1.1 Overview

Rocks that have solidified from a molten silicate liquid (magma) either underground (plutonic) or on the surface (volcanic) are called igneous rocks. How do geologists recognize rocks as igneous? If a rock is igneous, what is the proper name to use for it? This article presents the critera that are commonly used to recognize and name igneous rocks.

1.2 Recognizing Igneous Rocks

Geologists use many types of information to help them recognize and name rocks. Regional geology, field relations, outcrop features, hand sample textures, mineralogy, thin section textures, rock and mineral chemistry are all used if available. Some important features that can be used to identify igneous rocks are listed in the following table with "X" marks for common features by rock type. Rare but possible features have "(x)" marks. Click on the table entries to read information about these features and to see photos of rocks with them (🚧 under construction). If you are familiar with these features and believe you can recognize igneous rocks, visit the Next Page to confirm your understanding.

Rock Feature	Ign	Met(Ign)	Met(Sed)	Sed
Igneous Rock Only Features	X
elipsoidal holes (vesicles)	X
glass	X
volcanic features (e.g. lava flow, spatter cone, etc.)	X
pillow structures	X
cross-cutting relationships (e.g. dike)	X	(x)
xenoliths	X	(x)
certain Ign minerals (e.g. sanidine, leucite, pigeonite)	X
Igneous Rock Shared Features	X	X	X	X
mosaic texture (crystalized in place)	X	X	X
euhedral minerals (not rounded)	X	X	X
absence of layering	X	X
random orientation of grains (not aligned)	X	(x)	(x)	X
garnet present	X	X	X
angular grains in matrix: Ign (pyroclastic) Sed (immature), Met (cataclastic)	X	X	X	X
presence of layering: Ign (lava flows, pyroclastic rocks)	X	X	X	X
Features Igneous Rocks Do Not Have		X	X	X
ductile deformation (e.g. folding, stretching, flattening)		X	X
aligned mineral grains (foliation)		X	X
rounded grains				X
fossils or trace fossils			(x)	X
sedimentary features (e.g. cross-bedding, ripple marks)			(x)	X
carbonate mineral-rich	(x)		X	X
mica-rich			X
gneissic banding		X	X
certain Met minerals (e.g. staurolite, chloritoid)			X

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