Earth and atmosphere

The lithosphere

HintIt is what one tectonic plate is actually made of, and the crust on its own is far too thin for the job.

The whyThe crust is only about 5 km thick under the oceans and up to about 70 km under mountain ranges, far too thin to behave as a plate on its own. A plate is the crust welded to the cold, rigid top of the mantle, sliding slowly over hotter rock beneath — which is why earthquakes cluster along plate boundaries rather than along the edges of continents.

Nitrogen

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

The whyRoughly 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

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

The whyErosion 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

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

The whyCooling 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

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

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

Metamorphic rock

HintLimestone squeezed and baked becomes marble, and mud becomes slate.

The whyThe 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.

weathered, then eroded

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

The whyThe 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

HintIt is the job green leaves do in sunlight.

The whyRespiration, 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

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

The whyThe 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

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

The whyBecause 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

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

The whyCarbon 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

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

The whyRecycling 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.