Earth and atmosphere

The crust

提示On a boiled-egg model of the planet, it is the shell.

为什么It is astonishingly thin — only about 5 km under the oceans, against a planet radius of over 6,000 km. Everything we mine, farm and build on sits inside it.

Nitrogen

提示It is not the gas your lungs are after, but there is far more of it about.

为什么Roughly 78% of the air is this gas and about 21% is oxygen, leaving barely 1% for everything else — including the carbon dioxide that matters so much to the climate.

weathering

提示Water freezing in a crack, tree roots and slightly acidic rain all do it.

为什么Erosion is the next step: once the fragments are loose, wind, water and ice carry them elsewhere. The break-up happens first, and it happens in place.

Igneous rock

提示Granite and basalt both formed this way, one deep underground and one at the surface.

为什么Cooling slowly at depth grows the large crystals you see in granite; cooling fast after an eruption gives basalt its tiny ones. Crystal size records the speed.

Sedimentary rock

提示Sandstone and limestone are the usual examples, and fossils turn up in them.

为什么Layer upon layer of sand, mud or shell fragments is buried and compacted, which is why these rocks appear in visible bands.

Metamorphic rock

提示Limestone squeezed and baked becomes marble, and mud becomes slate.

为什么The original never actually melts — if it did, it would cool into the igneous family instead. It recrystallises while still solid, which is why marble is far harder than the limestone it came from.

It is weathered and eroded

提示Frost, rain and rivers get to work on it, and the fragments end up in the sea.

为什么The cycle has no beginning and no end: fragments are buried and compacted, may be heated and squeezed, may melt and cool again, and the loop can run for hundreds of millions of years.

Photosynthesis

提示It is the job green leaves do in sunlight.

为什么Respiration, decay and combustion all send that carbon straight back. For thousands of years the ins and outs roughly balanced, until fossil fuels began to be burned on a huge scale.

The Earth becomes warmer

提示Greenhouse gases let sunlight in but slow the escape of heat back out to space.

为什么The gas behaves like the glass of a greenhouse, letting short-wave sunlight through while absorbing the longer-wave energy the surface radiates. Adding more tips the balance.

Millions of years

提示They come from the buried remains of living things, squeezed underground over an unimaginably long stretch of time.

为什么Because they form so slowly, burning them draws on a store that is not being refilled on any human timescale. That is exactly what finite means.

The oxygen

提示Carbon is the more reactive of the two, so it grabs the part of the compound the metal was joined to.

为什么Carbon cannot do this for aluminium, which sits above it in the reactivity series and has to be extracted using electricity instead — which is why aluminium was once more precious than gold.

It uses far less energy

提示You skip the mining, the crushing and the huge furnace, and start from metal that is already pure.

为什么Recycling aluminium takes roughly a twentieth of the electricity that winning it from bauxite needs. It also leaves the ore in the ground for later, which matters because the store is finite.