The Rock Cycle

• Crustal rock components transform slowly from one form to another in a process summarized by the rock cycle.
• Earth's internal heat engine and the Sun drive the rock cycle.
• A planet requires a hot core, an atmosphere, and liquid water to maintain an active rock cycle.

The Rock Cycle

• Magma is created from rock heated to a molten state
• Intrusive igneous rock forms when magma cools slowly within the crust
• Extrusive igneous rock (volcanic rock) forms when lava cools rapidly on the surface
• Uplift and exposure of all rock types at the surface facilitates weathering by water, wind, ice, and gravity
• Weathering breaks/alters the rock physically and chemically, producing small fragments
• Erosion transports and deposits these fragments as sediments
• Sediments are buried, compressed, and cemented into sedimentary rock
• Deep burial subjects any rock type to increasing heat and pressure
• Tectonic forces add stress, changing the stability of minerals and the texture of the original rock forming metamorphic rock.

Igneous Rock

• Magma temperature ranges between 800° and 1600°C, based on its mantle origin.
• Magma cools slowly within the crust to make intrusive igneous rock or erupts and cools quickly to make extrusive igneous rock.
• Intrusive igneous rock crystallizes at depths of hundreds of meters to tens of kilometers below the surface.

Igneous Rock Classification

• Composition and texture are the two characteristics used to classify igneous rocks.
• Bowen's Reaction Series informs the classification of igneous rocks based on composition This describes crystallization sequence of silicate minerals from cooling magma.

Igneous Rock Composition

• Ultramafic, mafic, intermediate, and felsic are the four categories of igneous rock composition with compositions ranging from silica, potassium, sodium, calcium, iron, and magnesium
• Mafic composition has high concentrations of magnesium (Mg) and iron (Fe).
• Ferromagnesian minerals enrich mafic rocks with Fe and Mg.
• Felsic composition has feldspar minerals of sodium-rich plagioclase and potassium feldspar, plus quartz.
• Igneous rock compositions lie on a spectrum with transitional compositions.
• Mafic igneous rocks generally have calcium-rich plagioclase and ferromagnesian minerals like olivine, pyroxene (augite), and amphibole (hornblende).
• Felsic igneous rock typically contains sodium-rich plagioclase, biotite, potassium feldspar, muscovite, and quartz.
• Bowen's Reaction Series diagram helps classify igneous rocks based on mineral quantities
• The composition is quantitatively determined by the percentage of Bowen's minerals in a sample.
• Ultramafic rocks are mostly olivine with some pyroxene and calcium-rich plagioclase which is rarely found on Earth's surface These come from the upper mantle, are high in iron/magnesium, and poor in silica (<40%).
• Mafic rocks have an abundance of dark ferromagnesian minerals and gray calcium-rich plagioclase feldspar; makes up most of the oceanic crust and has low silica content (45-50%).
• Intermediate rocks have roughly equal amounts of light and dark minerals, like plagioclase feldspar and hornblende commonly associated with convergent plate boundary volcanoes.
• Silica content in intermediate rocks ranges from 55-60%.
• Felsic rocks are composed of light-colored minerals (feldspar and quartz) and minor amounts of dark-colored (ferromagnesian) minerals; it dominates the continental crust is rich in silica (65-75%) and poor in iron/magnesium.

Igneous Rock Texture

• Intrusive (plutonic) rocks cool slowly deep in the crust, allowing development of large crystals producing coarse-grained, phaneritic texture with visible mineral crystals.
• Extrusive (volcanic) rocks cool quickly from lava and have a fine-grained, aphanitic texture with grains too small to see unaided.
• Petrographic microscopes can view tiny crystals in extrusive rocks
• Rapid cooling of extrusive lava can create volcanic glass, a non-crystalline material common in volcanic ash and rocks like obsidian
• An igneous rock is named by determining composition and texture. Classification diagram are read in columns.
• Felsic igneous rocks include potassium feldspar, quartz, sodium-rich plagioclase, biotite, and amphibole
• Intrusive felsic rocks with coarse grains is granite; extrusive felsic rock with small mineral grains is rhyolite
• Diorite and andesite refer to intrusive and extrusive intermediate rock, respectively
• Gabbro (intrusive) and basalt (extrusive) are mafic igneous rocks
• Peridotite is ultramafic and intrusive
• Komatiite is the fine-grained extrusive equivalent but is rare because volcanic material directly from the mantle isn't common Earth history.

Sediments and Sedimentary Rocks

• The sedimentary rock cycle starts with pre-existing rock exposing to weathering and erosion, which can be physical (disintegration) or chemical (mineral chemistry changes)
• Water is the main agent in chemical and physical weathering
• Weathering creates sediment and ions in water
• Sediment is transported and deposited, then buried, compacted, and cemented into sedimentary rock (diagenesis/lithification)
• Ions in solution can cement sediment particles, support life, or precipitate through supersaturation.

