The rock cycle
A grain of quartz can sit in granite, wash onto a beach as sand, harden into sandstone and end up in quartzite.
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Rocks seem permanent, but over geological time they are recycled. The BGS describes the rock cycle as a picture of Earth as a dynamic system: all its parts are connected, and a change in one part produces a change in another, in a cycle that has run throughout geological time. The three types of rock are stops on that cycle, and the processes from the last three units are the roads between them.
The main roads
Start anywhere. Magma cools and crystallizes into igneous rock. Igneous rock lifted to the surface is weathered and eroded into sediment. Sediment is deposited, buried, compacted and cemented into sedimentary rock. Buried deeper, or caught where plates collide, sedimentary rock is heated and squeezed into metamorphic rock. Heated further, metamorphic rock melts into magma, and the loop is closed.
There are shortcuts too, and the cycle has no fixed direction. Igneous rock can be metamorphosed without ever becoming sediment: granite squeezed in a mountain belt becomes granite gneiss. Metamorphic rock pushed up into mountains is worn down into sediment just like any other rock. Sedimentary rock can be weathered back into sediment, as when sandstone cliffs crumble into sand that may one day become sandstone again. And any rock that gets hot enough will melt.
Following one grain
Quartz is hard (7 on the Mohs scale), and the BGS finds it both as crystals in granite and as grains in sandstone. Picture a quartz crystal that grew in a granite deep underground. Uplift and erosion bring the granite to the surface, where it is weathered. The rock falls apart, and the quartz grain is washed down a river and dropped on a beach. Buried under more sand, it is cemented into sandstone. Millions of years later the sandstone is caught up in a mountain-building collision and becomes quartzite. Each stop is a rock or sediment described in this course: granite, sand, sandstone, quartzite.
Try the same idea in the diagram. Tap Sedimentary rock and look at its ways out: it can be weathered back into sediment, or heated and squeezed into metamorphic rock. Then tap Metamorphic rock. It can melt into magma or be worn down into sediment. Igneous, sedimentary and metamorphic rock each have two exits in the diagram, which is why no two trips around the cycle need to look alike.
Why so little very old rock survives
Because rock is constantly recycled, Earth keeps few rocks from its earliest days. The USGS explains that Earth's oldest rocks have been recycled and destroyed by plate tectonics, and that if any of the planet's first crust is left in its original state it has not been found. Among Earth's oldest well-dated rocks are the metamorphic Acasta gneisses of north-western Canada, about 4.03 billion years old. Even those have already been around the cycle at least once.
| Magma → igneous | Cooling and crystallization |
| Any rock → sediment | Weathering and erosion |
| Sediment → sedimentary rock | Deposition, burial, compaction, cementation |
| Rock → metamorphic | Heat and pressure without melting |
| Rock → magma | Melting |
- Which process turns magma into igneous rock?
- Cooling and crystallization
- Cementation
- Weathering
- Which process turns metamorphic rock into magma?
- Melting
- Erosion
- Compaction
- Can igneous rock become metamorphic without first becoming sediment?
- Yes, by heat and pressure
- No, it must become sediment first
- Only if it melts first
- Order one path through the cycle for a quartz grain.
- Which mineral appears in granite, sandstone and quartzite?
- Quartz
- Calcite
- Talc
- Does the rock cycle have a fixed starting point?
- No; any rock can follow several routes
- Yes, always magma
- Yes, always sediment
- Why does Earth keep so little of its earliest rock?
- Plate tectonics has recycled and destroyed it
- It was all carried off by meteorites
- It is all under the oceans and has been dated
- The oldest dated rocks on Earth, the Acasta gneisses, are…
- metamorphic
- sedimentary
- volcanic glass
- Sandstone cliffs crumble into sand. Which arrow is that?
- Sedimentary rock → sediment (weathering, erosion)
- Sediment → sedimentary rock
- Metamorphic rock → magma
- Which pair are both processes, not rock types?
- Erosion and cementation
- Granite and erosion
- Marble and slate
- Magma → igneous (cooling) → sediment (weathering, erosion) → sedimentary (cementation) → metamorphic (heat, pressure) → magma (melting).
- Shortcuts and reverse routes exist; there is no fixed start.
- Quartz turns up in granite, sand, sandstone and quartzite.
- Plate tectonics has recycled Earth's earliest rocks; the oldest dated are about 4.03 billion years old.
Sources for this course (18)
- USGS FAQs (usgs.gov/faqs): What is the difference between a rock and a mineral? (mineral definition; common minerals quartz, feldspar, mica, amphibole, olivine, calcite; rock = aggregate of one or more minerals)
- What are igneous rocks? (Latin for fire; intrusive/plutonic slow cooling over thousands or millions of years, coarse grained, granite, diorite, gabbro, peridotite; extrusive/volcanic fine grained or glassy, vesicular, rhyolite, andesite, basalt, obsidian)
- What are sedimentary rocks? (layering; siltstone, sandstone, conglomerate, limestone, shale; clay, silt, sand, pebbles larger than 2 mm; chert; limestone precipitated from water)
- What are metamorphic rocks? (heat, pressure, hot mineral-rich fluids; not melted; denser; phyllite, schist, gneiss, quartzite, marble; foliation; contact metamorphism)
- USGS Find-A-Feature: Fossil (fossils almost exclusively in sedimentary rocks)
- USGS Fossils, Rocks, and Time (pubs.usgs.gov/gip/fossils: most rocks exposed at the surface are sedimentary; gravel to conglomerate, sand to sandstone, mud to mudstone or shale; Nicholas Steno mid-1600s; Law of Superposition; William Smith; Law of Fossil Succession)
- USGS Geologic Time: Age of the Earth (pubs.usgs.gov/gip/geotime/age.html: radiometric dating, half-lives 700 million to more than 100 billion years, Acasta Gneisses 4.03 Ga, Isua 3.7 to 3.8 Ga, zircons 4.3 Ga and 4.4 Ga in Western Australia, Moon rocks 4.4 to 4.5 Ga, meteorites 4.53 to 4.58 Ga, Earth 4.54 Ga with uncertainty under 1%, rocks over 3.5 billion years on all continents)
- USGS Mohs Hardness Scale image page and USGS Bulletin 1042-G Gem Stones of the United States (ten minerals, relative scale, diamond many times harder than corundum, much dust is silica, quartz 7)
- NPS Mohs Hardness Scale (Theophrastus, On Stones, about 300 BC)
- Physical Geology H5P Edition 5.6 Mineral Properties (opentextbc.ca, reference materials: natural fingernail 2 to 2.5, copper 3, steel nail 4, glass and knife 5 to 5.5, streak plate 6.5 to 7)
- Physical Geology 2nd ed. 7.2 Classification of Metamorphic Rocks (Steven Earle, opentextbc.ca: slate, phyllite, schist, gneiss by grade; grade table 150 to 300, 300 to 450, 450 to 550, above 550 °C; marble, quartzite, hornfels; migmatite)
- British Geological Survey, Discovering Geology: Rocks and minerals (rock cycle, Peterhead granite feldspar, quartz and biotite, limestone to marble, shale to slate, contact metamorphism, mineral definition)
- Weathering (biological, chemical, physical, freeze-thaw)
- Geological processes (erosion, deposition)
- BGS Geology dataset page (schist and gneiss recrystallized in the solid state)
- Geological Society of London, Compaction and Cementation (calcite, silica, iron oxides, clay minerals as cements)
- OpenStax Chemistry 2e 1.2 (oxygen 49.20% and silicon 25.67% of the mass of Earth's crust and atmosphere)
- USGS Water Density (ice about 9% less dense than water).
Checked: 2026-10-17