EESC `1000 Test 2

Rock

a coherent, naturally occurring solid that consists of an aggregate of minerals (or occasionally, glass)

Rock cycle

Igneous rocks → subjected to weathering and erosion at the surface → sediments → cementation → sedimentary rock → incorporated into a mountain range + increased temperature → metamorphic rock → more heat = melting → melts/magma (molten rock) → igneous rocks

Igneous rocks

• formed by crystallization of a cooling melt near Earth’s surface

  • Found in mid-ocean ridges (divergent) & volcanic arcs (convergent) → active tectonic plate boundaries with intense heat & pressure; molten rock rapidly cools

  • Smooth, homogeneous, no structure

Sedimentary rocks

• formed by cementation of rock fragments or by precipitation near Earth’s surface 

  • Found in deep-sea trenches → subduction of oceanic plate at convergent boundary scrapes sea floor

  • Bedding, layering

Metamorphic rocks

•  formed by transformation of an existing rock (i.e., protolith) in response to changes in temperature (T), pressure (P), and/or chemical conditions below the Earth’s surface

  • Unlike weathering (Earth’s surface), metamorphism occurs deep within the crust/upper mantle

  • Found in mountain belts → tectonic plates collide & compress crust; high temperatures & directional pressure 

  • Deformed → folded

Magma

• molten rock/melt underground

  • Produces intrusive igneous rocks inside Earth’s crust (magma chamber); slow cooling

Lava

• comes to the surface via fractures 

  • Produces extrusive igneous rocks on the Earth’s surface; rapid cooling → (ash cloud, lava flow)

Heat is required to produce magma/melts

• Primordial heat → generated by kinetic energy of impact and gravitational energy (more material accumulating = more gravitational energy)

• Radioactivity → heat-producing radioactive isotopes decaying in Earth’s crust

In most cases, “melting” = partial melting

only some parts of the protolith (i.e., the minerals with the lowest melting points) will melt; the rest forms a residue, the melting of which would require even higher temperatures

Crystallization is the opposite of melting

the minerals with the highest melting points will crystallize from the magma first; the ones with lower melting points will crystallize at lower temperatures

Fractional crystallization

when a mineral crystallizes from a magma, it removes chemical components because it requires them to grow (i.e., they are the mineral’s essential “nutrients”) → the melt will become depleted in these components (i.e., the melt composition changes over time as a result of crystallization)

Crystallization of minerals from magma is induced by

cooling

Quenching

very fast cooling; Earth’s surface → glassy rocks can be produced (e.g., obsidian)

Slow cooling

coarse-grained rocks (e.g., granite) → the chemical components (“nutrients”) required to crystallize the minerals have lots of time to assemble into large crystals

Three (3) locations where melting happens

• Rise of a mantle plume to form a hot spot

• Release of volatiles during subduction reduces melting point of overlying asthenosphere → volcanic arcs are associated with all subduction zones 

• Transfer of heat from mantle magma to the overlying lower crust

Dike

cuts across layers; magma that got stuck

Sill

pushes between layers; parallel to layers; weak boundaries

Xenolith

piece of wall rock that falls into magma (eventually igneous rock); occurs along the border of a pluton (border cools/solidifies faster + interior destroys/moves inclusions)

Viscosity

a fluid’s resistance to flow

Low-viscosity magma

runny; gases escape easily as bubbles; effusive; lava flows (sheets); shield volcanoes

High-viscosity magma

thick/sticky; gases get trapped = pressure builds; explosive; pyroclastics (loose igneous material); stratovolcanoes

Factors that control viscosity

• Temperature → high temperature decreases viscosity 

• Gas/volatile content → high content increases viscosity

• Crystal content → high content increases viscosity

• Chemical composition → high SiO₂ content increases viscosity

Pyroclastics are somewhat similar to sediments (loose material)

upon consolidation becomes a pyroclastic rock (e.g., tuff)

Some sequences of basaltic flows or pyroclastic rocks are distinctly layered

resemble sedimentary rocks (from afar)

