14.1 The Rock Cycle, Minerals, and Soils
Key Takeaways
- The rock cycle connects three rock families — igneous, sedimentary, and metamorphic — through melting, crystallization, weathering, lithification, heat, and pressure; any rock can become any other rock.
- Igneous rocks form from cooled magma; intrusive rocks cool slowly underground to form coarse grains (granite) while extrusive rocks cool quickly at the surface to form fine grains (basalt, obsidian).
- Sedimentary rocks form through compaction and cementation of sediment; metamorphic rocks form when existing rocks recrystallize under heat and pressure without melting.
- A mineral is naturally occurring, inorganic, solid, with a definite chemical composition and an ordered crystalline structure; Mohs hardness, luster, streak, cleavage, and fracture are the standard identification tests.
- Fossil fuels form from ancient organic matter buried under low-oxygen conditions; coal forms from swamp plant debris while petroleum and natural gas form from marine plankton buried and heated over millions of years.
The Three Rock Families and Their Interconversions
The rock cycle is the continuous set of processes that recycles Earth's rocky material. Three families — igneous, sedimentary, and metamorphic — interconvert through melting, crystallization, weathering, lithification, and metamorphism. Any rock can become any other rock given enough time, heat, pressure, or erosion.
Igneous Rocks
Igneous rocks crystallize from magma (below ground) or lava (at the surface). Two subtypes differ by grain size and cooling history:
- Intrusive (plutonic) rocks cool slowly underground, allowing large crystals to grow. Granite, with visible quartz, feldspar, and mica crystals, is the textbook example.
- Extrusive (volcanic) rocks cool quickly at the surface, producing fine grains or glass. Basalt is fine-grained and dark; obsidian is volcanic glass; pumice is frothy lava that traps gas bubbles.
The Texas Hill Country exposes Enchanted Rock, a granite batholith that cooled intrusively about a billion years ago and was exposed by long-term erosion — a single field site showing both igneous formation and destructive surface processes.
Sedimentary Rocks
Sedimentary rocks form through lithification — the combined processes of compaction (sediment squeezed by overlying weight) and cementation (minerals such as calcite or silica precipitating between grains). Common types:
- Clastic rocks form from broken rock fragments. Shale (mud), sandstone (sand), and conglomerate (rounded pebbles) span grain sizes from fine to coarse.
- Chemical rocks precipitate from solution; rock salt (halite) and some limestone form this way.
- Organic rocks accumulate from biological debris; coal and limestone made of shell fragments fit here.
Sedimentary rocks often preserve fossils and sedimentary structures (ripple marks, cross-bedding, mud cracks) that record past environments — a key 4-8 teaching point.
Metamorphic Rocks
Metamorphic rocks form when existing rocks recrystallize under heat and pressure without melting. Two textures distinguish the family:
- Foliated rocks show aligned mineral bands from directed pressure; slate (from shale), schist, and gneiss are examples.
- Nonfoliated rocks lack aligned bands; marble (from limestone) and quartzite (from sandstone) form under uniform pressure.
Comparison of the Three Rock Families
| Feature | Igneous | Sedimentary | Metamorphic |
|---|---|---|---|
| Origin | Cooling of magma or lava | Compaction and cementation of sediment | Heat and pressure on existing rock |
| Texture | Crystalline (interlocking grains) | Clastic, chemical, or organic; often layered | Foliated or nonfoliated; recrystallized |
| Common examples | Granite, basalt, obsidian | Sandstone, shale, limestone, coal | Slate, schist, gneiss, marble, quartzite |
| Fossils? | Rarely preserved (heat destroys them) | Commonly preserved | Usually destroyed |
Minerals: Definition and Identification
A mineral must satisfy five criteria: it is naturally occurring, inorganic, solid, has a definite chemical composition, and an ordered crystalline structure. A substance that fails any criterion — for example, glass (no crystal structure), coal (organic), or a synthetic ruby (not natural) — is not a mineral.
The standard identification tests rely on physical properties that do not require advanced equipment:
| Property | What it measures | Example |
|---|---|---|
| Hardness (Mohs scale) | Resistance to scratching, 1 (talc) to 10 (diamond) | Quartz (7) scratches glass (about 5.5) |
| Luster | How light reflects — metallic, vitreous, dull, pearly | Pyrite has a metallic luster |
| Streak | Color of the powdered mineral on an unglazed porcelain plate | Hematite leaves a reddish-brown streak regardless of surface color |
| Cleavage / fracture | Tendency to break along flat planes (cleavage) or irregularly (fracture) | Halite cleaves in three directions at 90°; quartz fractures conchoidally |
| Color | Least reliable property; impurities change it | Pure quartz is clear, but varieties include amethyst (purple) and citrine (yellow) |
Students should use multiple properties together because a single test rarely gives a unique identification — calcite and quartz can both look clear, but calcite fizzes in dilute acid and has a Mohs hardness of 3, while quartz does not fizz and has hardness 7.
Fossil Fuels: Coal, Petroleum, Natural Gas
Fossil fuels are non-renewable energy sources formed from ancient organic matter buried faster than it could decay. Low-oxygen conditions preserved the carbon, and later heat and pressure converted it into fuel over millions of years.
- Coal forms from terrestrial swamp plants that accumulated in anaerobic wetlands. Stages from soft to hard are peat → lignite → sub-bituminous → bituminous → anthracite, with increasing carbon content and heat value.
- Petroleum and natural gas form from marine plankton buried in ocean-floor sediments. As burial depth increases, temperature rises; oil forms in the oil window (about 60-120 °C) and natural gas forms at higher temperatures. The oil and gas migrate upward through permeable rock until trapped by an impermeable cap rock.
Because fossil fuels form so slowly on human timescales, they are classified as non-renewable; burning them releases stored carbon as CO₂, linking this section to the carbon cycle in Section 14.2.
Soil Formation and Soil Horizons
Soil forms through weathering of parent material combined with the accumulation of organic matter. Physical weathering breaks rock into smaller pieces without changing chemistry (frost wedging, root growth, abrasion); chemical weathering alters minerals (hydrolysis, oxidation, carbonic-acid dissolution of limestone). Over time, a vertical soil profile develops distinct layers called horizons:
| Horizon | Description |
|---|---|
| O horizon | Surface organic layer of decomposing leaves and humus |
| A horizon | Topsoil — mineral particles mixed with organic matter; where most plant roots grow |
| B horizon | Subsoil — accumulates minerals leached from above; often clay-rich and colored by iron oxides |
| C horizon | Weathered parent material, partly broken bedrock |
| R horizon | Unweathered bedrock |
Soil types reflect climate and parent material: pedalfers (iron- and aluminum-rich, leached soils of humid regions) and pedocals (calcium-carbonate-accumulating soils of dry regions, common in West Texas) are classic contrasts. Soil fertility, erosion control, and the link between soil and agriculture give 4-8 teachers a direct bridge to social studies and to the nutrient-cycle material in the next section, "Water and Nutrient Cycles in Earth Systems."
A student finds a rock with visible interlocking crystals of quartz, feldspar, and mica. The rock most likely formed by which process?
Which of the following is NOT one of the five defining criteria of a mineral?
A teacher scratches an unknown mineral against an unglazed porcelain plate and observes a reddish-brown powder. Which mineral property is being tested?