you know? The black sand beaches of Iceland, Hawaii and Tahiti are made of tiny grains of basalt. The basalt comes from local lava flows that have been worn down by waves over centuries. White sand beaches are usually made of weathered shell, coral or quartz; the dramatic black sand beaches owe their colour to ground-up igneous rock.
Deeper dive: how the Giant's Causeway formed

One of the most famous igneous rock features in the world is the Giant's Causeway on the north coast of Northern Ireland. It consists of approximately 40,000 hexagonal columns of basalt, neatly fitted together like a giant tiled pavement leading into the sea. The columns range from 30 cm to over a metre wide. According to legend, the causeway was built by an Irish giant called Finn MacCool so he could walk to Scotland to fight a Scottish giant. The real story is geologically interesting too.

Approximately 60 million years ago, North America and Europe were beginning to pull apart, opening up the early Atlantic Ocean. As the crust stretched, huge volumes of basalt magma erupted along the new boundary. One of these eruptions left a thick layer of lava across what is now northern Ireland and western Scotland.

The hexagonal column pattern formed as the lava cooled. As any liquid cools, it contracts. With a flat sheet of cooling lava, the contraction causes the surface to crack into regular geometric shapes. Hexagons happen to be the most efficient pattern (every line meets the next at 120 degrees, evenly distributing the stress). So the cracks at the surface formed roughly hexagonal patterns. As cooling continued downwards through the lava sheet, the cracks extended down with it, creating the tall hexagonal columns we see today.

You can see the same pattern in mud as it dries in the sun: cracks tend to meet at 120 degrees and produce hexagons. The Giant's Causeway is just the same thing on a much larger scale, with the bonus that the rock is forever set in stone.

For other rock types, see sedimentary rocks and metamorphic rocks.