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Rock formation sits at the heart of understanding Earth as a dynamic system. When you study how rocks form, you're really learning about the rock cycle—the continuous process that connects plate tectonics, weathering, erosion, volcanic activity, and mountain building into one interconnected system. The AP exam loves testing whether you can trace energy and matter through these transformations, not just identify rock types in isolation.
Here's the key insight: every rock on Earth is temporary. Granite that cooled deep underground millions of years ago can be uplifted, weathered into sediments, compressed into sandstone, and then metamorphosed into quartzite—all driven by Earth's internal heat and surface processes. Don't just memorize rock names—know what process formed each rock and what conditions that reveals about Earth's past. That's what earns you points on FRQs.
Igneous rocks form when magma (underground) or lava (at the surface) cools and solidifies. The rate of cooling determines crystal size—slow cooling allows large crystals to grow, while rapid cooling produces fine-grained or glassy textures.
Compare: Granite vs. Basalt—both are igneous, but granite's coarse texture reveals slow underground cooling while basalt's fine texture indicates rapid surface cooling. If an FRQ asks you to explain how texture reveals formation environment, these are your go-to examples.
Sedimentary rocks form through the weathering, erosion, deposition, and lithification of pre-existing materials. Compaction and cementation transform loose sediments into solid rock, often preserving layers that record environmental history.
Compare: Sandstone vs. Rock Salt—both are sedimentary, but sandstone forms from physical accumulation of weathered fragments while rock salt precipitates chemically from evaporating water. This distinction tests whether you understand clastic vs. chemical formation pathways.
Metamorphic rocks form when existing rocks (igneous, sedimentary, or other metamorphic) are transformed by heat, pressure, or chemically active fluids without melting completely. The original rock is called the protolith.
Compare: Regional vs. Contact Metamorphism—both transform existing rocks, but regional metamorphism involves large areas and tectonic pressure (producing foliation), while contact metamorphism is localized around heat sources (typically non-foliated). FRQs often ask you to identify metamorphic type based on texture and geologic setting.
The texture of a metamorphic rock reveals the conditions under which it formed. Pressure direction and intensity determine whether minerals align into layers or remain randomly oriented.
Compare: Schist vs. Marble—both are metamorphic, but schist's foliation indicates directed pressure during regional metamorphism, while marble's non-foliated texture suggests either contact metamorphism or a protolith (limestone) that can't develop foliation. Texture is your diagnostic clue.
| Concept | Best Examples |
|---|---|
| Slow cooling (coarse texture) | Granite, Diorite, Gabbro |
| Rapid cooling (fine/glassy texture) | Basalt, Obsidian, Pumice |
| Clastic sedimentation | Sandstone, Shale, Conglomerate |
| Chemical precipitation | Rock salt, some Limestone |
| Regional metamorphism (foliated) | Slate, Schist, Gneiss |
| Contact metamorphism (non-foliated) | Marble, Hornfels, Quartzite |
| Increasing metamorphic grade | Shale → Slate → Schist → Gneiss |
| Rock cycle connections | Any rock type can become any other |
Texture comparison: Both granite and basalt are igneous rocks with similar chemical compositions. What causes their dramatically different textures, and what does each texture reveal about formation environment?
Process identification: A geologist finds a rock with visible layers of aligned mica crystals. What type of metamorphism likely formed this rock, and what does the foliation indicate about pressure conditions?
Compare and contrast: How do clastic and chemical sedimentary rocks differ in their formation processes? Give one example of each and explain what environmental conditions each indicates.
Rock cycle thinking: Describe the sequence of processes that could transform a granite into a sandstone, then into a quartzite. What Earth systems are involved at each step?
FRQ-style application: A limestone formation is intruded by a magma body. Predict what metamorphic rock would form in the contact zone, explain whether it would be foliated or non-foliated, and justify your answer based on the conditions present.