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Earth and Space Science Unit 2: Rocks, minerals and the rock cycle

Rocks are made of minerals and constantly change from one type to another through the rock cycle.

Covers: Minerals and their properties, igneous, sedimentary, metamorphic rocks, the rock cycle, weathering and erosion

Study guide

A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an orderly crystal structure. Minerals are identified by their properties: color (unreliable), streak (the color of the powder), luster (metallic or nonmetallic), hardness measured on the Mohs scale from 1 (talc) to 10 (diamond), cleavage and fracture (how it breaks), density, and special properties like magnetism or fizzing in acid.

Rocks are made of minerals and come in three types. Igneous rocks form from cooled magma or lava. Intrusive igneous rocks cool slowly underground and have large crystals (granite); extrusive ones cool quickly at the surface and have small crystals or glassy textures (basalt, obsidian). Sedimentary rocks form when sediments are deposited, compacted and cemented, as in sandstone, shale and limestone. They form in layers and are the rocks most likely to contain fossils. Metamorphic rocks form when existing rocks are changed by heat and pressure without melting; examples are marble from limestone and slate from shale, and many show foliation (banding).

The rock cycle shows how any rock type can become another. Weathering and erosion break rocks into sediment that forms sedimentary rock; heat and pressure turn rocks into metamorphic rock; melting creates magma that cools into igneous rock.

Weathering breaks rock down in place. Mechanical weathering breaks it physically, through ice wedging (frost wedging), plant roots and abrasion. Chemical weathering changes its minerals, through oxidation (rusting) and acid dissolving limestone. Erosion moves the pieces by water, wind, ice and gravity, and deposition drops them in new places, building deltas, dunes and other landforms.

Porosity is the percentage of open space between grains, and permeability is how easily water flows through them. Larger, well-sorted, rounded grains have the highest permeability. Mixed sizes pack tightly and lower porosity, and clay is nearly impermeable. Faster-moving water can carry larger particles. When water slows, for example where a river enters the ocean, the largest and densest particles settle first, sorting the sediment and building deltas. Waves that hit a beach at an angle drive a longshore current that moves sand along the shore, building spits and sandbars.

Glaciers drag rocks frozen into their base across bedrock, carving parallel scratches (striations) and U-shaped valleys. When they melt, they leave unsorted mixtures of sediment called till and large out-of-place boulders called erratics. Vesicular texture (holes, as in pumice and scoria) forms when gas bubbles are trapped in quickly cooling lava. Mafic rocks like basalt and gabbro are dark and dense, rich in iron and magnesium. Felsic rocks like granite are light-colored and rich in silicon and aluminum, and ultramafic peridotite is an intrusive rock even denser than basalt that makes up much of the mantle.

Resistant layers such as sandstone or basalt weather slowly and form cliffs and ridges, while weaker layers like shale wear down into slopes and valleys. Bedrock structure shapes stream patterns: streams form radial patterns off domes and volcanoes, trellis patterns in folded ridges and valleys, and branching dendritic patterns on uniform rock. Some sedimentary rocks are made from the remains of organisms. Limestone made of shell fragments (like coquina) and coal from plant remains are examples. Gravity can move material downslope all at once in landslides, which are most likely on steep slopes with ground saturated by heavy rain.

Key ideas

• Minerals are natural, inorganic solids with a definite crystal structure.
• Igneous rock forms from cooled magma or lava.
• Sedimentary rock forms from compacted, cemented sediment and often holds fossils.
• Metamorphic rock forms from heat and pressure without melting.
• Weathering breaks rock down; erosion carries the pieces away.

Formulas and rules

Common mistakes

• Thinking metamorphic rock melts. It changes in the solid state.
• Mixing up weathering and erosion.

Words to know

Igneous rock

Forms from cooled magma or lava.

Sedimentary rock

Rock formed from compacted sediment; most likely to contain fossils.

Metamorphic rock

Formed by heat and pressure without melting.

Mohs hardness scale

Ranks minerals from 1 (talc) to 10 (diamond, the hardest).

Weathering vs erosion

Weathering breaks rock; erosion moves it.

Mineral

A natural, inorganic solid with a definite composition and crystal structure.

Streak

The color of a mineral's powder.

Luster

How a mineral reflects light: metallic or nonmetallic.

Cleavage / fracture

Breaking along flat planes / breaking unevenly.

Intrusive igneous rock

Cools slowly underground; large crystals (granite).

Extrusive igneous rock

Cools quickly at the surface; small crystals (basalt).

Foliation

Banding in metamorphic rock.

Rock cycle

How rocks change from one type to another.

Mechanical weathering

Breaking rock physically, like ice wedging.

Chemical weathering

Changing rock's minerals, like oxidation or acid dissolving limestone.

Deposition

Dropping sediment in a new place.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.