Rocks
Senior 1 · Geography
Rocks
Lesson 1 of 3
22 min
A student from Musanze picks up three stones on the way to school: a heavy black stone full of small holes, a pale speckled stone with pink, grey and black spots, and a flaky grey stone that glitters in the sun and splits into thin plates. All three are rocks, yet they look nothing alike. Before reading on, write two words to describe each one. By the end of this unit you will be able to name them and explain how each was made.
Definition
Rock: a naturally formed, solid mass made of one or more minerals (or of natural glass or once-living material) that forms part of the Earth's crust. Example: granite is a rock made of the minerals feldspar, quartz and mica.
Definition
Mineral: a natural, non-living, solid substance with a definite chemical composition and a regular crystal structure. Example: quartz is always silicon dioxide, ; cassiterite, the ore of tin, is tin dioxide, .
Think of a rock as a loaf of bread with raisins and nuts in it: the rock is the loaf, and the minerals are the ingredients. Most rocks contain several minerals; a few, such as quartzite (almost all quartz), contain mainly one. Figure 1 shows what a piece of granite looks like through a hand lens.
Figure 1. Granite under a hand lens: a rock is a mixture of mineral crystals locked together.
Only about a dozen minerals make up most of the rocks of the crust. Quartz and feldspar alone make up more than half of it. A few rarer minerals are valuable because metals can be extracted from them; these are called ores.
Definition
Ore: a rock or mineral from which a useful metal can be extracted at a profit. Example: cassiterite (tin), coltan (tantalum) and wolframite (tungsten) are the three ores most mined in Rwanda.
| Property | What to look for | Example |
|---|---|---|
| Colour | Light (rich in quartz and feldspar) or dark (rich in iron and magnesium minerals) | Granite is light; basalt is dark |
| Grain (texture) | Size and shape of the grains or crystals: coarse, medium or fine; interlocking crystals or rounded grains | Granite is coarse; basalt is fine |
| Hardness | Can it be scratched by a fingernail, a copper coin, a steel knife? | Quartz scratches glass; mica is scratched by a fingernail |
| Layers or bands | Flat layers (beds), or wavy bands of minerals | Sandstone has beds; schist has bands |
| Holes (vesicles) | Small gas holes left in cooled lava | Vesicular basalt from the Virunga area |
| Fossils | Remains of plants or animals | Found only in sedimentary rocks |
| Reaction with dilute acid | Fizzing shows calcium carbonate | Limestone fizzes |
Definition
Hardness (Mohs scale): how easily a mineral can be scratched, on a scale from 1 (talc, the softest) to 10 (diamond, the hardest). A fingernail is about 2.5, a copper coin about 3.5, glass and a steel knife about 5.5. Example: quartz (7) scratches glass; mica (2.5 to 3) can be scratched with a fingernail.
Worked example 1: identifying a mineral by scratching
A grain in a rock is scratched by a steel knife but not by a copper coin. Between which values does its hardness lie?
Step 1 Not scratched by a coin, so it is harder than the coin: hardness .
Step 2 Scratched by the knife, so it is softer than the knife: hardness .
Answer. . The mineral could be fluorite (4) or apatite (5), but not quartz (7) or feldspar (6).
Worked example 2: how heavy is a rock? (density)
A piece of rock has a mass of 270 g. When it is lowered into a measuring cylinder, the water level rises from 200 cm³ to 300 cm³. Find its density.
Interpretation. Granite has a density of about 2.7 g/cm³ and basalt about 3.0 g/cm³, so this sample is more likely to be granite. The dark volcanic stones of Musanze feel heavy for their size because basalt is dense, even with its gas holes.
Warning
Do not say "minerals" when you mean "rocks". In everyday Kinyarwanda and English, "minerals" often means anything dug from a mine. In geography, a mineral is one substance with a fixed chemical formula; a rock is a mixture of minerals.
Important
Case study: the stones under Rwanda
Most of Rwanda is built of very old rocks, more than a billion years old: quartzite, schist and shale, cut in places by bodies of granite. You can see them in the ridges of the Congo–Nile divide and the hills of the centre. In the north-west, the young Virunga volcanoes have covered the land with dark volcanic rock such as basalt. Along the shore of Lake Kivu and in the big river valleys lie loose, young sediments: sand, gravel and clay. The ores that Rwanda mines, cassiterite, coltan and wolframite, occur in veins and pockets linked to the granites in the old rocks.
Gems, Minerals, Crystals and Rocks: What's the Difference?, by the American Museum of Natural History (3.5 min). Watch for the museum's one-sentence definitions of a mineral and a rock.
Look again at the three stones from the opening: the black stone with holes, the speckled stone and the flaky glittering stone. For each, suggest which properties from the table you would test first, and why.
Tip
Rock = recipe, mineral = ingredient. To describe any rock in an exam, use C-G-H-L: Colour, Grain, Hardness, Layers (plus holes or fossils if you see them).
Practice
Remember
Sources and credits
The lesson text, granite diagram, table, worked examples and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
