Rock Cycle Stages

Why This Matters

The rock cycle isn't just a diagram you memorize—it's the story of how Earth constantly recycles its materials over millions of years. When you understand the rock cycle, you're really learning about energy transfer, Earth's internal heat engine, and the interplay between surface and subsurface processes. These connections show up repeatedly on exams, especially when questions ask you to trace a rock's journey or explain why certain rock types appear in specific geologic settings.

You're being tested on your ability to explain how and why rocks transform, not just label the stages. Each transition in the cycle—melting, cooling, weathering, burial—represents a change in environmental conditions. Don't just memorize that granite becomes gneiss; know that this transformation requires heat and pressure found deep in the crust. Master the mechanisms, and you'll handle any question thrown at you.

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From Melt to Solid: Igneous Processes

Igneous rocks represent the rock cycle's "starting point" in many diagrams, but they can form at any time when temperatures get high enough to melt existing rock. The key variable is cooling rate—it determines crystal size and texture.

Igneous Rock Formation

• Cooling rate determines crystal size—slow cooling underground produces large, visible crystals; rapid surface cooling creates fine-grained or glassy textures
• Intrusive (plutonic) rocks like granite form beneath the surface, while extrusive (volcanic) rocks like basalt form from lava at the surface
• Mineral composition reflects the chemistry of the parent magma, linking igneous rocks to tectonic settings like mid-ocean ridges or subduction zones

Melting

• Extreme heat transforms solid rock to magma—this occurs at plate boundaries, hotspots, and areas with high radioactive decay
• Parent rock composition determines magma chemistry, which in turn controls the type of igneous rock that eventually forms
• Tectonic significance is huge: melting drives volcanism, creates new oceanic crust, and recycles crustal material back into the mantle

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Breaking Down: Weathering and Erosion

Before any rock can become sedimentary, it must first be broken apart and transported. These destructive processes are actually constructive for the cycle—they liberate minerals and create the raw materials for new rocks.

Weathering and Erosion

• Weathering breaks rocks down through physical processes (frost wedging, root growth), chemical reactions (oxidation, acid dissolution), and biological activity
• Erosion moves the pieces—agents include water, wind, ice, and gravity, each leaving distinctive signatures in the landscape
• Soil formation and nutrient cycling depend on weathering, connecting the rock cycle to ecosystems and the biosphere

Sediment Transport

• Transport agents include rivers, glaciers, wind, and ocean currents—each moves particles differently based on available energy
• Particle size matters—larger, heavier sediments require more energy to move and settle out first when energy decreases
• Sorting occurs during transport, with well-sorted sediments indicating consistent transport conditions and poorly-sorted sediments suggesting rapid deposition

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Building Up: Sedimentation Processes

Once sediments stop moving, they begin accumulating in layers. The depositional environment—its energy level, chemistry, and biological activity—controls what type of sedimentary rock eventually forms.

Sediment Deposition

• Deposition occurs when transport energy drops—this happens where rivers slow, glaciers melt, or wind loses speed
• Energy level controls grain size—high-energy environments (mountain streams) deposit coarse gravels; low-energy settings (deep ocean) accumulate fine clays
• Landforms result from deposition patterns, including deltas, beaches, floodplains, and submarine fans

Sedimentary Rock Formation

• Lithification transforms loose sediment into rock through compaction (pressure squeezes out water) and cementation (minerals precipitate between grains)
• Three categories exist: clastic rocks (sandstone, shale) from rock fragments; chemical rocks (rock salt, some limestone) from precipitation; organic rocks (coal, some limestone) from biological remains
• Sedimentary structures like bedding, ripple marks, and fossils preserve evidence of ancient environments—key for interpreting Earth history

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Transformation Under Pressure: Metamorphism

When rocks get buried deep or caught in tectonic collisions, heat and pressure transform them without melting. Metamorphism represents a middle ground—conditions intense enough to reorganize minerals but not hot enough to create magma.

Metamorphism

• Heat and pressure drive recrystallization—minerals realign or transform into new minerals stable under the new conditions
• Foliated rocks (slate, schist, gneiss) show layered textures from directed pressure; non-foliated rocks (marble, quartzite) form under uniform pressure or from single-mineral parents
• Metamorphic grade indicates intensity—low-grade slate forms from shale under mild conditions; high-grade gneiss requires temperatures approaching melting

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Quick Reference Table

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Self-Check Questions

1. A rock has large, visible crystals of quartz and feldspar. What does this tell you about its cooling history, and what rock type is it likely to be?

2. Compare and contrast the roles of weathering and erosion in the rock cycle. Why do both processes need to occur before sedimentary rocks can form?

3. Which two sedimentary rock types—sandstone and rock salt—form through different processes? Explain the mechanism behind each.

4. A geologist finds schist in a mountain range. What can she infer about the pressure conditions during its formation, and what parent rock might it have come from?

5. Trace a possible path for a single grain of quartz through the complete rock cycle, starting as part of a granite pluton and ending up in a metamorphic rock. Identify at least four stages the grain passes through.