Geologic Time Power Point 2011Rev

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Transcript Geologic Time Power Point 2011Rev

Earth’s History
The Rock Record
Topic 1 – Telling Time
• Kinds of Time
– 2 types
1. Relative time – places an event in order
by comparing it with other events
– Example: Sedimentary Rock layers
» A – Oldest (Lower)
» B – Middle Age
» C – Youngest (Upper)
– Actual age not known, only relative!
• Relative time – place in a sequence
Topic 1 – Telling Time
2. Absolute time – specific age
–Example: Sedimentary Rock
»Rock Layer B deposited 30 Ma (mega anna =
million years ago)
»B is 10 million years older than C
»B is 5 million years younger than A
Rock Layer
Absolute Age
A
35 Ma
B
30 Ma
C
20 Ma
Two things told with absolute time
1. Actual age
2. Length of time between events
-- used to determine rate of geologic
process
List observations
Do not write down
interpretations
So... What’s the
story?
Grand Canyon, Arizona, Copyright Larry Fellows
http://www.earthscienceworld.org/imagebank/search/results.html?ImageID=hmwnq6
Finding Age with Relative Time
• Law of superposition – in a
sequence of horizontal
sedimentary rocks, the oldest
in on the bottom
Originally Horizontal
Timing
Younger
Older
Finding Age with Relative Time
http://www.classzone.com/books/earth_science/terc/content/investigations/es2903/es2903page03.cfm
Finding Age with Relative Time
• What happened here?
•
Originally layers deposited
horizontally
•
Then they were tilted
USGS/Jennifer Loomis, TERCTilted limestone beds in the Mojave Desert, California
http://www.classzone.com/books/earth_science/terc/content/investigations/es2903/es2903page04.cfm.
Finding Age with Relative Time
• Law of Cross-cutting relationships –
igneous rocks are younger than
rocks that they intrude into
Crosscutting
MOST recent
Finding Age with Relative Time
• Law of (fragmented) Inclusion
– Rock fragments found in another rock
must be older than the rock it is found
in
• Examples
– pebbles in a conglomerate
– Sand grains in a sandstone
http://www.geology.sfasu.edu/rocks/conglomerate02.jp
g
http://www.zionnationalpark.com/zioninfo/photos/Sandstone-2.jpg
Finding Age with Relative Time
• Unconformity –
break in the rock
record
– Eroded surface then
buried
– Parts of rock record
missing like pages
in a book
– Gap in geologic time
http://www.bamboo.hc.edu.tw/~sts/course-2003/course/textbook/text05/ch14/images/ch14-032.jpg
TIMING: RELATIVE vs. ABSOLUTE
Geologic Time
250 Ma
____________
___________
600 Ma
http://www.astro.lsa.umich.edu/users/cowley/grand33.jpg
Murder Mystery?
List observations
From your observations
make interpretations
How is this picture
explained using relative
dating?
Law of Superposition
Principles of: Original Horizontality
Cross-Cutting Rel.
Inclusions
Unconformities
M. d’Alessio, 2004
Topic 9 – Measuring Absolute Time
• Tree rings can be used to measure
specific dates
– 1 ring = 1 year
– Width of ring correlates to
temperature and rainfall
– Wide ring = high temperature and
rain
– Oldest tree is the bristle cone pine
4,862 – Prometheus
4,767 -- Methuselah
http://www.championtrees.org/champions/articles/AP21010.htm
http://home.earthlink.net/~colorado_hiking/1_Hiking_topics/pics/bristleconepineT.jpg
Radioactive Elements & Absolute Time
• Radioactive decay used to date
much farther back in time
• Some elements naturally “decay” in
the nucleus
– Radioactive decay occurs until a
stable element is formed
http://www.pbs.org/wgbh/nova/first/radiocarbonce.html
Topic 12 – Half-Life
• Half-life = rate of decay
–Time needed for ½ radioactive
items to decay
•Ranges from seconds to billions of
years
Radioactive
material
Found in
Half-life (years)
C-14
Wood, peat, coal ,
bones, shells
~5,700
K-40
Mica, feldspars
~1.3 billion
Rb-87
Mica, feldspars
~47 billion
U-235
Many rocks
713 million
U-238
Oldest igneous
rocks
4.5 billion
Fossil Preservation
• 1. Original Remains
