Geology 1 MT2: stress/strain, geochronology

stress

• compression => <= (squish)
• tension <= => (stretch)
• shear ^v (twist)

strain

• elastic
• plastic
• brittle

folds

anticline

syncline

fault

slickensides

attitude

strike

dip

strike-slip fault

dip-slip fault

• hanging wall
• foot wall

• compression occurs at the bend
• leads to reverse faults and mountains

a basin will form due to normal faults

sediments settle out and are deposited horizontally

• all beds will continue in all directions equally until:
• their edges pinch out
• they reach the edge of their deposition basin

• rocks and seds are generally deposited on top of other deposits
• without perturbation, younger rocks should be found above older

any intrusive igneous body must be younger than the rocks it cuts across

fragments of other rocks found in the host rock must be older than the host

a rock's maximum age is the same as the age of its youngest fossil

• Precambrian - origin of life; rise of oxygen in atmosphere
• Paleozoic - age of fish; Pangaea assembled
• Mesozoic - age of reptile; dinosaurs; Pangaea broke up
• Cenozoic - age of mammals;

numerical method involving the radioactive decay of elements

• alpha decay
• beta decay
• electron capture

• nucleus emits an alpha particle (2p⁺ + 2n)
• alpha particle is a helium nucleus
• atomic # down by 2
• atomic mass down by 4

• nucleus emits a negatron (massless negative charge)
• neutron becomes a proton
• atomic number up by 1
• atomic mass unchanged

• an electron is incorporated into the nucleus
• proton becomes a neutron
• atomic number down by 1
• atomic mass unchanged

original radioactive isotope

product of radioactive decay

the time it takes for 1/2 of the radioactive parents to become daughters

• 1. count the number of parents and daughters
• 2. add the numbers together to get the starting number of parents
• 3. divide your number of parents in half repeatedly until you get down to the number of parents in step 1
• 4. count number of half-lives and multiply by the duration of the half-life
• 5. that's the age

the splitting of a nucleus -- Ex: alpha decay

• if we know the half-life we can determine the amount of time that will pass between decays
• to get the age, we multiply the number of tracks by the time that passes between fission events

• radioactive isotopes created by nuclear reactions between atoms and cosmic rays (the sun)
• useful for dating ice, sediments, exposures, etc.

• alternating layers of seds that develop seasonally
• summer - high deposition due to river input
• winter - low deposition (only fines) due to lack of river input

tree rings

lichen: symbiotic organism that is fungus/algae

combination of numerical and relative dating techniques in order to determine the min/max ages of rocks and geologic features that we cannot directly date

a gap in the rock record due to erosional events

• nonconformity - unaltered sedimentary rocks on top of intrusive igneous or highly metamorphosed rocks
• angular unconformity - horizontal sed beds on top of tilted sed beds
• disconformity - young sed rocks on top of old sed rocks with erosion found between them

determining equivalency in age for geographically distant rocks