How do rocks change from one type to another, and how do we identify an igneous rock?
Describe the rock cycle and explain how igneous rocks form from cooling magma or lava, using the Reference Tables Scheme for Igneous Rock Identification to relate texture, composition, color and density to the rock name.
A Regents answer on the rock cycle and igneous rocks: the three rock families and the processes that link them, how cooling rate controls crystal (grain) size, how the Scheme for Igneous Rock Identification relates texture, mineral composition, color and density to a rock name (granite, basalt, obsidian and others), with worked exam questions.
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What this topic is asking
The Regents wants you to describe the rock cycle (how the three rock families convert into one another) and to identify igneous rocks using the Scheme for Igneous Rock Identification. The single most important igneous idea is that cooling rate controls crystal (grain) size.
The rock cycle
The Reference Tables include a rock cycle diagram showing every family connected by these processes. The key point the Regents tests: there is no fixed order. A metamorphic rock can melt to magma, an igneous rock can be weathered to sediment, and a sedimentary rock can be metamorphosed, all depending on the conditions.
How igneous rocks form: cooling rate sets texture
Igneous rocks crystallize from molten rock. The rate of cooling determines the crystal (grain) size, which is the rock's texture:
- Slow cooling, deep underground (intrusive/plutonic): atoms have time to organize into large, visible crystals (coarse texture). Example: granite.
- Fast cooling, at or near the surface (extrusive/volcanic): crystals have little time to grow, so they are small (fine texture). Example: basalt.
- Very fast cooling (quenching): no crystals form at all, giving a glassy texture. Example: obsidian.
- Trapped gas during eruption gives a vesicular (holey) texture. Example: pumice or scoria.
Composition: felsic to mafic
The Scheme also arranges igneous rocks by mineral composition, which controls color and density:
- Felsic (granitic): rich in quartz, potassium feldspar and plagioclase; light-colored, low density (about 2.7 g/cm cubed). Example: granite (coarse), rhyolite (fine).
- Mafic (basaltic): rich in pyroxene, olivine and plagioclase; dark-colored, higher density (about 3.0 g/cm cubed). Example: gabbro (coarse), basalt (fine).
- Felsic magmas are richer in silica and lighter; mafic magmas are poorer in silica, denser and darker.
So texture tells you where it cooled, and composition/color/density tells you what it is made of. You need both to name a rock from the chart.
Try this
Q1. State what controls the crystal size of an igneous rock. [1 point]
- Cue. The rate of cooling (slow cooling gives large crystals, fast cooling gives small crystals).
Q2. Explain how an igneous rock could become a metamorphic rock. [2 points]
- Cue. Heat and pressure (for example from deep burial or nearby magma) change the rock's minerals and texture without melting it, producing a metamorphic rock.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (style)1 marksPart B-1. Using the Scheme for Igneous Rock Identification, an igneous rock has very large, intergrown crystals (coarse texture) and a light color with low density. Which rock is it? (1) basalt (2) gabbro (3) granite (4) obsidian. Justify your choice.Show worked answer →
A 1-point Reference Tables question. The answer is (3).
A coarse (large-crystal) texture means slow cooling deep underground (intrusive). A light color and low density means a felsic (granitic) composition, rich in quartz and feldspar. On the chart, the coarse, felsic rock is granite (3). Basalt (1) is fine-grained and dark (mafic); gabbro (2) is coarse but dark (mafic); obsidian (4) is glassy (very fast cooling) and has no crystals. The trap is matching color without also matching texture, or vice versa.
Regents (style)3 marksPart C. (a) Explain why granite has large mineral crystals while basalt of similar composition has very small crystals. (b) Identify whether each rock cooled at or below Earth's surface. (c) Using the rock cycle, describe one way granite could become a sedimentary rock.Show worked answer →
A 3-point extended-response question.
(a) 1 point: granite cooled slowly, so atoms had time to form large crystals; basalt cooled quickly, so only small crystals formed (cooling rate controls crystal size).
(b) 1 point: granite (coarse, large crystals) cooled below the surface (intrusive); basalt (fine, small crystals) cooled at or near the surface (extrusive).
(c) 1 point: granite is weathered into sediments, the sediments are transported and deposited, then compacted and cemented (lithified) into a sedimentary rock such as sandstone or conglomerate.
Markers reward slow versus fast cooling for crystal size, the intrusive/extrusive placement, and a valid weathering, erosion, deposition, then compaction/cementation pathway.
Related dot points
- Define a mineral and explain how physical properties (hardness, cleavage, luster, streak, color and density) and chemical composition are used to identify minerals, using the relevant Reference Tables charts.
A Regents answer on minerals: the definition of a mineral, the physical properties used to identify them (hardness, cleavage and fracture, luster, streak, color, density), why composition and internal arrangement control those properties, and how to use the Reference Tables Properties of Common Minerals chart, with worked exam questions.
- Explain how sedimentary rocks form by compaction and cementation or by chemical and biologic processes, and how metamorphic rocks form by heat and pressure, using the Reference Tables charts to identify each by texture and composition.
A Regents answer on sedimentary and metamorphic rocks: clastic versus chemical and biologic sedimentary rocks, compaction and cementation, the role of fossils and sorting, foliated versus nonfoliated metamorphic rocks, contact and regional metamorphism, and how to use the Reference Tables identification charts, with worked exam questions.
- Describe the layered structure of Earth's interior and explain the theory of plate tectonics, including the evidence (sea-floor spreading, matching coastlines, fossils, magnetic stripes) and the calculation of plate spreading rate.
A Regents answer on Earth's interior and plate tectonics: the crust, mantle, outer and inner core and the Reference Tables inferred properties, mantle convection as the driver, the three boundary types, the evidence for sea-floor spreading (matching coastlines, fossils, magnetic stripes, age of sea floor), and a worked spreading-rate calculation.
- Explain where and why volcanoes form (boundaries and hot spots), describe how crustal rock is deformed by folding, faulting and tilting, and interpret evidence of crustal movement such as displaced rock layers and marine fossils on mountains.
A Regents answer on volcanoes and crustal deformation: why volcanoes form at subduction zones, divergent boundaries and hot spots, the Ring of Fire, how rock is folded, faulted and tilted, and the evidence that the crust has moved (displaced strata, tilted layers, marine fossils and rounded sediments now on mountains), with worked exam questions.
- Distinguish physical from chemical weathering, explain the factors that control the rate of weathering (climate, surface area, rock type), and describe how weathering and other processes form soil.
A Regents answer on weathering and soil: physical (mechanical) weathering such as frost wedging versus chemical weathering such as carbonation and oxidation, how climate, surface area and rock type control the rate, why warm wet climates weather chemically faster, and how soil forms as a mix of weathered rock and organic matter, with worked exam questions.
Sources & how we know this
- Reference Tables for Physical Setting/Earth Science (2011 edition) — New York State Education Department (2011)
- Regents Examination in Physical Setting/Earth Science — New York State Education Department (2026)