Geological Time Flashcards
age of earth and its oldest materials/rocks
- earth: 4.5 billion years old
- oldest material found is 4.4 billion years old -> zircon crystals from conglomerate
- Oldest rock 4.031 billion years old -> from NWT
- Can’t find rocks as old as earth due to weathering (and possibly plate tectonics?)
inaccurate estimates for earth’s age
- Archbishop James Usher worked backwards through scriptures and records to figure out when he thought the earth was created: October 23rd, 4004 BC
- Uniformitarians: measured rate of present processes to figure out how long it would take to build up earth’s crust to present form: est. Millions of years
- Lord Kelvin: more scientific approach than Usher; calculated rate of cooling from molten body the size of earth, but forgot that Earth isn’t completely cool: 20-40 million years old
evidence for age of earth
- Some things are older, some are younger:
- Oldest to youngest (in billion years):
1) Meteorites (4.55)
2) Earth (4.54)
3) Moon (4.53)
4) Zircon (4.44) -> oldest material from earth
5) Acasta gneiss (4.03) -> oldest rock on earth
relative age dating
- science of determining relative order of events without providing a specific age of features involved
- includes:
- law of superposition
- principle of original horizontality
- principle of cross-cutting relationships
- law of inclusions
- unconformities
- principle of faunal succession
law of superposition
- In an undeformed sequence of sedimentary rocks, the oldest rocks are at the bottom, and youngest at the top
- Also applies to lava flows and ash beds
principle of original horizontality
- Layers of sediment are generally deposited horizontally
- If layers are found otherwise, something happened to them during deposition
principle of cross-cutting relationships
If a rock unit (or fault) cuts other layers/units, the rock unit that cuts must be younger, and the layers that are cut must be older
law of inclusions
- If you see xenoliths (rocks broken off and melted into granite), the granite must be younger than the xenoliths
- If you see sandstone with granite, the granite is older than the sandstone -> it had to be weathered to get in there
2 types of contact surfaces between rocks
1) Parallel contacts are said to be conformable
2) Unconformable where contact is not parallel
principle of faunal succession
- Organisms have evolved through time and certain time periods can be recognized based on their fossil content
- Ex. Ammonite is always found in older rocks than triobite
unconformities where contact is not parallel
- Occur when a lot of time passes before deposition of next layer
- Represent a gap in rock record/geological time -> a time of erosion, not deposition
- Separate younger rocks from much older rocks
- 2 types:
1) Disconformity: erosion, but no tilting (rocks are horizontal)
2) Angular unconformity: erosion and tilting (rocks below erosion surface are tilted)
types of absolute age dating
- dendrochronology
- ice cores
- radioactive dating
dendrochronology
yearly growth rings on trees -> can use overlapping records to go back 14,000 years (but not useful to geologists)
ice cores
can be used in a similar way as dendrochronology, ice/snow deposit layers -> each fall of snow recording the years like the rings in a tree, can go back thousands of years (still not useful for geologists)
radioactive dating
- Provides numeric ages – specifying the actual number of years that have passed since an event occurred (aka: absolute age dating)
- Using decay products of radioactive elements in minerals to get at absolute ages
- Takes isotopes into account (variants of same parent atom -> differ in number of neutrons)
- Some isotopes are unstable and radioactively decay -> we use this
- can date igneous rocks (time magma crystallized) and metamorphic rocks (the time of its metamorphism, not age of parent rock)
