GEO 401 Midterm 2 | UT Austin

Metamorphic rock

Original mineralogy and/or texture changed by P and T WITHOUT melting

Does bulk chemistry change during metamorphism?

No, unless fluids add/remove components (metasomatism)

Geothermal gradient

~30°C per km

Confining vs directed pressure

Confining = equal in all directions; directed = one direction, causes folding and mineral alignment (foliation)

Sandstone metamorphoses to

Quartzite (texture change only)

Limestone metamorphoses to

Marble (texture change only; both calcite)

Granite metamorphoses to

Gneiss (texture change only)

Shale metamorphoses to

Slate → phyllite → schist; texture AND mineralogy change (clays → mica, garnet, staurolite)

Foliation

Parallel planes from planar minerals aligned under directed pressure; NOT bedding

Foliated sequence, low → high grade

Slate (fine, cleavage) → phyllite (mica sheen) → schist (visible micas) → gneiss (banded) → migmatite (partly melted)

Nonfoliated (granoblastic) rocks

Equant grains: quartzite, marble, hornfels

Contact metamorphism

Heat from an intrusion; small scale; high T, low P (hornfels)

Regional metamorphism

Heat + pressure over mountain belts from plate collision; most metamorphic rocks

Metamorphic facies

Rocks grouped by the P-T conditions they formed under; each has index minerals and a tectonic setting

Zeolite facies

Lowest P and T

Greenschist facies

Low-moderate T (~300-500°C)

Amphibolite facies

Moderate-high T (~500-700°C)

Granulite facies

Highest T (~700°C+)

Blueschist facies

HIGH P, LOW T; subduction zones

Eclogite facies

Very high P, high T; deep subduction

Shale index minerals, low → high grade

Chlorite → muscovite → biotite → garnet → staurolite → kyanite → sillimanite

Kyanite / andalusite / sillimanite

Same chemistry (Al2SiO5): kyanite = high P, andalusite = low P, sillimanite = high T

Prograde vs retrograde

Prograde = P and T increasing; retrograde = decreasing (exhumation)

Tension / compression / shear

Divergent → normal faults; convergent → reverse/thrust faults + folds; transform → strike-slip faults

Brittle conditions

Low P, low T, fast strain rate, dry; shallow crust (<~15 km); fractures, faults, breccia

Ductile conditions

High P, high T, slow strain rate, wet; deep crust (>~15 km); folds, mylonite

Strike vs dip

Strike = direction of bed's intersection with horizontal; dip = angle below horizontal

Joint vs fault

Joint = fracture with no offset; fault = fracture with offset

Normal fault

Hanging wall DOWN; FUN (Footwall Up Normal); tension, rifts

Reverse fault

Hanging wall UP; FDR (Footwall Down Reverse); compression; older over younger

Thrust fault

Reverse fault dipping <45°; big lateral displacement; repeated sequences

San Andreas fault

Right-lateral strike-slip (transform)

Anticline

Arch; OLDEST rocks in core

Syncline

Trough; YOUNGEST rocks in core

Dome vs basin

Dome: beds dip away, oldest in center; basin: beds dip inward, youngest in center

Superposition

Younger on top of older

Original horizontality

Sediments deposited flat

Cross-cutting relationships

Whatever cuts a rock is younger than it

Inclusions

Fragments are older than the rock enclosing them

Faunal sequence

Fossils appear in a definite order

Angular unconformity

Flat younger beds on eroded tilted/folded older beds

Disconformity

Erosion surface between PARALLEL sedimentary beds

Nonconformity

Sedimentary rock on eroded igneous or metamorphic rock

Kelvin's age of Earth

20-40 Myr from cooling; wrong because he didn't know about radioactivity

Patterson (1956)

U-Pb in meteorites → Earth is 4.56 billion years old

Radioactive vs radiogenic isotope

Radioactive = parent that decays; radiogenic = daughter that accumulates

Half-life

Time for half the parent atoms to decay to daughter

U-238 / U-235 → ?

Pb-206 / Pb-207; date zircon

K-40 → ?

Ar-40; half-life 1.3 Gyr; micas, hornblende

Rb-87 → ?

Sr-87; half-life 47 Gyr

C-14 → ?

N-14; half-life 5730 yr; only up to ~70,000 yr; organic material

Eras of the Phanerozoic

Paleozoic → Mesozoic (251 Ma) → Cenozoic (65 Ma)

Isostasy

Gravitational balance setting elevation; depends on density and thickness (P = ρgh)

Two ways continents grow

Magmatic addition (subduction-zone magma) and accretion (attaching arcs, fragments)

Orogeny vs epeirogeny

Orogeny = mountain building with folding/faulting/metamorphism; epeirogeny = broad up/down warping without folding/faulting

Grenville orogeny

1.3-1.0 Ga; assembly of Rodinia

Taconic orogeny

~450 Ma; island arc collision; first Appalachian phase

Caledonian / Acadian orogenies

~400 Ma; Laurentia + Baltica collide

Variscan orogeny

~340 Ma; Gondwana begins colliding with Laurussia (Europe)

Alleghenian orogeny

~300 Ma; final Appalachians as Gondwana and Laurussia collide

Pangea complete

~270 Ma (Early Permian)

Alpine-Himalayan orogeny

Started with Pangea breakup; still active today

Wilson cycle

Rift → ocean opens → subduction → ocean closes → collision → repeat

Craton

Stable, old, flat continental interior; survives due to thick cold lithospheric keel

Basaltic lava

Mafic, low viscosity, 1000-1200°C, rarely explosive; MORs and hot spots (Hawaii)

Andesitic lava

Intermediate; Mt. St. Helens, Krakatoa

Rhyolitic lava

Felsic (>68% Si), high viscosity, 600-800°C, most explosive; Yellowstone

Pahoehoe vs aa

Pahoehoe = fluid, ropy, gas-rich; aa = more viscous, cooler, rubbly

Shield volcano

Broad, thin basalt flows; Mauna Loa

Stratovolcano

Alternating pyroclastics and lava; Vesuvius, St. Helens

Caldera

Large depression from roof collapse into emptied magma chamber; Crater Lake, Santorini

Diatreme

Explosive breccia-filled pipe; Shiprock, NM

LIP (flood basalt)

Huge, rapid mafic eruptions, often >1 million km³ in a few Myr; from plume head

Pyroclastic flow

Hot ash + gas cloud racing downslope; Mt. Pelée 1902 (>25,000 dead)

Lahar

Volcanic mudflow; Nevado del Ruiz 1985, Armero (>25,000 dead)

Flank collapse

Side of volcano fails; Mt. St. Helens 1980

Gas hazard

CO2 suffocation; Lake Nyos, Cameroon

Volcanic tsunami

Krakatoa 1883 (>40,000 dead)

Tambora 1815

Largest recorded eruption; "year without a summer"

Pinatubo 1991

Dust cloud circled globe; ~0.5°C global cooling in 1992