Rocks and minerals
How does a grain of quartz end up in granite, then in sand on a beach, then in sandstone and finally in quartzite? Click through the rock cycle, learn to tell igneous, sedimentary and metamorphic rocks apart, test minerals on the Mohs scale and see how geologists date rocks 4 billion years old.
About this course
A piece of Peterhead granite is a mosaic of three minerals: pink feldspar, grey glassy quartz and black biotite mica. The same quartz grain can sit in granite, wash onto a beach as sand, harden into sandstone and end up in quartzite. Rocks are not fixed objects but stages in a long cycle, and this course lets you follow one grain through it. Zircon crystals from Western Australia are about 4.4 billion years old, older than any whole rock found on Earth.
The six units start with minerals, then sort rocks by how they formed. Igneous rocks come from cooled magma, and obsidian is volcanic glass, lava that cooled so fast its atoms had no time to line up into crystals. Sedimentary rocks, built grain by grain from older rock, make up most of what is exposed at Earth's surface. In metamorphic rocks, slate, phyllite, schist and gneiss can all start as the same mudrock; what differs is how hot it got, from about 150 °C to above 550 °C. The rock cycle unit joins the three families together, and the last unit shows how geologists date rocks. The lab is an interactive rock cycle. Tap any box or process for detail, press Trace a path to send a rock on a random route of six steps, and look at the Acasta gneiss, whose zircon crystals record when they grew.
Before you start
Nothing has to come first, since the course starts with what a mineral is. If you have seen Volcanoes and earthquakes, the igneous unit will feel familiar. A fingernail is enough if you want to try a Mohs scratch test on stones at home.
When you finish
You will be able to tell igneous, sedimentary and metamorphic rocks apart, explain why obsidian has no crystals and how the same mudrock becomes slate, schist or gneiss as the heat rises, and trace a quartz grain around the rock cycle. You can also read the oldest layer in undisturbed rock and compare ages such as 4.54 billion years for Earth and 4.4 billion for the oldest zircons.
- 1MineralsA piece of Peterhead granite is a mosaic of three minerals: pink feldspar, grey glassy quartz and black biotite mica.
- 2Igneous rocksObsidian is volcanic glass: lava that cooled so fast its atoms had no time to line up into crystals.
- 3Sedimentary rocksMost of the rock exposed at Earth's surface is sedimentary, built grain by grain from pieces of older rock.
- 4Metamorphic rocksSlate, phyllite, schist and gneiss can all start as the same mudrock; what differs is how hot it got, from about 150 °C to above 550 °C.
- 5The rock cycleA grain of quartz can sit in granite, wash onto a beach as sand, harden into sandstone and end up in quartzite.
- 6How old rocks areTiny zircon crystals from Western Australia are about 4.4 billion years old, older than any whole rock found on Earth.
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