Major Rock Types

Why This Matters

Understanding the three major rock types isn't just about memorizing names—it's about grasping the rock cycle and the dynamic processes that continuously reshape Earth's crust. Every rock tells a story of temperature, pressure, time, and environment, and your exams will test whether you can read that story. When you see granite, you should immediately think "slow cooling deep underground"; when you see shale, you should picture "quiet, low-energy water depositing fine particles."

The key concepts you're being tested on include crystallization rates and texture, depositional environments, metamorphic grade, and parent rock relationships. These principles connect to plate tectonics, Earth's internal heat engine, and surface processes like weathering and erosion. Don't just memorize that marble comes from limestone—know why heat and pressure transform calcite crystals and what that reveals about geologic history.

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Igneous Rocks: Born from Molten Material

Igneous rocks crystallize from magma (underground) or lava (at the surface), and their texture reveals their cooling history. Slow cooling allows large crystals to grow; rapid cooling produces fine grains or glass.

Granite

• Coarse-grained intrusive rock—the large, visible crystals of quartz, feldspar, and mica indicate slow cooling deep within the crust
• Felsic composition means it's rich in silica and light-colored minerals, forming from continental crust magmas
• Durability and abundance make it a key indicator of plutonic activity and continental geology

Basalt

• Fine-grained extrusive rock—the tiny crystals result from rapid cooling of lava at Earth's surface
• Mafic composition with high iron and magnesium content gives it a characteristic dark color
• Dominates oceanic crust—understanding basalt is essential for plate tectonics and seafloor spreading concepts

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Sedimentary Rocks: Records of Surface Processes

Sedimentary rocks form through weathering, erosion, deposition, and lithification. Their composition and grain size reveal the depositional environment—from raging rivers to quiet ocean floors.

Sandstone

• Clastic rock with sand-sized grains—typically 0.0625–2 mm, often composed of quartz due to its resistance to weathering
• High-energy depositional environments like rivers, beaches, and deserts produce well-sorted sand deposits
• Excellent reservoir rock—porosity and permeability make it critical for groundwater and petroleum geology

Shale

• Fine-grained clastic rock—clay and silt particles indicate low-energy environments like deep lakes or marine basins
• Fissility (ability to split into thin layers) results from parallel alignment of flat clay minerals during compaction
• Source rock for hydrocarbons—organic-rich shales generate oil and gas, making this economically significant

Limestone

• Chemical/biochemical sedimentary rock—primarily $CaCO_3$ (calcite) precipitated from water or accumulated from shells and coral
• Marine environments dominate limestone formation, making it an indicator of past ocean coverage
• Highly reactive—dissolves in weak acid, creating karst topography (caves, sinkholes) through chemical weathering

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Metamorphic Rocks: Transformed by Heat and Pressure

Metamorphic rocks form when pre-existing rocks (the protolith) are altered by heat, pressure, or chemically active fluids without melting. The degree of change—metamorphic grade—determines texture and mineral assemblages.

Slate

• Low-grade metamorphic rock derived from shale—the clay minerals begin recrystallizing but retain fine grain size
• Excellent cleavage allows it to split into thin, flat sheets along planes perpendicular to pressure direction
• Foliated texture develops as platy minerals align, representing the first stage of shale's metamorphic journey

Schist

• Medium to high-grade metamorphic rock—visible, shiny mica flakes create pronounced foliation and a sparkly appearance
• Index minerals like garnet, kyanite, or staurolite indicate specific pressure-temperature conditions during formation
• Progressive metamorphism from shale → slate → phyllite → schist demonstrates increasing metamorphic grade

Gneiss

• High-grade metamorphic rock—alternating light and dark mineral bands create distinctive compositional layering
• Forms from various protoliths—granite, shale, or other rocks subjected to intense heat and pressure near magma bodies
• Coarse-grained texture with segregated minerals (quartz/feldspar bands vs. biotite/hornblende bands) indicates extensive recrystallization

Marble

• Non-foliated metamorphic rock—forms when limestone or doloite recrystallizes under heat and pressure
• Interlocking calcite crystals create the smooth texture prized in sculpture; original fossils are typically destroyed
• Parent rock relationship is a classic exam topic—always connect marble back to its limestone protolith

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Quick Reference Table

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Self-Check Questions

1. What texture difference between granite and basalt reveals their cooling environments, and what does each texture indicate?

2. You find a rock with visible garnet crystals and shiny, aligned mica flakes. What rock type is this, and what does the presence of garnet tell you about metamorphic conditions?

3. Compare and contrast how sandstone and shale form—what do their different grain sizes reveal about their depositional environments?

4. Trace the metamorphic progression of shale as it experiences increasing heat and pressure. What rocks form at low, medium, and high grades?

5. An FRQ asks you to explain how limestone and marble are related. Describe the parent-product relationship and explain what happens to the original calcite during metamorphism.