The rock cycle

The rock cycle begins with magma
Forms from melting in crust & mantle
Less dense magma rises toward the surface
Erupts at surface as lava, cools within crust

Magma: generated by melting in the mantle, & some by melting the crust, & Rises because it is less dense than surrounding rock

lava: Magma reaches Earth’s surface

Cooling is crystallization or solidification

Igneous rocks are crystallized from
– Magma: within the crust
– lava: at Earth’s surface
Igneous rocks exposed at Earth’s surface undergo weathering
Atmosphere decomposes rock
Generates loose material or dissolves it

Loose material is called sediment
Transported by gravity, running water, glaciers, wind, waves, etc.
Most sediment is transported to the ocean, but some is deposited in other environments

lithification: Deposited sediment “conversion into rock” by Compaction & Cemention

Metamorphism: Deformed by great heat & pressure if deeply buried or incorporated into a mountain chain

heat can melt the rock & generate magma

Rocks aren’t stable unchanging masses over geologic time scales, & Rock cycle happens over millions or billions of years

Different stages of the rock cycle are occurring today all over Earth’s surface
– New igneous rocks are forming in Hawaii
– The Colorado Rockies are eroding & material is being carried to the Gulf of Mexico

Rocks don’t always go through the rock cycle from igneous to sedimentary to metamorphic
– Igneous remain deeply buried & become metamorphosed
– Sedimentary & metamorphic uplifted & eroded into sediment instead of melted

The rock cycle is driven by internal heat & external processes (weathering & erosion)

Processes for transform a rock
– Igneous form if molten magma crystallizes
– Sedimentary form if weathered particles are lithified (compaction & cementation)
– Metamorphic rocks form when other rocks are exposed to heat & pressure

Igneous Rocks

Formed by Fire, form when magma or lava cools & crystallizes

extrusive or volcanic rocks: Solidification of lava at Earth’s surface
– Abundant in the NW (Cascades, Columbia)
– Many oceanic islands are volcanic (Hawaii)
Most volcanic eruptions are not violent

intrusive or plutonic rocks: magma never reaches the surface, & solidifies
Exposed at the surface by uplift & erosion
– Mount Washington (New Hampshire)
– Stone Mountain (Georgia)
– Mount Rushmore & Black Hills (S-Dakota)
– Yosemite National Park (California)

Magma contains ions (Si, O) & gas (H₂O) confined by pressure, & solid crystals

Crystallization occurs as mobile ions arrange into orderly patterns during cooling

As cooling continues, more ions are added to the crystals until all of the liquid becomes a solid mass of interlocking crystals

Igneous classified by
Texture: results from cooling history, interloking texture
Mineral composition: derives from parent magma & environment of crystallization

The texture

described based on size, shape, & arrangement of grains

Used to make inferences about rock’s origin
Large crystals → slow cooling → common in magma chambers deep in the crust
Fine-grained → rapid cooling → at the surface or in small masses in the upper crust
– crystals small to see with the naked eye
vesicular voids left by gas bubbles that remained when lava solidified (scoria)
Coarse-grained →Solidified at depth while insulated by surrounding rock
– Masses of interlocking crystals roughly the same size (large to be seen by the naked eye)
Porphyritic Large crystals in matrix of smaller crystals
Different minerals crystallize under different T-P conditions & One mineral can reach large size before other minerals start to form
Glassy texture when rocks cool rapidly
Atoms freeze in place before they can arrange themselves in an orderly structure
More likely in silica-rich magmas
Form during volcanic eruptions

Composition

Igneous rocks mainly composed of silicate
Si & O + (Al, Ca, Na, K, Mg, Fe) make up 98% of most magmas, & includes small amounts of trace elements Ti, Mn, Au, Ag, U
During crystallization, these elements combine to form 2 major groups of silicate
Dark silicates: rich in Fe & Mg, low in Si such as Olivine, pyroxene, amphibole, biotite
Light silicates contain Na, K, Ca, richer in Si such as Quartz, mica, feldspars
Feldspars are most abundant mineral group 40% of most igneous rocks

