Geol Exam 2 part 1

Metamorphic grade

the degree to which the parent rock changes during metamorphism

Recrystallization

minerals change in size and/or shape (changes texture of the rock). can occur due to pressure solution or plastic deformation

Pressure Solution

minerals partially dissolve then recrystallize in response to change in conditions

Plastic Deformation

mineral grains soften and deform into a new aligned orientation (foliation) due to temperature and pressure increase

Neocrystallization

the original minerals undergo a phase change or become unstable and change to form new minerals. Can created polymorphs or change chemical composition

Polymorphs

minerals with the same chemical composition, but different crystalline structure

What drives metamorphism?

Heat, pressure, differential stress, and chemically active fluids

Heat

most important agent for that drives metamorphism. 2 sources Geothermal gradient or contact metamorphism

Geothermal Gradient

An increase in temperature with depth

Confining pressure

Forces are applied equally in all directions. rocks become compact with greater densities.

mineral grains are compacted and overall volume is reduced.

confining pressure does not cause deformation

Compressional Stress

Shortened in one direction, elongated in the other direction. In high pressure and temperature environments rock are ductile and will stretch, flatten or fold.

Differential Stress

Forces are unequal in different directions. Stressors are greater in one direction

causes deformation by applying a strong force in one direction and a weaker force in another.

Chemically Active Fluids

Water becomes a hot ion-rich fluid- Hydrothermal solution. Enhances migration of ions, aids in recrystallization and neocrystallization of existing minerals.

Protolith

Parent rock

Parent Rock and the importance

metamorphic rocks have the same overall chemical composition as the original parent rock. This mineral makeup determines the degree to which each metamorphic agent will cause change.

Foliation

Any planar arrangement of mineral grains or structural features within a rock. Can form rotation of platy minerals, recrystallization, or flattening of spherical grains.

–Parallel alignment of platy and/or elongated minerals

–Parallel alignment of flattened mineral grains or pebbles

–Compositional banding of dark and light minerals

–Cleavage where rocks can be easily split into slabs

Rock or Slaty Cleavage

▪Rocks split into thin slabs

▪Develops in beds of shale with low-grade metamorphism

▪Example: Slate

Schistosity

▪Platy minerals are discernible with the unaided eye

–Mica and chlorite flakes begin to recrystallize into large muscovite and biotite crystals

▪Exhibit a planar or layered structure

▪Example: Schist

Gneissic Texture

During high-grade metamorphism, ion migration results in segregation of minerals into light and dark bands

Although foliated, gneisses do not split as easily as slates and schists

Example:

Gneiss

Non-foliated

metamorphic rocks without a layered or banded appearance. ▪Typically form in metamorphic environments where compressional stress is minimal

▪Parent rock will develop equidimensional crystals rather than flat or tabular-shaped crystals

Porphyroblastic textures

▪Unusually large grains, called porphyroblasts, are surrounded by a fine-grained matrix of other minerals

Contact/Thermal Metamorphism

–Results from a rise in temperature when magma invades a host rock

–Occurs in the upper crust (low pressure, high temperature)

–The zone of alteration (aureole) forms in the rock immediately surrounding the magma

–Aureoles consist of distinct zones of metamorphism

Hydrothermal Metamorphism

Chemical alteration caused by hot, ion-rich water circulating through pore spaces and rock fractures

Typically occurs along the axes of mid-ocean ridges

▪Black smokers are the result of the fluids gushing from the seafloor

Also occurs associated with hot springs and geysers

Burial Metamorphsim

–Associated with very thick sedimentary strata in a subsiding basin

▪Confining pressure and heat drive recrystallization

Subduction Zone Metamporphism

–Sediments and oceanic crust are subducted fast enough that pressure increases before temperature

▪Differential stress drives metamorphism

Regional Metamorphism

–Common, widespread type of metamorphism

–Produces the greatest quantity of metamorphic rock

–Associated with mountain building and the collision of continental blocks

–Crust is shortened, thickened, folded, and faulted

Metamorphism along Fault Zones

–Occurs at depth and high temperatures

–Pre-existing minerals deform by ductile flow

▪Minerals form a foliated or lineated appearance

▪Rocks formed in these regions are called mylonites

Impact Metamorphism

–Also called shock metamorphism

–Occurs when meteoroids strike Earth’s surface

▪Product of these impacts (called impactites) are fused fragmented rock plus glass-rich ejecta that resemble volcanic bombs

Stress

force per unit area that is exerted on rocks or other materials

Strain

deformation induced by stress

examples would be

• monocline

• reverse fault

• anticline

• thrust fault

Compressional Stress

Squeezes a rock (shortening); associated with convergent plate boundaries

Tensional Stress

pulls the rock apart; associated with divergent plate boundaries

Shear Stress

involved the movement of one part of a rock body relative to another; associated with transform plate boudaries

Elastic deformation

occurs when a shape temporarily responds to stress but return to its original shape when the stress is removed

Brittle Deformation

occurs when the rock is deformed beyond its ability to respond elastically, usually resulting in breaks or bends in the rock

Ductile Deformation

occurs when an object changes shape without breaking (clay is an example).

