3.2 Rocks & the Rock Cycle

Three Rock Families

Every rock belongs to one of three groups based on its origin: igneous, sedimentary, or metamorphic. The Earth and Space Sciences Regents tests how processes such as cooling, weathering, and heating move material between these groups, and it provides identification charts in the reference tables so you reason from evidence rather than memorizing rock names.

Igneous Rocks

Igneous rocks crystallize from molten rock. The most important variable is cooling rate, because it controls crystal (grain) size.

• Intrusive (plutonic) rocks cool slowly deep underground, giving atoms time to grow large, visible crystals (coarse texture). Granite is the classic example.
• Extrusive (volcanic) rocks cool quickly at or near the surface as lava, producing small or invisible crystals (fine texture). Basalt is the classic example. Extremely fast cooling can form glassy obsidian with no crystals.

This is why granite shows large visible mineral grains while basalt looks fine and uniform even though both can share similar chemistry.

Felsic versus Mafic Composition

The second variable is composition, summarized by the felsic–mafic spectrum:

Felsic rocks are light-colored, lower-density, and silica-rich; mafic rocks are dark, denser, and rich in iron and magnesium. The ESRT Scheme for Igneous Rock Identification lets you cross grain size (cooling environment) with mineral composition / color / density to name a rock: read down for texture and across for composition until you land on granite, basalt, gabbro, rhyolite, and so on.

Sedimentary Rocks

Sedimentary rocks form at or near the surface when sediment is compacted (squeezed) and cemented (glued by minerals). They divide by how the sediment originated.

• Clastic sedimentary rocks are made of broken fragments (clasts) of older rock, classified by grain size: clay-sized particles form shale, sand-sized form sandstone, and pebble-sized fragments form conglomerate.
• Chemical sedimentary rocks form when dissolved minerals precipitate out of water, such as rock salt (halite) and some limestone.
• Organic (biochemical) sedimentary rocks form from the remains of once-living things, such as coal (compressed plant material) and fossil-rich limestone.

Two features the Regents loves: sorting and fossils. Sorting describes how uniform the grain sizes are; well-sorted sediment (similar sizes) usually indicates long transport or steady water energy, while poorly sorted sediment (mixed sizes) suggests rapid deposition such as a glacier dropping its load. Fossils are found almost exclusively in sedimentary rock, because the heat and pressure that make igneous and metamorphic rock destroy delicate remains.

Metamorphic Rocks

Metamorphic rocks form when existing rock is changed in the solid state by heat and pressure without fully melting. The classic split is by texture.

• Foliated metamorphic rocks show banding or aligned minerals caused by directed pressure. Shale metamorphoses to slate, then to schist, then to gneiss as conditions intensify.
• Non-foliated metamorphic rocks lack banding because they are made largely of one mineral or formed under uniform pressure. Limestone becomes marble; sandstone becomes quartzite.

Metamorphism also varies by setting. Contact metamorphism affects rock baked locally next to a hot magma body, producing a narrow altered zone. Regional metamorphism affects large areas during mountain building, where intense directed pressure typically produces foliation.

The Rock Cycle

The rock cycle ties all three families together: with enough time and energy, any rock can become any other. The cycle has no single starting point.

• Melting and cooling → igneous rock.
• Weathering, erosion, deposition, compaction, and cementation → sedimentary rock.
• Heat and pressure (short of melting) → metamorphic rock.
• Re-melting returns any rock to magma, restarting the cycle.

On the Regents, expect a cycle diagram with one arrow labeled “?” and a question asking which process fits. Match the arrow's start and end rock to the process: an arrow from igneous rock toward sediment is weathering and erosion, while an arrow from any rock into magma is melting.

Reading the Cycle as Processes

It helps to memorize which process connects each pair of rock types, because the Regents diagram tests transitions rather than rock names:

A single rock can take many paths through the cycle over geologic time. For example, a granite (igneous) exposed at the surface can weather into sand, that sand can be buried and cemented into sandstone (sedimentary), and the sandstone can later be heated and squeezed into quartzite (metamorphic) — then melted and cooled back into a new igneous rock, with no fixed beginning or end.

Connecting Rocks to Earth Systems

The rock cycle is not an isolated topic; it links directly to other Regents strands. Weathering and erosion that produce sediment are surface processes you study elsewhere in Earth's Systems, plate tectonics (3.3) supplies the heat and pressure for igneous and metamorphic rock at boundaries, and fossils preserved in sedimentary rock are the foundation of the History of Earth strand. When you see a rock on the exam, ask three questions: how did it form, what does its texture reveal about that formation, and where in the cycle could it go next.

That habit turns rock identification from memorization into reasoning, which is exactly what three-dimensional NYSP12SLS questions reward.