Pet Rock Family

Figure 00. Are these rocks metamorphic? If so, how should they be named?

Naming Metamorphic Rocks

1.1 Overview

Rocks that have changed their mineralogy and texture in response to changes in temperature and/or pressure are called metamorphic rocks. How do geologists recognize rocks as metamorphic? If a rock is metamorphic, what is the proper name to use for it? These pages present the critera that are commonly used to recognize and name metamorphic rocks.
1.2 Recognizing Metamorphic 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 metamorphic 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. If you are familiar with these features and believe you can recognize metamorphic rocks, visit the to confirm your understanding.

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