Igneous vs Sedimentary
The three main categories of rock — igneous, sedimentary, and metamorphic — are defined by how they formed. Igneous rocks crystallise from cooled magma or lava. Sedimentary rocks form from deposited and compacted layers. Metamorphic rocks are existing rocks transformed by heat and pressure. Together they make up the rock cycle that has shaped Earth's surface for billions of years.
Last reviewed on 2026-04-27.
Quick Comparison
| Aspect | Igneous | Sedimentary |
|---|---|---|
| How it forms | Cools and solidifies from magma or lava | Deposited as layers, then compacted and cemented |
| Where it forms | Underground (intrusive) or at surface (extrusive) | On surface, often in water |
| Texture | Crystalline (large or small crystals depending on cooling rate) | Layered, often with visible grains or fossils |
| Common examples | Granite, basalt, obsidian, pumice | Sandstone, shale, limestone, conglomerate |
| Fossils? | No (heat would have destroyed them) | Yes — sedimentary is the main fossil-bearing category |
| Shows layers? | Sometimes (lava flows) | Yes — almost always |
| Hardness | Generally hard | Varies — soft (chalk) to hard (well-cemented sandstone) |
Key Differences
1. Formation
Igneous rocks form when molten rock cools. Intrusive igneous rock cools slowly underground, growing large crystals (granite). Extrusive igneous rock cools quickly at the surface from lava, with small crystals or no crystals at all (basalt, obsidian, pumice).
Sedimentary rocks form at the surface from accumulated sediments — sand, mud, the remains of organisms, mineral precipitates. Layers accumulate over thousands to millions of years; pressure and cementation lithify them into solid rock.
2. A third category: metamorphic
Metamorphic rocks come from heat and pressure transforming existing rock without melting it. Limestone becomes marble; shale becomes slate; granite becomes gneiss; sandstone becomes quartzite.
The transformation can be regional (large-scale tectonic forces) or contact metamorphism (heat from a nearby igneous intrusion).
3. Visible features
Igneous rocks show interlocking crystals — large in slowly-cooled granite, small in basalt. Some, like obsidian, are glassy because they cooled too fast for crystals to form.
Sedimentary rocks show layers (bedding), often grains of varying size, and frequently fossils. Sandstone's grains are visible to the eye; shale's are too fine to distinguish without magnification.
4. Common examples
Igneous: granite (kitchen counters, gravestones), basalt (oceanic crust, volcanic islands), obsidian (volcanic glass, used historically for tools), pumice (light enough to float).
Sedimentary: sandstone (cliff faces, building stone), shale (often produces oil and gas), limestone (cathedral stone, cement raw material), conglomerate (cemented gravels). Metamorphic: marble (sculpture, decorative stone), slate (roofing), schist, gneiss, quartzite.
5. Fossils
Igneous rocks don't contain fossils. Anything organic was destroyed in the molten state.
Sedimentary rocks are the main reservoir of fossils. Organisms buried in sediment have a chance of preservation as the surrounding material lithifies. Metamorphic rocks rarely preserve fossils — heat and pressure usually destroy them, though distorted fossils sometimes survive in lower-grade metamorphic rocks.
6. The rock cycle
Each category can become any of the others. Igneous rock weathers into sediments that become sedimentary rock; sedimentary rock can be metamorphosed into metamorphic rock; both can melt and become igneous again.
The cycle operates on timescales of millions to billions of years. The Earth's surface is constantly recycling its rocks through plate tectonics, erosion, and volcanism.
When to Choose Each
Choose Igneous if:
- Geology of volcanic regions, ocean crust, deep mountain roots.
- Building materials made from granite or basalt.
- Identifying igneous rock by texture and crystal pattern.
Choose Sedimentary if:
- Geology of sedimentary basins, where most fossil fuels are found.
- Construction with sandstone, limestone, slate.
- Paleontology — fossil-bearing rocks are sedimentary or low-grade metamorphic.
Worked example
A river flowing across granite (igneous) breaks down crystals through weathering, carrying grains downstream. The grains settle in a delta and accumulate over millennia, becoming sandstone (sedimentary). Tectonic forces later push that sandstone deep underground; heat and pressure transform it into quartzite (metamorphic). Sufficient heat melts the quartzite back to magma, which cools as new granite. The cycle has run once.
Common Mistakes
- "Sedimentary rocks are weak." Some are; well-cemented sandstones and limestones are strong building materials.
- "Igneous rocks are always volcanic." Volcanic rocks (extrusive) cool at surface; many igneous rocks (intrusive) cooled deep underground.
- "Metamorphic rocks are just transformed sedimentary." Metamorphic rocks can come from any starting type — sedimentary, igneous, or other metamorphic.
- "Once a rock, always that type." The rock cycle continues; rocks transform through geological time.