What Is Actually Under Your Boots? A Layer-by-Layer Walk Down

I came to geology through earthworms, which is not the usual route. I was standing in my vegetable patch one evening, holding a fork, wondering where all this soil had come from. Had someone delivered it? Had it always been there? That small question turned out to be a very long hole.

So let's go down it together. I'll start at the grass (or gravel, or the lino of your kitchen, if you're reading indoors) and descend one layer at a time. For each layer I'll say what it's made of and how anyone knows, because the second part is where it gets interesting.

A note on scale first, since the numbers get silly fast. Imagine a five-minute drive to the shops, about 5 kilometres. I'll keep coming back to that. And if you could drive straight down at motorway speed, 100 km/h, it would take about 64 hours to reach the centre of the Earth. That's nearly three days of nonstop driving, with no toilet stops and no flat white.

Layer one: topsoil (the first few centimetres to a spade's depth)

Topsoil is a mixture, and the proportions matter. It holds tiny grains of broken-up rock, decayed plant and animal matter (called humus), water, air, and an astonishing amount of living things: fungi, bacteria, insects, and yes, worms.

My first guess, years ago, was that soil is basically ground-up rock. It's partly that, but the dark, crumbly stuff in a healthy garden gets its colour and its goodness from the organic part. Charles Darwin was so taken with worms that he spent years watching them and wrote a whole book on them, published in 1881. His main point still stands: worms keep pulling leaves down and mixing material through the soil, so the ground is being slowly turned over by creatures you can barely see.

How do we know all this? The easy way: dig a hole and look. Topsoil is the one layer you can examine with a trowel and a cup of tea.

Layer two: subsoil (roughly 30 cm to a metre or so)

Dig down a bit further and the colour usually changes. It gets paler, often more orange, yellow or grey, and it feels different between your fingers: stickier, more clay-like, with fewer roots and less life.

That clay is a clue about what's coming next. Clay isn't just finely crushed rock. Many clay minerals are new minerals, made when water and dissolved gases chemically change the original ones. Rain is slightly acidic (it picks up carbon dioxide from the air as it falls), and over long spans of time that gentle acidity changes minerals such as feldspar into clay. Some of that clay and other material gets washed down from the topsoil and collects in the subsoil.

So the subsoil is a bit of a collecting-box. The exact depth and colour vary a lot from place to place, which is why a spade goes in easily in one garden and bounces off in another.

Layer three: weathered rock (from about a metre to tens of metres)

Below the subsoil you often find rock that has gone soft and crumbly but still holds some of its original structure. Geologists call this weathered rock, or regolith if they mean the whole loose blanket on top of solid rock.

Here the rock is being broken down in place. Water seeps into cracks, freezes and expands where it's cold enough, and roots wedge their way in. Meanwhile chemical reactions keep working on the minerals. The thickness of this layer varies enormously. On a steep, freshly exposed slope it can be almost nothing. In warm, wet places that have been stable for a long time, it can be many metres deep.

You can often see this layer yourself. Roadside cuttings and eroded banks show the sequence neatly: dark soil on top, then paler soil, then crumbly rock, then harder rock. (Please admire these from a safe distance, off the road. Cuttings can drop bits, and so can traffic.)

Layer four: solid bedrock (down to the base of the crust)

Eventually the digging stops, because you hit unbroken rock. This is bedrock, the stuff the rest of the landscape sits on. What it's made of depends entirely on where you are. In New Zealand, for instance, much of the country's mountainous backbone is a hard, grey, layered sandstone-type rock called greywacke, while parts of the North Island sit on volcanic rock. A geological map for your area will tell you what's beneath you.

How do we know what's down there when we can't see it? Partly from places where it pokes out at the surface, partly from drill cores (long cylinders of rock pulled up from boreholes), and partly from clever indirect methods we'll get to shortly.

The deepest hole humans have ever drilled is the Kola Superdeep Borehole in Russia, which reached just over 12 kilometres. That's roughly two and a bit trips to the shops, in a straight line, downward. It took years of drilling. And here's a humbling comparison: the Earth's radius is about 6,371 kilometres, so that record-holding hole covers about 0.2% of the way to the centre. It's a pinprick. (I confess I once assumed it had reached the mantle. It hadn't, not even close.)

The crust: the Earth's thin skin

Bedrock is the top of the crust, the outermost shell of the planet. Under the oceans it's thin, roughly 5 to 10 kilometres. Under the continents it's thicker, often something like 30 to 50 kilometres, and thicker again beneath big mountain ranges. At 100 km/h, a typical continental crust is a drive of about twenty minutes.

If you think of the Earth as an apple, the crust is thinner than the skin. People often say "eggshell", but even that is generous. Everything we've ever built, farmed or walked on is in that thin skin.

Now for the part I find genuinely lovely, because the rest of the descent depends on it: how can anyone know where the crust ends when we can't drill there?

How earthquake waves let us see the unseeable

When an earthquake happens, it sends waves of energy through the planet. Seismometers, sensitive instruments around the world, record when those waves ar