2.3 Rock cycle and major rock types

Rock Types

Igneous Rocks

Igneous rocks form from the cooling and solidification of magma (molten rock beneath Earth's surface) or lava (molten rock that has reached the surface). Their classification depends on two things: mineral composition and texture.

By composition:

• Felsic igneous rocks like granite are rich in silica and tend to be light-colored
• Mafic igneous rocks like basalt are rich in magnesium and iron, giving them a darker color

By texture (which reflects where they cooled):

• Intrusive (plutonic) rocks cool slowly beneath the surface, giving crystals time to grow large. Granite and diorite are common examples.
• Extrusive (volcanic) rocks cool rapidly at the surface, producing small crystals or even a glassy texture. Basalt has tiny crystals; obsidian cools so fast it forms volcanic glass.

The key idea: cooling rate controls crystal size. Slow cooling = large crystals. Fast cooling = small or no crystals.

Sedimentary Rocks

Sedimentary rocks form from the accumulation and lithification (compaction and cementation) of sediments, which are loose particles of rock, mineral, or organic matter. They're classified into three main types:

• Clastic sedimentary rocks form from cemented fragments of pre-existing rocks. Sandstone, for example, is made of sand-sized grains glued together by mineral cement.
• Chemical sedimentary rocks form when dissolved minerals precipitate out of solution. Limestone often forms this way, as calcium carbonate precipitates in warm, shallow seas.
• Organic sedimentary rocks form from accumulated and compressed organic material. Coal forms from layers of ancient plant matter buried and compacted over millions of years.

Sedimentary rocks are the only rock type that commonly contains fossils, the preserved remains or traces of once-living organisms. This makes them essential for reconstructing Earth's biological history.

Metamorphic Rocks

Metamorphic rocks form when pre-existing rocks are transformed by high temperature, high pressure, or both, while remaining in a solid state. They never melt during this process; if they did, the result would be an igneous rock instead.

By texture:

• Foliated metamorphic rocks like gneiss and schist display a layered or banded appearance because minerals align in response to directed pressure
• Non-foliated metamorphic rocks like quartzite and marble have a uniform texture without visible layering, typically forming under more even pressure conditions

Metamorphic grade describes the intensity of metamorphism. Low-grade metamorphism (lower temperatures and pressures) produces rocks like slate. High-grade metamorphism produces rocks like gneiss, with more dramatic changes to mineralogy and texture.

Weathering and Erosion

Weathering Processes

Weathering is the breakdown of rock at or near Earth's surface. There are three main types:

Physical (mechanical) weathering breaks rock into smaller pieces without changing its chemical composition.

• Freeze-thaw weathering: Water seeps into cracks, freezes, and expands (by about 9% in volume), wedging the rock apart over repeated cycles.
• Thermal expansion and contraction: Repeated heating and cooling causes the outer layers of rock to expand and contract at different rates, eventually cracking the surface.

Chemical weathering alters the rock's chemical composition through reactions with water, air, or organic compounds.

• Dissolution: Minerals dissolve directly in water. This is how caves and sinkholes form in limestone regions, since calcite dissolves in slightly acidic rainwater.
• Oxidation: Minerals react with oxygen. Iron-bearing rocks "rust," producing the reddish-brown color you see on many exposed rock surfaces.

Biological weathering is driven by living organisms. Tree roots pry apart rock as they grow into cracks. Lichens and mosses produce organic acids that chemically break down rock surfaces.

In practice, these three types almost always work together.

Erosion and Transportation

Erosion is the removal and transport of weathered material by water, wind, ice, or gravity.

• Water erosion is the most widespread agent. Rivers carve valleys and canyons (the Grand Canyon took roughly 5–6 million years to form). Waves erode coastlines into cliffs and beaches. Glaciers grind the land beneath them, sculpting U-shaped valleys and fjords.
• Wind erosion is most effective in arid environments with little vegetation to anchor sediment. It creates features like sand dunes and sculpted rock formations (Arches National Park).
• Mass wasting is the downslope movement of rock and soil under gravity. This includes landslides, rockfalls, and slow-moving creep. It doesn't require a transporting agent like water or wind; gravity alone does the work.

