Deep time: How old is old

Week 4 [part 2]

Geologic time

Earth has history → billions yrs old + discovering it way major discovery for human → changed our perception of time and Universe

→ provides a frame of reference for understanding rock, fossils, geologic, structure, landscapes, tectonic events…

→ deep time = immense span of geologic time + concept is so vast, difficult to grasp → human history «« geological history

James Hutton

Scottish physician & framer

→ called father of modern geology

→ first to articulate the “principle of uniformitarianism”

James Hutton’s principle

“The present is the key to the past”

→ geologic change = slow, same processes as those in past

Dating geological materials

→ relative ages = based upon order of formation → qualitative method hundred yrs old + permit determination of older vs. younger relationships

→ numerical ages = actual number of yrs since an event → quantitative method not so old

Physical principles - origins

Principles of geology - Lyell (1830-33)

→ principles for deciphering Earth history + used to establish relative ages of earth material

Principle of uniformitarianism

processes observed today were same in the past

mudcracks in old sediments formed like mudcracks today

physical principles

→ allow us to sort out relative ages + is possibleeven in complex situations

- principle of original horizontally

- principle of superposition

- principle of lateral continuity

-principle of cross-cutting relations

-principle of baked contacts

-principle of inclusions

principle of original horizontally

sed settle out of fluid by gravity → causes sed to accumulate horizontally ( not favored on a slope) → tilted sed rocks must be deformed

principle of superposition

is an underformed sequence of lqyered rocks → each bed is older than the one above and younger than one below

→ younger strata on top and older at bottom

principle of lateral continuity

strata often form laterraly extensive horizontal sheets → subsequent erosion dissects once-continous layers => lateral continuity

principle of cross-cutting relations

younger features cut across (truncate) older features → faults/ dikes / erosion must be younger than material that is cut through

principles of baked contact

igneous intrusion cooks the invaded country rocks → baked rock must have been there first (bc older)

principles of inclusions

= rock fragmet within another

inclusions are always older than the enclosing material

→ weathering rubble must have come from the older rock + fragments (xenolith) older than igneous intrusion

Principle of Fossil Succession

fossils = often preserved in sed. rocks + are time markers useful for relative age-dating

→ fossils speak of past depositional env & specific fossil only found within limited time span

+ fossils correlate strata locally, regionally and globally

fossil range

the first and last appearance

→ each fossil ha unique range & range overlap narrows time

Index fossils are diagnostic of particular geologic time

Uncomformities

= time gap in the rock record → from erosion / nondeposition

→ correlations allow us to interpolate through uncomformities

3 types of unconformities: angular, noncomformity, and disconformity

Angular unconformity

represents a huge gulf in time

→ horizontal marine sed deformed by orogenesis / mountains eroded completly away / renewed marine invasion and new sed deposited

s/o Siccar Point, Scotland = common destination for geologists for angular unconfor.

Nonconformities

= igneous/metamorphic rocks capped by sed. rocks

→ igneous/metamorphic rocks exposed by erosion + sed was deposited on the eroded surface

Disconformities

= parallel strata bounding nondeposition

→ due to interruption in sedimentation: → pause in deposition/ sea level falls & rises/ erosion

→ often hard to recognize

Correlating formations

Earth history in sed. strata

→ ex: Grand Canyon => thick layers → formations can be correlated over long distances

→stratigraphic column describes sequence of strata → can be traced over long distances

Stratigraphic correlation

Lithologic correlation (=based on rock types) = regional

→ sequence = relative order in which rocks occur

+marker beds have unique characteristics to aid correlation

→ fossils correlation = based on fossils within rocks → applicable to much broader areas

Geologic Maps

Strata can be matched across distances (s/o William “strata” Smith)

→ similar rock types in a similar order + rock layers contained the same distinctive fossils

The Geologic Column

→ composite stratigraphic column can be constructed (assembled from incomplete sections across the globe + brackets almost all Earth history)

→ composite column divided into time blocks

Geologic Time Scale

Eons → largest subdivision of time (100’s to 1000’s of Ma) → major glob. environment changes

Eras → subdivisons of an eon 65 to 100’s of Ma

Periods → subdivions of an era 2-70 Ma

Epochs → subdivison of a period 0.011 to 22 Ma

based on: nature of life (“zoic” means of life) + characteristic of the time period + specific locality

Name of the Eons

Hadean -”hell” → 4.6-4.28 Ga

Archean -”ancient-begining” → 4.28-2.5 Ga

oxygen abondance in the atmoshpere

Proterozoic -”before-early life” → 2.5-0.542 Ga

first shell fossiles in between two periods

Phanerozoic -”visible life 542-0 Ma

Namess of the Eras

Paleozoic -”ancient life”

Mesozioic -”middle life”

Cenozoic -”recent life”

Geologic time and life

life appeared ~3.8-4.0 Ga on Earth - early life = single-celled organisms

O2 from cyanobacteria built up in atmosphere by 2 Ga

→ ~700 Ma: mutlicellular life evolved

→ ~542 Ma: 1st appearance of invertebrates → (shells increased fossils preservation) + life diverified as the ‘Cambrain Explosion”

Phanerozoic

Palzeozoic → Mesozoic (dinosaur → ended with dinosaur extinction) → Cenozoic (most recent period)

permian → everyone died at the end possibly because of big eruption in what is now Siberia

Stratigraphic correlation

Numerical age

→ give age of rocks in years

→ based on radioactive decay of atoms in minerals

→ study of age of rocks = geochronology

Radioactive decay

→ progresses along a decay chain

decay creates new unstable elements that also decay → goes on until stable element endpoint

Isotopic dating

age of a rock can be determined by:

→ ratio of of parent to daughter isotopes

→ calculating amount of time by using known half life

BUT isotopic dating = time consuming and expensive

Uranium lead-dating

= most trusted decay system for ancient rocks

[s/o 235U ; 238U → 207Pb and 206Pb]

U parents and Pb daughters behave almost identically and forms minerals

→ two clocks ticking within zircon and one checks the other

→ then we plot Concordia diagram

Concordia diagram

ratios 207Pb/235U and 206Pb/238U according to the amounts of each isotope measured by mass-spectroscopy and then you plot one ratio against the other

Isotope Date Mean

→ gives the time a mineral cooled below its “closure temperature” → cooling of magma of lava to solid, OR metamorphic rock temperatures drop below closure temp

/!\ sedimentarty rocks cannot be directly dated

Other Numerical ages

→ growth rings (annual layers for tress or shells)

→ rhythmic layering (annual layer in sediments or ice!)

Numerical Ages and Geologic Time

geochrology = less useful for sedimentary deposits

→ sediments ages can be bracketed by numerical ages (→ data adjacent ignous & metamophic rocks / apply principle of cross-cuting)

Age of Earth

before radiometric dating → age estimated widely (s/o Lord Kelvin)

after radioactivity discovered → estimation = much more precise

Acasta Gneiss dates to 4.03 Ga (oldest rocks found on Earth) BUT Nuvuaggituq faux amphibolites is belived to be older (4.28 Ga?)

Zircons in ancient sandstones date 4.4 Ga and so age of Earth baed on correlation = 4.57 Ga