Rock Cycle Notes

Rock Cycle Overview

  • The rock cycle connects Earth's surface and internal processes into one dynamic system.
  • Rocks are continuously broken down, transformed, and reformed, linking processes like volcanism, weathering, erosion, and mountain building.
  • The three major rock types are igneous, sedimentary, and metamorphic; they form under very different conditions.
  • The cycle is not a closed loop with a single starting point; it is better thought of as a network or web where any rock type can become another given the right conditions.
  • Earth has two main energy sources that drive the cycle:
    • Heat from interior: leftover heat from formation (approximately 4.5 × 10^9 years ago) and ongoing radioactive decay of unstable isotopes inside Earth. Heat drives mantle convection, which powers tectonics, volcanism, mountain building, and metamorphism.
    • Sun’s energy at the surface: solar radiation powers the hydrologic cycle (evaporation, precipitation, rivers) and fuels weathering and erosion, moving sediment across the landscape.
  • Compared to Mars, Earth has much more internal heat and an active hydrologic cycle, enabling a dynamic rock cycle; Mars has much less internal heat and no active hydrologic cycle, so its rock cycle is largely inactive.
  • In summary, Earth’s rock cycle is a dynamic interaction of surface and interior processes that can recycle any rock into another through time.

What is a Rock?

  • Rocks are naturally occurring aggregates of minerals, not pure substances.
  • A rock is made of many mineral grains stuck together.
  • Example: Granite is composed of quartz, feldspar, and mica.
  • This composition is a common exam point (granite example used repeatedly).

Igneous Rocks

  • Igneous rocks form when magma cools and crystallizes.
  • Texture reflects cooling rate:
    • Intrusive (plutonic): magma cools slowly underground → coarse-grained rocks (crystals grow large).
    • Extrusive (volcanic): lava erupts to the surface and cools quickly → fine-grained or glassy rocks.
  • Granite is a classic example of a felsic, coarse-grained intrusive rock; quartz and feldspar are visible in granite.
  • Obsidian is a volcanic glass formed from rapid surface cooling.
  • The cooling history is key: slow cooling underground yields intrusive textures; rapid cooling yields extrusive textures; mixed cooling can yield porphyritic textures.
  • When classifying igneous rocks, two main criteria are used:
    • Composition: silica and light-colored minerals versus darker minerals.
    • Texture: grain size (coarse vs fine) and presence of glassy material.
  • Composition categories:
    • Felsic: rich in silica and light-colored minerals (e.g., quartz and feldspar); typically high silica content.
    • Intermediate: balanced light and dark minerals.
    • Mafic: higher iron and magnesium, darker color minerals.
    • Ultramafic: very high Fe and Mg, very low silica; mantle-derived; examples include peridotite; rare extrusive equivalents exist (komatiite).
  • Common rock names by composition and texture:
    • Intrusive: granite (felsic), diorite (intermediate), gabbro (mafic).
    • Extrusive: rhyolite (felsic), andesite (intermediate), basalt (mafic).
    • Obsidian represents glassy extrusive rock.
  • Bowen’s reaction series (conceptual framework linking temperature, mineral stability, and rock composition):
    • Discontinuous branch: olivine → pyroxene → amphibole → biotite, as temperature decreases, with each mineral crystallizing from the melt and then reacting to form the next.
    • Continuous branch: plagioclase feldspar changes from calcium-rich to sodium-rich variants as temperature drops.
    • Residual phase at low temperatures includes potassium feldspar, muscovite, and quartz.
  • Mineralogical implications:
    • Quartz and potassium feldspar often crystallize late and are commonly found together.
    • Olivine and pyroxene are common in basalts that solidify from hotter magma.
  • Silica content and rock classification:
    • Ultramafic: high Fe/Mg, low silica, e.g., peridotite (mantle-derived).
    • Mafic: ~45–50% silica; dark minerals predominate; e.g., basalt (extrusive) and gabbro (intrusive).
    • Intermediate: ~55–65% silica; e.g., andesite (extrusive) and diorite (intrusive).
    • Felsic: >65% silica; light-colored minerals dominate; e.g., granite (intrusive) and rhyolite (extrusive).
  • The compositional spectrum links to tectonic settings:
    • Basalt is typical of ocean ridges and hot spots.
    • Granite is typical of continental crust.
  • Igneous texture deeper dive:
    • Phaneritic: coarse-grained texture (granite as a classic example).
    • Aphanitic: fine-grained texture (basalt is a common example).
    • Glassy: no crystals (obsidian).
    • Porphyritic: large crystals set in a finer-grained groundmass.
  • Intrusive vs extrusive equivalents by composition:
    • Felsic: granite (intrusive) vs rhyolite (extrusive).
    • Intermediate: diorite (intrusive) vs andesite (extrusive).
    • Mafic: gabbro (intrusive) vs basalt (extrusive).
    • Ultramafic: peridotite (intrusive) with rare extrusive counterparts (komatiite).
  • Practical implications:
    • Texture reveals cooling history and thus the tectonic setting.
    • Composition reveals magma chemistry and potential tectonic context.
  • Specific igneous rock examples:
    • Granite: felsic, coarse-grained, intrusive; visible quartz and feldspar; dominant in continental crust.
    • Basalt: mafic, fine-grained, extrusive; forms much of the ocean floor.
    • Andesite: intermediate; common at subduction zone arcs (convergent boundaries).
    • Obsidian: volcanic glass; rapid surface cooling; tool-use in ancient cultures.

