Study guide: Sedimentary and metamorphic rocks and geologic time

(See also Pat's basic geology study guide

Weathering

destructive process at or near the Earth’s surface where rocks are broken down either mechanically or chemically without erosion; the end products are particles (clasts) and solutions

 

Physical weathering

is the mechanical breakdown or disintegration of rocks in place that results in clasts or fragments:

 

The main mechanisms for physical weathering are…

insolation

freezing of water

unloading

salt crystal formation

root wedging

 

Chemical Weathering

is the chemical breakdown of rocks that results in solutions

 

The main mechanisms for chemical weathering are…

oxidation

hydrolysis

dissolution

hydration

biochemical weathering

 

Sedimentary rocks

form when particles (clasts) and/or solutions that result from weathering are eroded, transported, deposited, and lithified.

lithification is result of cementation and compaction

main cements: oxides (commonly reddish), calcite (commonly effervesces), or silica

 

Environments of deposition

 are places where sediment can accumulate; main types: terrestrial, marine, nearshore marine

other types: lacustrine (lakes);

 

higher energy (big clasts) vs. low energy (fine particles settling out of suspension)

 

terrestrial includes alluvial fans, rivers, dunes caves, peat bogs, (harder to preserve terrestrial ones owing to erosion)

marine and nearshore marine include reefs, abyssal ocean, continental slope and fan, beach, delta, turbidity currents, and nearshore

 

 

Clastic sedimentary rocks (aka detrital sedimentary rocks)

Made of particles or clasts (detritus); includes such rocks as conglomerate (w/ rounded gravel and larger clasts), breccia (w/ angular larger clasts), sandstone, siltstone, shale, mudstone

know sediment name based on clast size: e.g. boulder > 256 mm; cobble 256–64 mm; pebble 64–2 mm; sand 2–1/16 mm; silt 1/16–1/256 mm; clay <1/256 mm;     gravel—general term for an assortment of larger particles regardless of size.

 

Chemical sedimentary rocks

Precipitated from solutions; include limestones (many types—see lab handout) gypsum, chert (deep sea silica), halite, diatomite

 

Organic sedimentary rocks

Includes coal and oil shale

Classification

rounded vs. angular

well-sorted vs. poorly sorted

size

color

composition

 

 

boulder >1/256 mm

 

 

Sedimentary structures/features

bedding

contact

stratum/strata

cross bed (How do these form? What can they tell us?)

ripple marks (assymentrical or current; symmetrical or oscillation)

graded beds

raindrop imprints

mudcracks

preferred orientation

tool markings

load structures

fossils

 

contacts

unconformities, angular unconformity

 

Metamorphic Rocks

form via recrystallization in the solid state owing to heat and/or pressure

 

Regional—form over huge areas where mountain building (convergent boundaries) causes thickening crust and/or faults that carry rocks down to depth and then back up.

 

Contact—form adjacent to hot intrusion, e.g. plutons (batholiths and stocks), dikes, sills, and laccoliths. Examples include hornfels

 

hydrothermal—form at mid-ocean ridges mainly

 

What are the geothermal gradient and pressure gradient?

parent rock or protolith

foliated (platy or rod_shaped crystals) vs. non-foliated (calcite or quartz of other equant-shaped mineral)

metamorphic grade, index minerals, metamorphic facies (ex: blueschist facies, the zeolite facies)

foliated metamorphick rocks, examples: slate, phyllite, schist, gneiss, migmatite

non-foliated metamorphic rocks: marble, greenstone, hornfels, quartzite

 

Web resources on metamorphic rocks

http://facstaff.gpc.edu/~pgore/geology/geo101/meta.htm

 

Geologic time

 

relative dating—simply ways to determine if rocks are older or younger than others; uses principles of stratigraphy (see below)

absolute dating—ways of using the radiometric clocks of unstable isotopes to date rocks or organic material: e.g. Uranium-lead dating; radiocarbon dating, half life, etc.

 

Principles of stratigraphy

original horizontality – tilted beds have been deformed by folding and/or faulting

superposition – what’s on top is younger

cross-cutting relationships – what cuts through is younger

inclusion – what’s picked up or included is older than what it’s in