The Grammar of Rocks

Rock names are discrete labels imposed on continuous geological manifolds. The mineral composition, grain size, temperature, pressure, and formation history of a rock all vary continuously, yet petrology assigns each specimen a discrete name. Three domains (igneous, sedimentary, metamorphic) each use their own classification grammar: different axes, different boundaries, different decision logic.

Igneous Classification

The primary axis is silica content. SiO₂ percentage partitions the space into ultramafic (<45%), mafic (45–52%), intermediate (52–63%), and felsic (>63%). The secondary axis is texture: plutonic (coarse-grained, slow cooling at depth) versus volcanic (fine-grained, rapid cooling at surface). For ultramafic plutonic rocks, a ternary classification based on olivine, orthopyroxene, and clinopyroxene modal proportions further partitions the space.

Sedimentary Classification

Clastic sedimentary rocks are classified by grain size following the Wentworth scale: clay (<0.004 mm), silt (0.004–0.063 mm), sand (0.063–2 mm), gravel (>2 mm). Within each size, modifiers refine the classification: fissility distinguishes shale from mudstone, rounding separates conglomerate from breccia, feldspar content yields arkose, poor sorting gives graywacke.

Chemical and organic sedimentary rocks follow different logic entirely: composition (carbonate, siliceous, evaporite) and biogenic origin (plant → coal, shell → coquina, diatom → diatomite).

Metamorphic Classification

Metamorphic classification depends on three variables: protolith (what the rock was before), grade (how much temperature and pressure it experienced), and the pressure regime. The same protolith at different P-T conditions yields different rocks. A pelitic (clay-rich) protolith follows the sequence slate → phyllite → schist → gneiss → migmatite with increasing grade. A mafic protolith takes a different path: greenschist → amphibolite → granulite at normal pressure, but blueschist → eclogite under high pressure.

The Rock Cycle

All three domains connect through geological processes. Weathering breaks down igneous and metamorphic rocks into sediment, lithification compresses sediment into sedimentary rock, metamorphism transforms any rock under heat and pressure, and melting returns material to magma. Every rock is potentially every other rock given enough time and the right conditions. The cycle has no beginning and no end.

Boundaries

Some classification boundaries are sharp: dunite requires >90% olivine. Others are fuzzy: the granite–granodiorite boundary depends on the ratio of alkali feldspar to plagioclase, which varies continuously. Still others are purely conventional: the 63% SiO₂ line separating intermediate from felsic is a human decision, not a physical discontinuity. The playground makes these different boundary types visible: nature is continuous, nomenclature is discrete.