Types of volcanic structures → Crater eruption

• crater = depression containing the central volcanic conduit, connecting to the magma chamber (e.g., Mt. Vesuvius (Italy))

  • Convergent boundary/subduction zone → magma builds steep mountains with a conduit

Types of volcanic structures → Fissure eruption

• conduit is a fissure → pillow basalts (at MORs) and/or flood basalts (e.g., Reykjanes Peninsula (Iceland))

  • Divergent boundary/MOR → crust pulled apart under tension; runny basalt through cracks

Fissure eruption → Pillow basalts

• subaqueous eruptions 

  • Divergent boundary/MOR

Fissure eruption → Flood basalts

• subaerial eruptions covering vast areas; repeated lava flows over prolonged period of time

  • Hotspots/mantle plume

Types of volcanic structures → Shield volcano

• large with gentle slopes; low viscosity (basaltic); effusive; layered, runny lava flows (e.g., Mauna Loa, Hawaii)

  • Hotspot → moving oceanic plate

Types of volcanic structures → Stratovolcano

• low/high viscosity; effusive/explosive; layers of lava flows & pyroclastic ash (e.g., Mt. Fuji (Japan), Mt. Vesuvius (Italy))

  • Convergent boundary/subduction zone

Pyroclastic flows

ash cloud; ash, pumice, rock fragments → column collapse of explosive eruption; high temperature at high speeds

Lahar

volcanic debris flow or mud flow, volcanic debris + water → intense rainfall on loose ash or volcano melting snow/glacier; acts like liquid/solid concrete

Volatiles

H₂O (steam), CO₂, SO₂, and H₂S

Atmospheric cooling

• Sulfate aerosols → explosive eruptions blast SO₂ into stratosphere; gas + water vapor = sulfuric acid; lingers for 1-3 years (e.g., Mt. Pinatubo) 

• Volcanic ash → dust & ash block sunlight; few days to weeks (e.g., Mt. Tambora)

Atmospheric warming

CO₂ → volcanoes release CO₂ (greenhouse gas); stays for centuries

Cooling periods after major volcanic eruptions

documented in tree rings, ice layers, fossil shells, extinctions, etc.

Volcanic hazards → Pyroclastic flows

200 km; ash cloud; ash, pumice, rock fragments → column collapse of explosive eruption; high temperature at high speeds

Volcanic hazards → Lahar

150 km; volcanic debris flow or mud flow, volcanic debris + water → intense rainfall on loose ash or volcano melting snow/glacier; acts like liquid/solid concrete

Volcanic hazards → Lava flows

50 km; streams of molten rock; destroys all

Volcanic hazards → Volcanic ash

2500 km; pulverized rock, glass shards, and pumice blasted into the air; drifts with wind over vast areas

Limnic eruptions

CO₂ eruption in lake → CO₂ is heavier than air and suffocates animals & humans

79 CE

Mt. Vesuvius eruption destroys Pompeii, Rome → stratovolcano (high viscosity) → eruption column of ash, pumice, and volcanic debris → wind blew volcanic ash over Pompeii → accumulation of ash collapsed buildings → series of pyroclastic flows (moving hundreds of miles per hour) → increased temperature & toxic gases

Benefit of volcanic eruptions

fertile soils → intensive agriculture

Weathering

weakens rocks → loose fragments

Physical weathering

• breaking a rock into smaller fragments (clasts) without changing its mineral/chemical composition

  • Fractures rocks = increase surface area

  • Cold, moist climates

Physical weathering → Frost wedging

water freezes in cracks and expands

Physical weathering → Root wedging

plant roots force cracks open

Chemical weathering

• chemical decomposition of minerals and rocks

  • Requires water (or humidity in the air); some require oxygen

  • Weakens rock’s crystalline structure 

  • Warm, wet climates

Chemical weathering → Dissolution

acidic rainwater dissolving minerals like calcite

Chemical weathering → Oxidation

oxygen reacting with iron-rich minerals to create rust/hematite

Chemical weathering influenced by

• Composition of rock and constituent minerals

• Presence or absence of water

• Composition of the water (rain)

