Tukuk-OSGeologists sort rock three ways by origin, not look. Igneous rock froze from molten material — every ocean floor, most of the crust, and the basalt of Hawaii. Sedimentary rock is reassembled debris or precipitate: sandstone from sand, limestone largely from shells and reef debris, shale from mud — the library layer-cake where fossils and Earth’s past climate live. Metamorphic rock is either of the first two, cooked and squeezed into something new underground — marble from limestone, slate from shale. The cycle runs because plate tectonics keeps recycling crust into magma and back up again, which is why Earth’s oldest intact rocks are about 4 billion years and meteorites (never recycled) are what actually give the planet’s 4.54-billion-year age.
The scale of deep time is the hard part to internalise. Compress Earth’s whole history into a single calendar year: the planet forms on 1 January, the first unmistakable life appears somewhere around March, complex cells with nuclei arrive in late October, the Cambrian explosion of animal bodies bursts open around 17 December, the dinosaurs’ reign ends about Christmas Day — and behaviourally modern humans show up in the last hour or so of 31 December. Everything you have ever heard about as “ancient history”, pyramids included, is the final seconds after midnight of the last New Year.
Reading it is like reading tree rings: stratigraphy — older layers below younger — plus absolute dating by radioactive decay (uranium to lead, carbon-14 for recent organics). The two methods agree across oceans and continents, which is why “4.54 billion years” is not a guess held loosely but a number checked by independent clocks that refuse to disagree.