Classification of Sedimentary Rocks

• Clastic sedimentary rock (detrital) - sediment pieces from weathered bedrock that are cemented together.
• Clastic sediment is siliciclastic because it comes from crustal rocks that are silicate minerals from igneous rock
• Classified/named by the size and composition of the clasts from boulders to clay, referred to as the sediment grain size
• Clast size, shape, and composition vary based on the source rock and how long the sediment moved
• Chemical sedimentary rock forms by processes without mechanical weathering/erosion Chemical weathering provides dissolved ions in water.
• Inorganic chemical sedimentary rocks precipitate from dissolved ions due to changing ion concentration, dissolved gasses, temperatures, or pressures
• Biochemical sedimentary rocks extracted from water by living organisms to build shells and bodies
• Biochemical rocks are commonly calcite (from sea life) or silica (from single-celled microorganisms called radiolarian)
• Organic sedimentary rock contains remains of organic material that is transformed through burial/heat, to create coal, oil, and methane (natural gas).

Classification of Sedimentary Rocks chart

• Conglomerate consists of rounded rock fragments with granular grain size
• Breccia consists of angular rock fragments with granular grain size
• Quartz sandstone consists of mostly quartz grains and has sand grain size
• Arkose has mostly quartz with typically a minimum 25% feldspar content with sand grain size.
• Graywacke consists clay, quartz, feldspars with a sand grain size.
• Siltstone has silt-sized particles making it lack any fissility.
• Shale contains silt and clay-sized particles with breaks or planes making it fissile.
• Claystone has clay-sized particles.
• Classifications for chemical and organic sedimentary rocks are based on what particle and mineral type contributes the most to the rock.

Metamorphism and Metamorphic Rocks

• Metamorphism- changes within a rock caused by conditions different from those in which it formed where higher temperatures and pressures put it in disequilibrium
• Typically includes recrystallization of minerals, formation of new minerals and different textures, while retaining overall chemical composition
• Protolith is the parent or source rock
• Tectonic forces during mountain building (orogenic) episodes introduce stress and heat that take place over vast areas and last millions of years.

Types of Metamorphism

• Regional metamorphism- occurs over wide expanses of the crust from the collision and compression of tectonic plates. Effects pre-existing rock of the crust and upper mantle.
• Contact metamorphism- baking of pre-existing rock by contact with molten rock (magma intrusion or lava extrusion) can be small and close to the surface, or large and deep in the crust.

Metamorphic Processes

• Mineral composition of parent rock
• Temperature
• Pressure
• Types of fluids (mostly water)
• Time

Parent Rock (Protolith)

• Any rock type can be a protolith
• The stability of minerals determines what happens when metamorphism takes place.
• Minerals may become unstable and begin to recrystallize in size or completely new minerals under increased temperatures.

Temperature and Pressure

• Minerals gain stability as temperature cools
• Quartz is stable under all surface environmental temperatures and up to 1800°C.
• Clay minerals are generally only stable up to 150-200°C, then transform into micas.
• Feldspar is generally stable up to approximately 1200°C.
• Burial/tectonic stresses exert pressure on the protolith.
• Pressure that affects metamorphic rocks include confining pressure and directed differential stress.
• Stress is a scientific term indicating force while strain refers to any metamorphic changes within minerals
• Confining (lithostatic) pressure- when pressure is exerted from rocks above, it is balanced from below/sides, causing chemical reactions just like heat.
• Differential stress is when there is unequal balance of forces on the protolith in one or more directions which is most commonly associated with tectonic movement/orogeny. Differential stress modifies the parent rock at a mechanical level, changing the arrangement, size, and/or shape of the mineral crystals creating an identifying texture known as foliation

Foliation

• Foliation occurs from rock textures developed with a visible parallel alignment.
• Mineral crystals rotate/recrystallize in a solid state is a process that occurs because of the changing orientation in space.
• Atoms migrate to locations of lower stress which leads to minerals increasing in grain size
• The mineral biotite easily reorients when stress is applied.

Fluids and Time

• Water is the main fluid present within rocks of the crust.
• Water facilitates the movement of ions, assists in mineral growth, and acts as a catalyst in metamorphic reactions.
• Most metamorphic reactions take place very slowly.
• Growth of new minerals within a rock during metamorphism is estimated at 1 millimetre per million years.

Classification of Metamorphic Rocks

• Texture and composition determine metamorphic rock classifications.
• Texture is the most readily observed characteristic.
• Foliated rocks are subjected to differential stress for a distinct alignment of minerals Named based on the style of their foliations.
• Non-foliated rocks are not subjected to directed pressure, or the dominant mineral doesn't tend to display alignment of two main types

Classification of Metamorphic Rocks chart

• Slate-Fine-grained foliation (micorscopic grains). Contains fine, microscopic clay or mica.
• Phyllite- Fine-grained (wavy surfaces)- Contains dark silicates and micas.
• Mica schist/Garnet Schist- Medium to coarse-grained. Contains common minerals (chlorite, biotite, muscovite, garnet and hornblende).
• Gneiss- Medium to course grained (banding). Contains light-colored quartz and feldspar, dark ferromagnesian minerals.
• Amphibolite- Medium to course grained. Contains crystals of amphibole (hornblende) in blade-like crystals.
• Anthracite coal- Fine-grained. Contains fine, tar-like, organic makeup.
• Hornfels- Fine-grained. Contains microscopic dark silicates.
• Serpentinite contains serpentine.
• Soapstone/Tale Schist contains Talc.
• Quartzite contains quartz.
• Marble contains calcite/dolomite crystals.
• Meta-Conglomerate contains granite.