Sedimentary Rocks

  • Sedimentary rocks form through weathering and erosion of existing rocks, followed by transport, deposition, and lithification.
  • They are key records of surface processes and environments, including deserts, oceans, and past life through fossils.
  • The three main categories:
    • Clastic (detrital): formed from fragments of preexisting rocks.
    • Chemical (or inorganic/evaporite): formed when minerals precipitate from solution.
    • Biochemical/organic: formed from biological activity or plant/animal remains.
  • Surface coverage and importance:
    • Sedimentary rocks cover about 75% of Earth's continents and form at the surface where weathering and deposition are active.
  • Sedimentary textures and structures:
    • Ripple marks, cross-bedding, and fossils are often preserved, giving clues to depositional environments.
  • Clastic rocks and particle sizes (examples):
    • Conglomerates: clasts > 2 mm, rounded clasts indicating transportation over long distances by rivers or waves.
    • Breccias: clasts > 2 mm, angular clasts indicating short transport distances.
    • Sandstones: sand-sized grains, commonly quartz; arkose (feldspar-rich); graywacke (muddy matrix).
    • Siltstone and shale: very fine grains; shale splits along bedding planes.
  • Chemical sedimentary rocks:
    • Halite (rock salt) from evaporated seawater.
    • Gypsum formed in evaporative environments.
    • Chert: microcrystalline quartz formed from silica-rich skeletal remains and detrital silica; valuable for recording chemistry and biology of ancient oceans and lakes.
  • Biochemical/organic sedimentary rocks:
    • Limestone: composed largely of shells and skeletal fragments cemented by calcite; chalk is a type of limestone from microscopic plankton remains.
    • Coal: formed from plant material, progressing from peat to lignite to coal to anthracite with burial, heat, and pressure.
    • Oil shales: organic-rich rocks that can generate oil and natural gas when buried and heated, stored as source rocks and can migrate into reservoirs.
  • Practical interpretation of sedimentary records:
    • Clastic rocks reveal transport and energy of environments (floods, rivers, deserts).
    • Chemical/biochemical rocks reveal water chemistry and biological activity.
    • Organic rocks reveal past life and ecosystem conditions.
  • Notable details:
    • Chalk and chert provide important paleooceanic records.
    • Sedimentary rocks form the surface record and preserve layered histories of environments.