Factors that influence how a rock deforms

temperature, confining pressure, type of rock and time.

Temperature

When temperatures are high (deep in the Earth), rocks undergo ductile deformation. Near the surface rocks are more brittle.

Confining Pressure

makes rocks harder to break. Higher temperatures enhance ductile behavior and greater pressures help keep the rock intact, thus more likely to bend rather than fracture.

Rock Type

mineral composition and texture greatly influence how a rock responds to stress

▪Strong brittle rocks tend to break when stresses exceed their strength (typically igneous and some metamorphic rocks)

▪Weakly cemented sedimentary and foliated metamorphic rocks more readily show ductile deformation

Glacial ice will also deform easi

Time

Folded rocks of mountain belts show that tens of kilometers of compressional strain can be accommodated by ductile deformation

Faults

– a fracture in crustal rock involving displacement of rock on one side of the fracture with respect to the rock on the other side

THESE ARE THE RESULT OF BRITTLE DEFORMATION!!

Different kinds of faults

Normal, reverse and thrust faults, and strike-slip faults.

Oblique-slip faults

exhibit both a strike-slip and a dip-slip movement

Fault Scraps

Vertical displacements along faults may produce long low cliffs

Slickensides

–On some fault surfaces the rocks became highly polished and striated (grooved) as crustal blocks slid past each other

Joints

fractures in a rock where there has been no appreciable displacement

–One of the most common rock structures

–Most joints appear in parallel groups

–Produced when rocks in the outermost crust are deformed and experience brittle failure

Folds

are a series of wave-like undulations

–Most folds result from compressional stress

–Results in lateral shortening and vertical thickening of the crust

Hinge Line

imaginary axis that each rock layer is bent around. can be horizontal or inclined

Axial Plane

surface that connects all hinge lines of the folded strata

Anticlines

upfolfed or arched sedimentary layers. the oldest strata are in the center, and the youngest are towards the edge

Synclines

Downfolds or troughs of rock layers. youngest strata are in the center, while the oldest layers are on the edge.

Symmetrical

the limbs of the fold are mirror images of each other

Asymmetrical

the limbs of the fold are not identical. Overturned (recumbent) - one or both limbs are tilted beyond vertical. Plunging- the axis of the folds penetrates the ground

Domes

structures that occur when a broad upwarping of basement rock deforms the overlying sedimentary strata

▪Produce a circular or slightly elongated bulge

▪Oldest rocks are in the center

▪Can form due to intrusion of a laccolith

Basins

downwarped circular features

▪Youngest rocks are in the center

▪Can form from subsidence of large sedimentary basins

Monoclines

are large, steplike folds in otherwise horizontal sedimentary strata

▪Uniquely coupled with faults

▪As blocks of basement rocks are displaced upward, the ductile sedimentary strata drape over them

Geologic map

is a representation of Earth’s surface, as viewed from above, that shows the locations and orientations of the rock units that outcrop at the surface.

Block diagram

is a three-dimensional view of a portion of Earth’s crust that allows you to visualize rock layers at the surface and underground.

Orogeny

an episode of mountain building

Compressional Mountains

mountains that display faulted and folded rocks.

Orogenesis

–The process that collectively produces a mountain belt

▪An episode of mountain building is called an orogeny

–Mountains that display faulted and folded rocks are compressional mountains

▪Display visual evidence of compressional forces

▪Including metamorphism and some igneous activity

–Plate tectonics provides a model for orogenesis

▪Earth’s major mountains have formed along convergent plate boundaries

Volcanic Arcs

–The subducting slab partially melts the overlying mantle wedge

–Melt migrates upward through the overlying oceanic lithosphere and forms a growth called a volcanic island arc or island arc

–When the melt migrates through continental lithosphere, a continental volcanic arc is created

Deep Ocean Trenches

–Created when oceanic lithosphere bends as it descends into the mantle

–Trench depth is related to the age of the subducting lithosphere

▪Old lithosphere is cold and dense

–Plates subduct at a steep angle, producing a deep trench

▪Young lithosphere is warm and buoyant

–Plates subduct at a shallower angle and produce shallower trenches (if at all)

Forearc and Back-Arc Regions

–The forearc region is the area between the trench and the volcanic arc

–The back-arc region is located on the side of the volcanic arc opposite the trench

▪Both regions consist of accumulated pyroclastic material and eroded sediments

▪Tensional forces prevalent in these regions, causing stretching

Extension and Back-Arc Spreading

–Two plates converging, but not necessarily dominated by compressional forces

–When the subducting plate is cold, the plate sinks vertically as it descends along an angled path

▪This causes the trench to “roll back” away from the overlying plate

–Consequently, the overlying plate is stretched

–Tension and thinning may initiate seafloor spreading, enlarging the back-arc basin

