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
35 Ma
30 Ma
20 Ma
Two things told with absolute time
1. Actual age
2. Length of time between events
-- used to determine rate of geologic
List observations
Do not write down
So... What’s the
Grand Canyon, Arizona, Copyright Larry Fellows
Finding Age with Relative Time
• Law of superposition – in a
sequence of horizontal
sedimentary rocks, the oldest
in on the bottom
Originally Horizontal
Finding Age with Relative Time
Finding Age with Relative Time
• What happened here?
Originally layers deposited
Then they were tilted
USGS/Jennifer Loomis, TERCTilted limestone beds in the Mojave Desert, California
Finding Age with Relative Time
• Law of Cross-cutting relationships –
igneous rocks are younger than
rocks that they intrude into
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
• Examples
– pebbles in a conglomerate
– Sand grains in a sandstone
Finding Age with Relative Time
• Unconformity –
break in the rock
– Eroded surface then
– Parts of rock record
missing like pages
in a book
– Gap in geologic time
Geologic Time
250 Ma
600 Ma
Murder Mystery?
List observations
From your observations
make interpretations
How is this picture
explained using relative
Law of Superposition
Principles of: Original Horizontality
Cross-Cutting Rel.
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
– Oldest tree is the bristle cone pine
4,862 – Prometheus
4,767 -- Methuselah
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
Topic 12 – Half-Life
• Half-life = rate of decay
–Time needed for ½ radioactive
items to decay
•Ranges from seconds to billions of
Found in
Half-life (years)
Wood, peat, coal ,
bones, shells
Mica, feldspars
~1.3 billion
Mica, feldspars
~47 billion
Many rocks
713 million
Oldest igneous
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 &
– Ex: Shells
Fossil Preservation
• Trace Fossils
• Evidence of life other than remains
– trails
– Footprints
– Burrows
– copralites
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:
• Theory of Evolution: Scientific
explanation for the past and
present diversity of life
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
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
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
Topic 7 – Rock Correlation
• Correlation: matching of rock
layers from one area to another
• Index Fossils
• Key Beds
• Allow correlating over great
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
4. Oil exploration
Microfossils – seen only with
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
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
time scale
can be
in many
Geologic Timetable
Geologic Timetable
Archean (Beginning Earth)
Began ~4.6 Ga
Beginning of Earth, almost no
Proterozoic (Earliest Life)
Began 2.5 Ga
Simple plants & animals
No life on land
Paleozoic (Ancient Life)
Began 570 Ma
Plant & Animal Fossils
Land & Ocean
Began 250 Ma
Middle Life
Age of Reptiles (dinosaurs)
Cenozoic (Recent Life)
Began 65 Ma
Age of Mammals