• Frozen
• Resin (Amber)
– Sticky substance from evergreens
– Usually insects
• Tar Pits
– LaBrea
Fossil Preservation
• Replaced Remains
• Hard parts replaced by minerals
• Usually done by groundwater
– Ex: petrified Wood
• Mold or Cast
• Mold – shape of where fossil was
• Cast – new material fills mold &
hardens
– Ex: Shells
Fossil Preservation
• Trace Fossils
• Evidence of life other than remains
– trails
– Footprints
– Burrows
– copralites
http://www.scienceviews.com/photo/thumb/SIA0651.jpg
http://www.delargy.com/images/2004_7_Colorado/dinosaur%20footprint.JPG
http://csd.unl.edu/csd-esic/ResourceNotesImages/volume16/page-24.jpg
Topic 5 – Fossils as Evidence for Evolution
• Oldest rocks with fossils show
simple life forms (only)
• As time goes by, some simpler life
forms become more complex
• This process of change is called:
EVOLUTION
• Theory of Evolution: Scientific
explanation for the past and
present diversity of life
http://www.thezreview.co.uk/posters/posterimages/e/evolution1.jpg
Topic 5 – Fossils as Evidence for Evolution
• Charles Darwin – “The Origin of
Species”, 1859
•
- theory states that species
evolve slowly over geologic time
• Modern science
- species remain fairly constant for
millenia, then abruptly change
within a relatively short time, often
less than 1 million years
http://earth.ast.smith.edu/courses/ast215/darwin.jpg
Topic 6 – Index Fossils and Key Beds
•
•
•
Index fossils (guide fossils)
Lived for a “short geologic” time
period in a lot of places
Allow dating of rock layers with
relative age
3 Characteristics
1. Easily recognized (unique)
2. Found over large geographic area
–
Continental drift theory evidence
3. Limited in time
–
Only lived over short period of time, therefore,
only found in FEW rock layers
http://www.paleocurrents.com/img/2002_09_13FI/HTML/138-3848_img_std.jpg
Topic 6 – Index Fossils and Key Beds
• Key Bed
• Single rock layer that acts like an
index fossil
• Easily recognized
• Large area
– Ex: Ash from volcanic eruptions
• Iridium layer from Chicxulub impact (K-T)
Iridium Layer
Clay with high rare element
(Ir) found in Mexico, Italy,
Denmark & New Zealand
http://www.astro.uva.nl/encyclopedie/images/chicxulub.gif
Topic 7 – Rock Correlation
• Correlation: matching of rock
layers from one area to another
• Index Fossils
• Key Beds
• Allow correlating over great
distances
Iridium Layer
Clay with high rare element (Ir) found in Mexico,
Italy, Denmark & New Zealand
Topic 8 – Other Uses for fossils
1. Determine relative age of rocks
2. Correlate rock layers
3. Indicate past climate
•
Ex: coal only from warm, swampy
areas
4. Oil exploration
•
Microfossils – seen only with
microscope
•
Help correlate layers of rock to oil rich
layers in other places
Topic 10 – Varves
• Varve
• Any sediment that
shows a yearly cycle
• Sedimentary layers
resemble tree rings
• Can date ~15,000
years ago
– Often associated
with lakes annual
turnover
Geologic Timetable (p. 600-601)
• Eon – greatest division of geologic time
• Era – 2 or more of these form an eon
• Period – basic unit of geologic time in
which a single type of rock system is
formed (named for specific occurrence)
• Epoch – subdivision of a period
• Age – subdivision of an epoch
– All changes based on significant changes in the
fossil record
• The greater the changes, the larger division
• Fossil – ANY evidence of earlier life or
environments preserved in rock record
Geologic Timetable
• The
geologic
time scale
can be
represented
in many
ways
Geologic Timetable
Geologic Timetable
ERA
Attributes
Archean (Beginning Earth)
Began ~4.6 Ga
Beginning of Earth, almost no
fossils
Proterozoic (Earliest Life)
Began 2.5 Ga
Simple plants & animals
(worms)
No life on land
Paleozoic (Ancient Life)
Began 570 Ma
Plant & Animal Fossils
Land & Ocean
Mesozoic
Began 250 Ma
Middle Life
Age of Reptiles (dinosaurs)
Cenozoic (Recent Life)
Began 65 Ma
Age of Mammals