• CO2 content of atmosphere

• Climatic conditions

• Plant life

• Time

Chemical weathering most intense in the tropics

high temperatures (increase rate of chemical reactions), rainfall (adding acidic rainwater), and dense rainforests (abundant organic material)

Chemical weathering weakest at poles or high altitudes

lack of water (trapped in ice), extreme cold (decreased reaction rates), and lack of soil/organic acids (no decaying organic matter)

Selective weathering

weathering depends on rock types → not a homogeneous process

Rocks that are being weathered were formed under different conditions

they and their components (i.e., minerals) are less stable when exposed to water and air at the Earth’s surface

Regolith

inorganic materials

Soil

unconsolidated inorganic and organic material → supports plants and their root systems

Erosion

• transport of material away from place of origin

  • Rock fragments and soil are removed from their original site and moved elsewhere by gravity and/or by mobile transport agents (wind, water, ice)

Transport changes the properties of original materials

• Progressively smaller (pebbles → sand → silt → mud)

• More similar in size

• Progressively more rounded

• Increasingly monomineralic → materials are not resistant enough to survive

Sediment

unconsolidated; loose; broken down pieces of rocks, minerals, or organic matter

Sedimentary rock

consolidated/cemented/lithified; compacted under intense pressure; solid, cohesive rock

Sediment → rock counterparts

• Gravel/pebbles/cobbles → conglomerate

• Sand → sandstone

• Mud/clay/slit → mudrock/shale

Clastic sedimentary rocks

• cemented rock fragments (clasts)

  • E.g., sandstone, conglomerate

Biochemical sedimentary rocks

• cemented shells or shell fragments of organisms

  • E.g., shell-rich limestone, coral-rich limestone

Organic sedimentary rocks

• cemented carbon-rich relics of organisms

  • E.g., peat → coal

Chemical sedimentary rocks

• precipitated from solutions

  • Water evaporates → concentrations of dissolved ions in water increase until water is saturated with respect to certain minerals → minerals precipitate/crystallize

  •  E.g., halite, gypsum (i.e., evaporites)

River flows fast →

high energy → carries large sediment load → slows down → coarse/heavy fraction (e.g., sand) is dumped (e.g., in a delta) & fine fraction (e.g., clay) carried far → mud in abyssal plains & pebbles/sand in river mouth (i.e., delta)

Bedding/layering

determination of ancient depositional environments

Bedding/layering → Shore environment

• deltas form when the velocity of a river slows down as it enters a lake/ocean

  • Coarse fraction dropped → main body of delta

  • Fine fraction carried into ocean → abyssal plains 

  • Symmetrical ripple marks → fossil ripple marks (perfectly symmetrical) → shaped by gentle sloshing motion (not one-direction river current)

Bedding/layering → Reef environment

• Fore-reef (steep slope facing ocean) → powerful ocean waves → coarse fraction 

  • Tells us exactly where the open ocean was located millions of years ago relative to the coastline 

• Back-reef → fine fraction (e.g., muds, shell fragments)

Bedding/layering → Continental margin

• steep drop-off beyond continental shelf

  • Underwater canyons → narrow & funnel-shaped → easily transport high volumes of material from continental shelf to abyssal plains

    • Turbidity currents (storm currents) → sediment accumulates on top of canyons until unstable → rushes down canyon as violent underwater landslides → spill onto ocean floor

Bedding/layering → Deep-marine environment

• no coarse clastic sediments; only microscopic organisms

  • Plankton shells (silica or carbonate) → accumulation & lithification → chalk

Metamorphism

• solid-state transformation of a protolith → no melting

  • Temperature highest → partial melting → magma

Metamorphic changes

change in mineral content of the protolith & change in texture (i.e., size and shape of minerals, arrangement in three dimensions, deconstruction of fossils)

Chemical composition of protolith determines

the type of minerals that will be present after its metamorphism

Confining pressure

• identical in all directions

  • Created by plate tectonic movements → continent-continent collision → compresses rocks in one direction → shape of object must change

