Earth's layers, minerals and rocks

The geosphere

• Scientists divide the Earth system into four connected parts: the air (atmosphere), the water (hydrosphere), living things (biosphere) and the solid Earth (geosphere).
• The geosphere is all the solid, non-living ground of the planet: every rock, mineral, grain of soil and landform, from mountains to sea-floor trenches.
• It does not stop at the surface. It reaches right down through the Earth's interior to the very centre.
• Aoraki, the Canterbury Plains and the sand on Piha beach are all part of it.

Four layers inside the Earth

• The crust is the thin, rocky outer skin we live on. It is thinner under the oceans than under the continents.
• The mantle lies under the crust. It is very hot rock that is mostly solid but can flow extremely slowly, like very stiff toffee.
• The outer core is a layer of liquid metal, mostly iron and nickel.
• The inner core, at the centre, is also iron and nickel. It is the hottest part, but the huge pressure squeezes it into a solid ball.
• No one has drilled through the crust. We know about the deeper layers mainly from how earthquake waves travel through the Earth.

Try it: which layer?

What is a mineral?

• The crust is made of rocks, and rocks are made of minerals.
• A mineral is a solid that forms naturally, not made by people, and has its own fixed chemical make-up.
• Its particles are packed in a neat pattern that repeats over and over. That repeating pattern is why minerals grow as crystals with flat faces and straight edges.
• The pattern also gives each mineral its own properties: colour, shine (lustre), the way it breaks, and hardness.
• Quartz, feldspar, mica and calcite are common minerals. Ice is a mineral too; plastic and glass are not, because their particles have no repeating pattern.

Testing hardness

• Hardness means how well a mineral resists being scratched.
• A harder material scratches a softer one, never the other way round, so you can rank minerals by testing which scratches which.
• Geologists use the Mohs scale, which ranks ten minerals from talc, the softest, at 1 to diamond, the hardest, at 10.
• A quick field test uses everyday objects: a fingernail, a copper coin, a steel nail and a piece of glass, from softest to hardest.
• Talc can be scratched by a fingernail; quartz is hard enough to scratch glass.

Rocks are mixtures of minerals

• A rock is a solid mixture of one or more minerals. Granite is mostly quartz, feldspar and mica, while limestone is mostly the single mineral calcite.
• Which minerals a rock contains, and how they are arranged, decide what it looks and feels like.
• Colour comes from the minerals. Texture describes the grains: coarse grains you can see easily, or fine grains too small to see.
• Structure describes how the grains sit: interlocking crystals, grains cemented together, or layers like the pages of a book.
• Hardness depends on how hard the minerals are and how strongly they are held together.

Classifying soils

• Soils can be compared in the same way, by grain size and feel.
• Sandy soil has big grains you can see; it feels gritty and water drains through it fast.
• Clay soil has tiny grains; it feels sticky when wet, can be rolled into a thin sausage, and holds water.
• Silty soil sits in between and feels smooth, a little like flour.
• A loam is a mix of all three with plenty of rotting plant matter, which is why gardeners prize it.

Try it: sandy, clay or silty?

Fossils are clues to the past

• A fossil is the preserved remains or trace of a living thing from long ago, such as a shell, a bone, a leaf print or a footprint.
• Fossils tell you what lived at the time the rock formed, and so what the place was like then.
• Sea shells in a rock mean that rock formed under the sea, even if it is now high on a hill.
• Limestone in many parts of New Zealand, such as around Ōamaru and Waitomo, is packed with fossils of sea creatures, so that land was once under the sea.