Metamorphic Rocks

  • Metamorphism transforms rocks while remaining solid, driven by heat, pressure, and fluids.
  • Protolith concept:
    • The original rock from which a metamorphic rock formed; it can be igneous, sedimentary, or another metamorphic rock.
  • Agents of metamorphism:
    • Heat increases mineral stability and promotes recrystallization.
    • Pressure includes confining pressure (equal from all directions) and directed stress (unequal, common during collisions).
    • Fluids (water-rich) circulate through rocks, transporting ions and speeding reactions to form new minerals.
    • Time is important; long durations allow substantial mineral changes and growth.
  • Metamorphic textures:
    • Foliated: minerals align in parallel layers or bands due to directed pressure; shows directional stress.
    • Nonfoliated: lack of preferred orientation; typically dominated by a single mineral that recrystallizes into an interlocking mosaic.
  • Regional vs contact metamorphism:
    • Regional metamorphism occurs on large scales during tectonic collisions and mountain-building, producing foliation (eg, slate to phyllite to schist to gneiss).
    • Contact metamorphism is localized near heat sources such as intruding magma; results in nonfoliated rocks (eg, quartzite, marble).
  • Mineral stability and metamorphic grade:
    • Mineral assemblages reflect stability fields at higher temperatures and pressures; progression with increasing grade yields minerals such as garnet and kyanite in higher-grade rocks.
  • Common metamorphic rocks and their textures:
    • Foliated rocks: slate (fine, slaty cleavage), phyllite (glossy sheen), schist (visible mica), gneiss (coarse with light-dark banding).
    • Nonfoliated rocks: marble (from limestone, recrystallized calcite), quartzite (from sandstone, interlocking quartz grains), anthracite coal (high-grade coal).
  • Protolith to metamorphic path examples:
    • Shale can become slate (low-grade), then phyllite, schist, and finally gneiss at higher grades.
    • Limestone can become marble; sandstone can become quartzite.
  • Practical significance:
    • Metamorphic rocks record deep crustal history, including plate tectonics and mountain-building processes.
    • Textures and mineral assemblages provide clues to the pressure-temperature trajectory of rocks.
  • Quick field implications:
    • Slate exhibits slaty cleavage and breaks along plains; Schist shows shiny mica crystals; Gneiss displays distinct light-dark banding; Marble and Quartzite are resistant to weathering as building stones.

The Rock Cycle in Practice

  • The rock cycle is a dynamic, interconnected system rather than a simple loop.
  • Uplift exposes rocks to surface processes; subduction transports rocks downward, leading to metamorphism or melting.
  • Volcanic activity returns materials to the surface, closing the surface part of the cycle.
  • Plate tectonics is a key driver of the cycle: it creates mountain belts, volcanic arcs, ocean basins, and uplifted terrains.
  • The cycle links internal energy (Earth’s heat) with external energy (solar energy and hydrological cycle).
  • Practical implications and relevance:
    • Sedimentary rocks record surface environments and life; igneous rocks reveal magma evolution and crust formation; metamorphic rocks tell deep crustal histories and tectonic processes.
    • The rock cycle informs resource exploration, such as building stones (granite, marble, quartzite) and hydrocarbon resources (oil shales, coal-derived deposits).
  • The big takeaway is that the rock in your hand could have originated as lava, beach sand, or deep crustal rock, illustrating Earth as a system in constant change.

Quick Takeaways

  • The rock cycle demonstrates that Earth materials are constantly recycled through igneous, sedimentary, and metamorphic processes.
  • Each rock type forms in characteristic ways:
    • Igneous: crystallize from magma or lava.
    • Sedimentary: form from weathered material or precipitation; cementation and compaction lithify sediments.
    • Metamorphic: form from heat and pressure without melting; new minerals and textures arise.
  • Classification hinges on texture and mineral composition:
    • Texture refers to grain size and foliation.
    • Composition ranges from ultramafic to felsic (ultramafic, mafic, intermediate, felsic).
  • Rocks are records of Earth processes, plate tectonics, and past environments, including life.
  • Studying rocks is about reconstructing Earth’s history, often one rock at a time, by interpreting textures, mineralogy, and depositional or tectonic context.
  • Good luck on your quiz next week.