  • Low compression (P) → low density of minerals

Directed pressure

• stress

  • Existing minerals/clasts align themselves in one plane → foliation → sedimentary layers fold

    • Foliation = perpendicular to stress → minerals naturally rotate, grow, and deform along the path of least resistance to relieve the intense squeezing force

  • Change in volume & shape → change in density

Fluids generally speed up the growth of new minerals during metamorphism because

the transport of the “nutrients” (i.e., cations, anions) required for mineral growth is faster if they are carried by fluids rather than via diffusion through solids

Types of metamorphism → Subduction zone

• high pressure, low temperature (e.g., blueschist)

  • Oceanic plate plunges into mantle

Types of metamorphism → Mountain-belt

• high pressure, high temperature (e.g., greenschist)

  • Convergent plate boundaries → continent-continent

Types of metamorphism → Contact metamorphism

• low stress, high temperature → no foliation

  • Heat transfer from hot melt/magma (pluton) to surrounding rocks → metamorphic zone = aureole

Types of deformation → Displacement

• rock changes location

  • Vertical displacement → normal fault

  • Horizontal displacement → strike-slip fault (i.e., transform plate boundary)

Types of deformation → Rotation

rocks change orientation → tilted sedimentary layers

Types of deformation → Distortion

• rock changes shape → folding from compression

  • Dykes → compressed vertically

Brittle deformation

• fractures, cracks, shatters; creates faults & fractures; high earthquake potential

  • Cold rocks = rigid → snap under stress → shallow crust

  • Surface → low pressure → rock fractures

  • Sudden stress → rock shatters

  • Rock made of hard/rigid minerals (e.g., quartz, feldspar) → brittle

Plastic deformation

• stretches, bends, flows smoothly; creates folds (i.e., anticlines/synclines); low earthquake potential

  • High temperature + high confining pressure

  • Hot rocks = high thermal energy → bonds break easily → atoms reform/slide past each other → plastic → deep in Earth

  • Deep in Earth → high pressure → rock squeezed tightly → prevents rock cracking

  • Slowly applied stress (e.g., tectonic plate movement) → rock adjusts & bends

  • Rocks made of soft/sheared minerals (e.g., clay, halite) → plastic

Fold components → Hinge line/fold axis

line of maximum curvature along folded bed (i.e., ridge of crest or bottom of trough)

Fold components → Limbs

curved sides of fold → one fold = two limbs

Fold components → Axial plane

imaginary flat surface connecting all hinge lines; cuts fold in half

Fold shapes → Anticline

arch-like; rock layers bend upward/tilted away from each other; oldest rocks exposed in center

Fold shapes → Syncline

trough-like; rock layers buckle downward/tilted towards each other; youngest rocks exposed in center

Fold symmetry → Symmetrical fold

axial plane is vertical; limbs dip away at the same angle

Fold symmetry → Asymmetrical fold

axial plane is tilted; one limb dips more steeply than the other; stronger compression from one direction

Plunging fold

hinge line tilted downward; V-shaped pattern across landscape

Compression

• inward squeeze; shortens & thickens rock layers; convergent boundaries (e.g., colliding continents, subduction zones); reverse & thrust faults; folds (i.e., anticlines/synclines)

  • Directed pressure → anticlines & synclines

Thrust fault

compression; moves up; low angle; shortens crust; convergent boundaries

Extension

outward pulling; lengthens & thins rock; divergent boundaries (e.g., MORs, continental rifts); normal faults

Normal fault

extension; moves down; high angle; lengthens; divergent boundaries

Fold-and-thrust belt

• highly deformed network of folds and low-angle thrust faults

  • Crust subjected to intense horizontal squeezing → upper layers of rock buckle into wavy folds & break along angled fractures → shortens & thickens continental crust → massive mountain ranges 

  • E.g., continent-continent collisions → collisional orogen (orogen = highly deformed structural zone that forms during mountain-building) → fold-and-thrust belts + mountain belt + crust thickens