Crystallization influenced by
Magma composition
Dissolved gas
Rate of cooling
– Slow cooling → fewer, large crystals
– Quick cooling → tiny intergrown crystals
– Instantaneous cooling → randomly distributed atoms, no crystal growth (formation of volcanic glass)

Volcanic ash: tiny shards of glass

Sediments

Weathering & Erosion

Weathering break down of rocks, transformation of rock to reach equilibrium with environment, form Sediment
– Natural response of materials to a new environment
– Generally occur simultaneously

2 basic categories of weathering
mechanical (physical) process of breaking down rocks into smaller pieces, Each piece retains same physical properties of original material & increases surface area
chemical alters internal structure of minerals, Elements are removed or added

Ex. Mechanical Weathering
Frost wedging
Sheeting
Biological activity

How does mechanical weathering break rocks?
1. Break into smaller pieces but chemical composition does not change
2. Increases surface area for chemical weathering

How does chemical break rocks down?
1. Oxidation or dissolution by carbonic acid
2. changes mineral’s crystalline structure

Erosion transports weathered rock

Sedimentary Rocks: Compacted and Cemented Sediment

form after weathering breaks rocks down, gravity & erosional agents transport & deposit the sediment, & then sediment lithified

Most sedimentary rock is deposited by solid material settling out of a fluid

Sedimentary rocks make up:
5% of Earth’s outer 10 miles g
75% of all continental rock outcrops

Used to reconstruct details about Earth’s history

Economically important: Coal, Petroleum, Gas, Metals, Fertilizer, & Construction Materials

Classification

classified in 2 groups
Detrital from weathered solid particle (rock)
Chemical from ion in solution
biochemical considered a type of chemical, Materials precipitated by organisms

Lithification Process

process by which sediment is transformed into sedimentary rock, include:
1. Compaction occurs when grains are pressed closer together so that pore space is reduced
– Weight of accumulated sediment
– Most significant in fine-grained rocks
2. Cementation occurs when water containing dissolved minerals moves through pores
– Cement precipitates, fills pores, & joins particles together
– common cements: Calcite, Silica, & Fe-oxide

Characteristic of sedimentary rocks

Strata or Beds layers of sedimentary rocks
Thickness range [microscopic – tens m]
Mark the end of one episode of sedimentation & the beginning of another

Fossils Traces or remains of life
Important clues of ancient environment
Can be used to match up rocks of the same age found in different places

Metamorphic Rocks: New from Old

produced when preexisting parent rock is transformed
Parent rock: can be Igneous, Sedimentary, or Metamorphic rocks

Metamorphism

Elevated T & P, occurs if rock is subjected to a different environments, Equilibrium with new environment

Most metamorphism occurs in 2 settings:
1. Contact or thermal: Rock T increases because of intruding magma
2. Regional: P & T, during mountain building, produce most of metamorphic rocks

What causes metamorphism: occurs because parent rock is not in equilibrium

How does metamorphism affect rocks: Heat & pressure change texture, mineralogy, & sometimes chemical composition

Metamorphism can change the texture
Low-grade (Contact) compact, & denser
High-grade (Regional) recrystallization & growth of visible crystals

Agents of metamorphism

Heat: from intrusion of magma or burial, reactions & recrystallization of new minerals
Confining pressure (contact): equal in all directions due to burial, Compaction & recrystallization
Differential stress (regional): greater in one direction due to mountain building, Deformation & development of metamorphic textures
– Rocks can react by breaking (brittle)
– Rocks can react by bending (ductile)
– brittle Vs ductile: depending on T
Chemically active fluids: hydrothermal fluid rich in Fe, Catalyze recrystallization reactions
– Dissolve mineral from one area & precipitate it in another, & change composition of surrounding rocks

Classification

Foliation rocks (regional)
– Driven by compressional stress 
– Causes grains to develop parallel alignment
– Includes:
1. Parallel alignment of micas
2. Parallel alignment of flattened pebbles
3. Separation of light & dark minerals
4. Development of rock cleavage

Nonfoliated rocks
– occur when deformation is minimal & parent rock is composed largely of stable minerals

Summary

Foliated regional, High-grade, Differential P
Nonfoliated Contact, Low-grade, Confining P

The End