Short answer
Igneous rock forms when molten rock solidifies; sedimentary rock forms from accumulated deposits or, in some cases, material precipitated from water. The central distinction is formation history, not whether a specimen looks grainy or lies at the surface. 1 2
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At a glance
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| Question or attribute | Igneous rock | Sedimentary rock |
|---|---|---|
| Defining origin | Solidified molten rock | Accumulated deposits or precipitation from water |
| Formation setting | Within Earth or at/near its surface | Deposits accumulate at Earth’s surface |
| Common texture clue | Coarse crystals, fine grains, or glassy texture | Bedding or fragments in finer material |
| Examples | Granite, diorite, gabbro | Sandstone, shale, limestone |
These are formation categories; the texture clues are not standalone definitions. 1 2
What each thing is
Igneous rock has two main groups: intrusive, also called plutonic, and extrusive, also called volcanic. Sedimentary rock includes material derived from older rocks and biological remains, as well as limestone formed directly from water. Neither category names a single composition or a single-looking material. 1 2
Key differences
For igneous rock, cooling conditions help explain texture: crystals have more time to grow in intrusive rock, while rapid cooling produces fine-grained or glassy extrusive rock. For sedimentary rock, the source emphasizes accumulation and deposition, often involving sediment transported by rivers into lakes or oceans. These are different routes to rock formation. 1 2
How to tell them apart
Start with evidence of origin. Bedding or rock fragments surrounded by finer material supports a sedimentary interpretation; coarse mineral crystals or a glassy texture can support an igneous interpretation. Use these as clues, not verdicts: the sources describe several textures and say sedimentary bedding is frequent, not universal. 1 2
Where they overlap
Both belong to the broader category of rock, which the USGS describes as mineral aggregates or undifferentiated mineral matter. Both can also originate at or near Earth’s surface: extrusive igneous rock solidifies there, while sedimentary deposits accumulate there. Surface formation alone therefore does not separate them. 1 2
Edge cases
Limestone is a useful boundary case within the sedimentary category. It can form through biological accumulation or by precipitation directly from water. A definition limited to broken pieces of older rock would wrongly exclude these pathways; sedimentary does not simply mean a collection of visible pebbles or sand grains. 2
Why the distinction exists
The labels preserve different geological histories: solidification of melt versus formation from deposits or precipitation. Metamorphic rock provides the third-class relationship: the supplied related FAQ says it began as another rock type. That identifies a changed prior rock rather than either formation pathway described here, although the excerpt does not explain the change fully. 1 2
Common misconceptions
Igneous does not mean exclusively volcanic: granite is an intrusive example. Sedimentary does not mean invariably layered: USGS describes bedding as common rather than required. The paired terms intrusive/plutonic and extrusive/volcanic describe igneous subgroups in these sources, not documented regional naming differences. 1 2
Examples
Two cases apply the distinction:
- Granite is classified as intrusive igneous rock: its relevant history is molten rock solidifying inside Earth, not sediment deposition. 1
- Leesburg Conglomerate is sedimentary: the source describes rock fragments among finer material and deposition as a fan beside the Culpeper Basin. Its fragments record a depositional origin rather than solidification from melt. 2