Types of Subduction zones

Volcanic island arcs, and continental volcanic arcs

Volcanic island arcs

form when oceanic lithosphere subducts beneath oceanic lithosphere

Continental volcanic arcs

–form when oceanic lithosphere subducts beneath continental lithosphere

–If subduction continues long enough, ocean basins may close and two continents will collide

Island Arc-Type Mountain Building

–Results from the steady subduction of oceanic lithosphere

–Continued growth can result in topography consisting of parallel belts of igneous and metamorphic rocks

–Just one phase in the development of mountain belts

–Example: Japan

Andean Type Mountain Building

–Subduction beneath a continent rather than oceanic lithosphere

▪Associated with long-lasting magmatic activity and crustal thickening

–Exemplified by the Andes Mountains

▪Starts with a passive continental margin

–Thick platform of shallow-water sedimentary rocks

▪Eventually, the forces that drive plate tectonics change direction and a subduction zone forms

–Oceanic lithosphere must be dense enough to sink

Batholiths

Most magma crystallizes underground as massive plutons

Accretionary wedge

▪is chaotic accumulation of deformed and thrust-faulted sediments and scraps of ocean crust

Forearc Basin

The region of relatively undeformed layers of sediment and sedimentary rock

Alpine-Type Mountain

Continental Collisions –Named for the Alps—two continental masses collide

–The zone where two continents collide is called a suture

▪Typically contains slivers of oceanic lithosphere

▪May also include accreted terrane(s)

–Most compressional mountains exhibit the deformation of a thick sequence of sedimentary rocks called a fold-and-thrust belt

Suture

the zone where two continents collide

Escape tectonics

much of the remaining penetration into Asia caused lateral displacement of large blocks of the Asian crust

The Appalachians

–Of a similar origin to the mountains in the British Isles, Scandinavia, northwest Africa, and Greenland

–Formed from three main orogenic events that cumulated with the formation of Pangaea

▪formed from 3 major orogenies Taconic, Acadian, and Alleghanian Orogenies

Taconic Orogeny

▪Volcanic arc located east of North America was thrust over the continental block 450 million years ago

▪The volcanic rocks and marine sedimentary rocks were metamorphosed and are exposed in New York

Acadian Orogeny

▪Continued closing of the ocean basin resulted in a micro-continent colliding with North America 350 million years ago

▪Thrust faults, metamorphism, and granite intrusions are associated with this event

▪Substantially added to the width of North America

Alleghenian Orogeny

▪Africa collided with North America 250–300 million years ago

▪Material was displaced 250 km inland on North America

▪Pangaea began rifting 180 million years ago

–Rift was eastward of the suture, leaving a remnant of Africa welded to North America

Cordilleran-Type Mountains Building

–Associated with the Pacific Ocean

▪Highly likely that subduction zones will form island arcs which will eventually collide with a continental crust

–The collision and accretion of small slivers of continental crust form the mountainous regions that rim the Pacific

Terranes

accreted blocks of crust

Microcontinents

terranes

Fault-block mountains

continental rifting and can produce uplift and the formation of mountains

Delamination

results in the upwelling and lateral spreading of hot mantle rocks, producing tensional forces in the crust

Weathering

involves the physical breakdown and chemical alteration of rock at or near Earth’s surface

Mechanical weathering

(disintegration)—physical forces breaking rocks into smaller pieces. ex = frost wedging

Chemical Weathering

(decomposition)—chemical transformation of rock into one or more new compounds

Erosion

is the removal and transport of weathered rock by water, wind, or ice

Dissolution

– The process of dissolving into a homogeneous solution – Simple Solution ▪ Example: halite (common salt) – A small amount of acid in water increases the corrosive force of water, causing dissolution ▪ Carbonation – Carbonic acid is created when carbon dioxide dissolves in raindrops – Calcite is easily attacked by weakly acidic solutions – This process is responsible for the formation of limestone caverns

Carbonation

Acidic water dissolves carbonate rocks • Calcium Carbonate (Limestone) • Physical form: Granular Disintegration

Oxidation

Metals combine with oxygen Most Common: Fe • Physical form Granular disintegration

Hydrolysis

Chemical reaction between water & rock Most common in Granite By-product: Clay minerals Physical form Granular Disintegration

Soil

portion of the regolith that supports the growth of plants

Humus

organic matter in soil that is produced from the decomposition of plants and animals

Soil texture

refers to the proportions of different particle sizes ▪ Strongly influences the soil's ability to transmit and retain water and air

Soil Strucutres

▪ Platy, prismatic, blocky, and spheroidal ▪ Influences how easily the soil can be cultivated, how susceptible it is to erosion, porosity and permeability

Bioturbation

Earthworms and other burrowing animals mix mineral and organic portions of the soil,

Slope Orientation

important in soil formation. Southern-facing slopes in the Northern Hemisphere receive the most sunlight are optimal for soil formation