How do sedimentary and metamorphic rocks form, and how do we tell them apart?
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.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
The Regents wants you to explain how sedimentary rocks form (from sediments by compaction and cementation, or by chemical and biologic processes) and how metamorphic rocks form (from existing rock by heat and pressure), and to identify each using the Reference Tables charts. The key tests are the clastic versus chemical split for sedimentary rocks and the foliated versus nonfoliated split for metamorphic rocks.
Sedimentary rocks
The Reference Tables chart classifies clastic sedimentary rocks by particle size (largest to smallest): pebbles and larger give conglomerate (rounded) or breccia (angular); sand gives sandstone; silt gives siltstone; clay gives shale. Chemical and biologic rocks are classified by composition: halite gives rock salt, calcite gives limestone, and plant remains give coal (bituminous). Two clues that a rock is sedimentary: layering (bedding) and the presence of fossils, which only form in sedimentary rocks.
Metamorphic rocks
Metamorphism comes in two settings:
- Regional metamorphism: broad areas changed by the heat and directed pressure of mountain building or burial. It produces foliation (aligned mineral bands).
- Contact metamorphism: rock baked by the heat of nearby magma, with little directed pressure, often producing nonfoliated rock.
Telling the families apart
| Feature | Clastic sedimentary | Foliated metamorphic |
|---|---|---|
| Texture | Separate cemented grains or fragments | Aligned mineral bands or layers |
| Layering | Flat bedding from deposition | Banding from directed pressure |
| Fossils | Often present | Destroyed by heat and pressure |
| Origin | Compaction and cementation at the surface | Heat and pressure at depth |
Try this
Q1. State the two processes that turn loose sediments into a clastic sedimentary rock. [2 points]
- Cue. Compaction (squeezing under overlying weight) and cementation (dissolved minerals gluing the grains together).
Q2. Explain why fossils are found in sedimentary rocks but not in most metamorphic or igneous rocks. [2 points]
- Cue. Fossils form when remains are buried gently in sediment; the heat and pressure of metamorphism and the melting of igneous rock destroy them.
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 A. A rock is made of rounded pebbles cemented together in a finer matrix. This rock is best classified as (1) igneous (2) clastic sedimentary (3) foliated metamorphic (4) chemical sedimentary. Justify your choice.Show worked answer →
A 1-point multiple-choice question. The answer is (2).
Rounded pebbles cemented together are fragments (clasts) of older rock that were transported, deposited, then compacted and cemented, which defines a clastic sedimentary rock (this one is conglomerate). It is not igneous (1, no crystals from cooling), not foliated metamorphic (3, no banding from pressure), and not chemical sedimentary (4, which forms from minerals precipitating out of water, like rock salt). The rounding of the pebbles is the clue that they were transported by water. The trap is calling any rock with separate grains "igneous".
Regents (style)3 marksPart C. (a) Describe the two main processes that turn loose sediments into a sedimentary rock. (b) Explain how the metamorphic rock gneiss could form from the sedimentary rock shale. (c) State one way to tell a foliated metamorphic rock from a clastic sedimentary rock.Show worked answer →
A 3-point extended-response question.
(a) 1 point: compaction (the weight of overlying material squeezes the sediments together) and cementation (dissolved minerals precipitate and glue the grains together).
(b) 1 point: shale is buried and subjected to heat and pressure, which recrystallizes its minerals and aligns them into bands; with increasing grade, shale becomes slate, then schist, then gneiss.
(c) 1 point: a foliated metamorphic rock shows aligned mineral bands or layers produced by pressure, whereas a clastic sedimentary rock shows separate cemented grains or fragments (and may contain fossils).
Markers reward compaction and cementation, a valid heat-and-pressure recrystallization pathway, and a distinguishing feature (foliation/banding versus cemented clasts).
Related dot points
- 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.
- 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.
- 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.
- Explain how deposition occurs as transporting agents lose energy, and use the Reference Tables relationship of particle size to water velocity, together with particle size, shape and density, to predict settling order and sorting.
A Regents answer on deposition and sorting: how sediment is dropped when a transporting agent slows, the Reference Tables graph of transported particle size versus water velocity, why larger and denser particles settle first, horizontal and vertical sorting, graded bedding, and how rounded versus angular shape affects settling, with worked exam questions.
- Explain how fossils form, what index fossils are, and how fossils are used to correlate rock layers between distant locations and to infer past environments, using the Reference Tables.
A Regents answer on fossils and correlation: how fossils form, the features of a good index fossil (widespread, short-lived, easily recognized), how index fossils and matching rock match (correlate) layers between distant outcrops, what fossils reveal about past environments and evolution, and how to read the Geologic History of New York State chart, 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)