stratigraphy
How do we understand what different stratum look like for historical dating, etc.,
uniformtarianism
“the past history of our globe must be explained by what seems to be happening right now.”
- an unconformity: a gap in the stratographic record
- if you look at the layers of the earth, if the lower older layers is not flat but the top layers are flat, then the lower layers have been squeezed / deformed in time
basic principles
- superposition: lower is older
- original horizontal: original is flat, tilt is later
- original continuity: if there’s a hole, it was a block before and something cut it
- cross-cutting relationships: if something cuts another thing, the thing that cuts it must be younger
unconformities
if there is some point where there’s a “sharp turn”, we call that an unconformity.
- *disconformity: edges between rocks not matching
- angular unconformity: angles don’t match
- non-conformity: young sedimentary rock above old igneous rock; latter is much loter, so there’s a gap
How to put years on things
Biostratigraphy
Fossils and their use in dating rock formations: trilobites in 2 places can be used to correlate as existing during the same period of time.
(Use fossils, their appearance / disappearance, to build geological timescale.)
fossil
Types:
- The preserved remains of an organism.
- Casts: an impression of the organism gets filled with sediment and then deposited
Note: rarity of fossil makes things seems rarer than it perhaps actually is.
“Golden Spike”: body fossil can give us exact age, and thus
absolute dating
Use radio dating!
- carbon: time to short, not in all rocks, only in biology
- zircon: Zircon is a minor mineral, in most Felic rocks; it lock Uranium in, but it doesn’t allow lead in. So the resulting lead must be from uranium decay.
Basic logistics is to speed it really far and see how much it curves: this measures the weight of the material.
texture words
- phaneritic: visible mineral crystals
- aphanitic: not visible mineral crystals
- porphyritic: intrusive holse