Sedimentary Processes

Deposition

Deposition is the settling and accumulation of sediments in a new location, and it happens when the transporting agent loses energy.

• Water deposits sediments in layers, building features like deltas (the Mississippi River Delta) and floodplains
• Wind deposits sediments in arid environments, forming sand dunes and loess (fine-grained wind-blown silt) deposits
• Ice deposits sediments as glaciers melt, creating moraines (ridges of till) and outwash plains

The energy of the environment controls what gets deposited where. High-energy environments (a fast-flowing river) deposit large, angular fragments like gravel. Low-energy environments (a calm lake) deposit fine, rounded particles like clay and silt. This relationship between energy and grain size is one of the most useful tools for interpreting ancient environments from sedimentary rocks.

Lithification

Lithification converts loose sediments into solid sedimentary rock through two main steps:

1. Compaction: The weight of overlying sediments squeezes the grains together, reducing pore space and forcing out water.
2. Cementation: Minerals like calcite, silica, or iron oxides precipitate from groundwater flowing through the remaining pore spaces, binding the grains together.

This process can take millions of years, depending on factors like sediment composition, burial depth, and the availability of cementing fluids.

Igneous and Metamorphic Processes

Melting and Magma Generation

Rocks melt when conditions push them past their melting point. But that doesn't always mean "add more heat." There are three main mechanisms:

1. Decompression melting: Hot mantle rock rises toward the surface and experiences lower pressure. Lower pressure reduces the melting point, so the rock melts without any added heat. This is the dominant process at mid-ocean ridges.
2. Flux melting: Water or other volatiles are introduced to hot rock, lowering its melting point. This is the key mechanism at subduction zones, where water released from the descending plate triggers melting in the overlying mantle wedge.
3. Heat transfer: Hot magma or a nearby intrusion heats surrounding rock enough to cause melting.

The composition of the resulting magma depends on the source rock and the extent of melting. Partial melting tends to produce more silica-rich (felsic) magmas because silica-rich minerals have lower melting points and melt first. More complete melting of mantle rock produces silica-poor (mafic) magmas like those that erupt as basalt.

Crystallization and Igneous Rock Formation

As magma cools, minerals crystallize in a predictable sequence based on their melting points. High-temperature minerals like olivine crystallize first, while low-temperature minerals like quartz crystallize last. This sequence is described by Bowen's Reaction Series, a foundational concept for understanding igneous rock diversity.

The rate of cooling determines the rock's texture:

• Slow cooling (deep underground) produces large, visible crystals. This is called phaneritic texture, and granite is a classic example.
• Rapid cooling (at or near the surface) produces tiny crystals (aphanitic texture, as in basalt) or even glass (obsidian).
• Mixed cooling histories can produce porphyritic texture, where large crystals sit within a fine-grained matrix, indicating the magma cooled slowly at first, then rapidly.

Together, the composition and texture of an igneous rock tell you about the conditions and tectonic setting where it formed.

Metamorphism and Metamorphic Rock Formation

Metamorphism transforms existing rocks through changes in temperature, pressure, or chemical environment, all while the rock stays solid. Different settings produce different types:

• Regional metamorphism affects large areas, typically during mountain-building events like the formation of the Himalayas. Directed pressure from tectonic forces produces foliated rocks (slate → phyllite → schist → gneiss, in order of increasing grade). The intensity increases with depth, creating a sequence known as a metamorphic facies series.
• Contact metamorphism occurs in a zone around a magma intrusion, where heat bakes the surrounding rock. Because the pressure is relatively even (not directed), this tends to produce non-foliated rocks like hornfels and marble.
• Hydrothermal metamorphism involves hot, mineral-rich fluids circulating through rock, often altering its chemical composition. This process, called metasomatism, can concentrate economically valuable minerals.

The mineralogy and texture of any metamorphic rock reflect three things: the type of metamorphism, its intensity, and the composition of the original parent rock (called the protolith).