Rocks keep a record of what happened to them

Word	What it means
Mineral / rock	A natural, non-living solid with a definite chemical make-up and crystal structure, such as quartz / a solid mixture of minerals, such as granite.
Hardness / Mohs scale	How well a mineral resists scratching / a ranking from 1 (talc, the softest) to 10 (diamond, the hardest), in which each mineral scratches the ones below it.
Lustre / streak	How a mineral's surface reflects light: metallic, glassy, pearly, dull / the colour of its powder, rubbed on unglazed white tile.
Cleavage / fracture	Breaking along flat, smooth surfaces / breaking unevenly or along curves.
Magma / lava	Melted rock underground / melted rock that has reached the surface.
Igneous rock	Rock formed when magma or lava cools. Intrusive igneous rock cools slowly underground; extrusive cools quickly at the surface.
Sedimentary rock	Rock made from layers of sediments — bits of rock, shell and plant — squeezed (compaction) and glued by minerals (cementation).
Metamorphic rock	Rock changed by heat and pressure deep underground, without melting.
Crust / mantle / core	Earth's layers: a thin rocky crust; the mantle, hot rock that flows very slowly; and a core of mostly iron and nickel.
Plate tectonics	The theory that Earth's surface is broken into large plates that move a few centimetres a year.
Weathering / erosion / deposition	Breaking rock down where it is / carrying the pieces away, by water, wind, ice or gravity / dropping them somewhere else.
Fossil	The remains or traces of a living thing from long ago, preserved in rock: a bone, a shell, a leaf print, a footprint.
Relative dating / geologic time scale	Putting layers and fossils in order of age, without years / Earth's 4.6-billion-year history, divided into long spans.

1 talc · 2 gypsum · 3 calcite · 4 fluorite · 5 apatite · 6 feldspar · 7 quartz · 8 topaz · 9 corundum · 10 diamond

Worked: narrowing it down. Glass is about 5.5 on the scale. An unknown mineral scratches glass, but quartz scratches it. So it is harder than 5.5 and softer than 7: feldspar, at 6, fits. The scale only puts minerals in order; its steps are not equal, and diamond is far more than one step harder than corundum.

Why colour fools you. Pyrite, “fool's gold”, is gold-coloured but leaves a greenish-black streak; real gold's streak is gold-yellow. Mica has perfect cleavage and peels into thin sheets; quartz breaks with curved, shell-shaped fractures, like thick glass.

Rock	How it forms	Clues, and examples
Igneous, intrusive	Magma cools slowly underground	Large crystals you can see: granite
Igneous, extrusive	Lava cools quickly at the surface	Tiny crystals, glassy, or full of gas bubbles: basalt, obsidian, pumice
Sedimentary	Layers of sediment compacted and cemented	Layers, rounded grains, often fossils: sandstone, shale, limestone, coal
Metamorphic	Heat and pressure change a rock without melting it	Bands, or crystals flattened and lined up: slate from shale, marble from limestone

crust, 5 to 70 km thick  →  mantle, about 2900 km  →  liquid outer core, about 2300 km  →  solid inner core, about 1200 km to the centre

Moving plates. The plates move a few centimetres a year, about as fast as your fingernails grow. That sounds like nothing, but at 3 cm a year a plate moves 30 km in a million years.

Plates	What happens	Where
Moving apart	Magma rises into the gap and makes new crust	The middle of the Atlantic, through Iceland
Pushing together	Rock folds up into mountains, or one plate sinks under the other, with earthquakes and volcanoes	The Himalayas; off the coast of British Columbia
Sliding past	The plates catch, then slip suddenly in earthquakes	The San Andreas Fault in California

How the Rockies got there. The Canadian Rockies are mostly sedimentary rock laid down in shallow seas — which is why sea fossils turn up high on the mountains — folded, broken, stacked and pushed up as plates to the west pressed against North America. Alberta itself sits far from any plate boundary, so it has no volcanoes and few strong earthquakes.

Weathering	How it works	Example
Mechanical	Breaks rock into pieces without changing what it is made of	Frost wedging: water in a crack freezes, expands and pries the crack wider, over and over
Chemical	Changes the minerals in the rock	Slightly acidic rainwater slowly dissolves limestone; iron in rock rusts and crumbles
Biological	Living things break the rock	Roots growing in cracks; lichens giving off acids on bare rock

Erosion and deposition. Rivers carry sand and mud, wind lifts dust, glaciers drag rock along, and gravity pulls it downhill. Where the movement stops, the load is dropped: a river entering a lake leaves its sand in a delta, and a melting glacier leaves ridges of rock and gravel called moraines.

Worked: reading Alberta's landscape.

• The badlands near Drumheller were carved by water eroding soft sedimentary rock, exposing its fossils. Hoodoos are pillars of soft rock left standing where a harder cap of stone shields them from the rain.
• Big Rock, the Okotoks Erratic, is a boulder bigger than a house that does not match the rock around it: a glacier carried it from the mountains near Jasper during the last ice age.
• The Frank Slide. In 1903 a huge mass of limestone broke off Turtle Mountain, in the Crowsnest Pass, and buried part of the town of Frank in less than two minutes, killing at least 70 people. Cracked rock layers, frost wedging in the cracks, and perhaps the coal mine at the mountain's foot all played a part: erosion by gravity, all at once.

How a fossil forms. Most living things rot or are eaten. A fossil forms when a body is buried quickly, in mud at the bottom of a lake or sea, before it can decay. The soft parts rot away; over thousands to millions of years the mud becomes rock, and water seeping through replaces the bone or shell with minerals, copying its shape in stone. So fossils are found almost only in sedimentary rock: melting destroys remains, and so do the heat and pressure that make metamorphic rock.

Worked: which is older? A cliff shows four flat layers, A at the top down to D at the bottom. A fish fossil is in layer C and a leaf fossil in layer A. Each layer was laid down on top of the one before, so unless something has flipped them, D is oldest and A youngest: the fish is older than the leaf. If a crack filled with igneous rock cuts through all four layers, the crack is younger than all of them — rock cannot be cut before it exists. This is relative dating: an order, not an age.

Span	Began about	Life
Precambrian	4.6 billion years ago	For most of it, only single-celled life
Paleozoic (“ancient life”)	540 million years ago	Shelled animals, fish, the first land plants, insects and amphibians
Mesozoic (“middle life”)	252 million years ago	Dinosaurs, the first mammals, birds and flowering plants
Cenozoic (“recent life”)	66 million years ago	Mammals spread; humans appear at the very end

Worked: Earth's history as one day. Squeeze all 4.6 billion years into 24 hours, and each hour stands for about 190 million years. The Precambrian lasts until about 9:10 p.m., the dinosaurs die out about 21 minutes before midnight, and humans, about 300 000 years old, arrive in the last 6 seconds.

Alberta's fossils. The Royal Tyrrell Museum near Drumheller is named after Joseph Burr Tyrrell, who in 1884 found the skull of a meat-eating dinosaur in the Red Deer River valley — the one later named Albertosaurus. Dinosaur Provincial Park, near Brooks, is a UNESCO World Heritage Site for its wealth of dinosaur fossils from about 75 million years ago.

• Naming a mineral by its colour. Use streak, hardness, lustre and cleavage.
• Treating the Mohs scale as equal steps. It is an order, not a measurement.
• Mixing up weathering and erosion. Weathering breaks rock; erosion moves it.
• Swapping intrusive and extrusive. Slow cooling underground makes big crystals.
• Saying the top layer is the oldest. In undisturbed layers the bottom is oldest.
• Looking for fossils in igneous rock. Look in sedimentary rock.