2.4 Rock cycle & broad geologic processes

The rock cycle

The rock cycle is the conceptual model describing how the three great rock classes — igneous, sedimentary, and metamorphic — form from and transform into one another through Earth's internal and external processes. No rock is permanent: given enough time and the right conditions, any rock can be converted into any other. The cycle links deep-Earth heat, surface climate, and tectonics into a single framework, and it is a favorite source of ASBOG questions.

The three rock classes and their transitions

• Igneous rocks crystallize from molten rock. Magma cooling slowly below ground forms coarse-grained intrusive (plutonic) rocks such as granite, while lava cooling quickly at the surface forms fine-grained extrusive (volcanic) rocks such as basalt; very rapid cooling can produce glass. Composition ranges from felsic (silica-rich, light-colored, e.g., granite and rhyolite) to mafic (silica-poor, dark, e.g., gabbro and basalt), and minerals crystallize in the predictable order of Bowen's reaction series.
• Sedimentary rocks form at or near the surface. Weathering breaks down existing rock, erosion and transport move the debris, deposition lays it down, and lithification (compaction plus cementation) hardens it. Clastic rocks (sandstone, shale) are made of particles, whereas chemical and biochemical rocks (limestone, rock salt, chert) precipitate from solution or accumulate from organisms.
• Metamorphic rocks form when heat, pressure, and chemically active fluids alter a pre-existing protolith in the solid state, without melting. Foliated rocks (slate to phyllite to schist to gneiss) develop aligned minerals under directed pressure; non-foliated rocks (marble from limestone, quartzite from sandstone) recrystallize without strong mineral alignment.

Paths through the cycle

The rock cycle is not a fixed one-way loop. Any rock buried and heated to melting becomes magma and then igneous rock. Any rock exposed at the surface is weathered and may become sediment and then sedimentary rock. Any rock subjected to elevated heat and pressure short of melting becomes metamorphic rock. For instance, a granite can weather to quartz sand that lithifies into sandstone, which can be metamorphosed to quartzite, then uplifted, weathered, and recycled again.

Internal (endogenic) processes

Processes driven by Earth's internal heat — leftover accretionary heat plus ongoing radioactive decay — build topography and drive the rock cycle from within.

Plate tectonics

Plate tectonics is the unifying theory of modern geology. The rigid lithosphere is broken into plates that move over the weaker, ductile asthenosphere, driven mainly by mantle convection, slab pull, and ridge push. Three boundary types dominate:

Continental collision builds great mountain belts such as the Himalaya; ocean-continent subduction builds volcanic arcs such as the Andes and Cascades. Tectonics also generates most earthquakes, which release stored elastic strain by elastic rebound along faults, and most volcanism, and it continually recycles oceanic crust back into the mantle.

Earthquakes and Earth's interior

Earthquakes radiate energy as seismic waves: fast P-waves (compressional, able to travel through solids and liquids) and slower S-waves (shear, blocked by liquids), whose differing travel times reveal Earth's layered interior. That interior is divided compositionally into crust, mantle, and an iron-rich core, and mechanically into the rigid lithosphere, ductile asthenosphere, deeper mantle, liquid outer core, and solid inner core. The inability of S-waves to cross the outer core is the primary evidence that it is molten. This internal architecture supplies the heat and convection that move the plates, so the deep Earth ultimately powers mountain building, volcanism, and the igneous and metamorphic limbs of the rock cycle.

External (exogenic) processes

Processes powered by solar energy and gravity sculpt the surface and supply the sediment for new rock.

• Weathering is the in-place breakdown of rock. Mechanical (physical) weathering — frost wedging, thermal expansion, exfoliation, and salt or root growth — disintegrates rock without changing its chemistry, whereas chemical weathering — dissolution, hydrolysis of feldspar to clay, oxidation of iron, and carbonation — alters mineral composition. Warm, wet climates accelerate chemical weathering, while cold or arid climates favor mechanical weathering. The residue of weathering forms soil and supplies dissolved ions to rivers and oceans.
• Erosion is the removal and transport of weathered material by running water (the dominant agent), glacial ice, wind, and gravity-driven mass wasting (landslides, slumps, creep).
• Deposition occurs wherever transport energy drops — in river channels, deltas, lakes, dunes, and the sea — building the sediment layers that later lithify. Sediments generally become more mature — better sorted and more rounded — with longer transport distance.

Internal and external processes act in constant opposition: tectonics and volcanism raise the land while weathering and erosion wear it down. The balance between uplift and denudation governs landscape evolution over geologic time. This interplay, organized by the rock cycle and powered by both internal and external energy, connects deep-Earth dynamics to the surface features geologists map — and it underlies the broad-process questions that appear throughout the ASBOG FG and PG exams.