Transcript Geologic Time

The Age of the Earth
• Prior to the 19th century, accepted age of Earth
based on religious beliefs
– ~6,000 years for Western culture (Biblical)
– Old beyond comprehension (Chinese/Hindu)
• James Hutton, the“father of geology”, realized
geologic processes require vast amounts of time
• Charles Lyell popularized Hutton’s concepts in
book Principles of Geology
– Uniformitarianism/actualism: same processes operating in past are
operating at present - “The present is the key to the past”
History of Early Geology
Catastrophism (James Ussher, mid 1600s) - He interpreted the Bible
to determine that the Earth was created at 4004 B.C. This was
generally accepted by both the scientific and religious communities.
Subsequent workers then developed the notion of catastrophism,
which held that the Earth’s landforms were formed over very short
periods of time.
Uniformitarianism (James Hutton, late 1700s) - He proposed that
the same processes that are at work today were at work in the past.
Summarized by “The present is the key to the past.” Hutton, not
constrained by the notion of a very young planet, recognized that
time is the critical element to the formation of common geologic
structures. Uniformitarianism is a basic foundation of modern
geology.
Geologic Time and Clocks
Lifespan of a human
~ 100 years
Human civilization
~10,000 years
Modern humans
~100,000 years
Stone tools
~1,000,000 years
Age of oceanic crust
~100,000,000 years
Precambrian Explosion
~540,000,000 years
Oldest Rocks
~3.8 Billion years
Age of the Earth
~4.3 Billion years
Age of the Solar System
~4.6 Billion years
Age of the Universe
~14 Billion years
Relative vs. Numerical Age
• Relative age - the order of events or objects, from first
(oldest) to last (youngest)
– Determined by applying simple principles, including original
horizontality, superposition, lateral continuity, cross-cutting
relationships, inclusions, unconformities, and correlation of rock
units and fossils
• Numerical age - the age of events or objects, expressed as
a number or numbers
– Determined using radiometric dating (determining how much
radioactive decay of a specific element has occurred since a rock
formed or an event occurred)
Absolute Ages
• Numerical dating - puts
absolute values (e.g., millions
of years) on the ages of rocks
and geologic time periods
– Uses radioactive decay of
unstable isotopes
– Only possible since radioactivity
was discovered in 1896
– Radioactive isotopes decay in
predictable manner, giving a
characteristic half-life (time it
takes for a given amount of
radioactive isotope to be
reduced by half)
Radioactive Decay
N = N0 e-kt
Where N is the amount of the
radioactive element in the rock
now; N0 is the amount originally
in the rock, e ~ 2.718 (natural
logarithm); k is the decay
constant of the radioactive
element, and t is time.
Half-life when N/N0 = 0.5
Absolute Time: Radiometric Clocks
Absolute (Radiometric) Dating: Using radioactive decay of
elements to determine the absolute age of rocks. This is done using
igneous and metamorphic rocks.
Carbon-14
half-life ~ 5730 years
Used in anthropology
Relative Age Determination
• Contacts - surfaces separating
successive rock layers (beds)
• Formations - bodies of rock of
considerable thickness with
recognizable characteristics
allowing them to be distinguished
from adjacent rock layers
• Original horizontality - beds
of sediment deposited in water are
initially formed as horizontal or
nearly horizontal layers
Relative Age Determination
• Superposition - within an
undisturbed sequence of
sedimentary or volcanic
rocks, layers get younger
from bottom to top
• Lateral continuity - original
horizontal layer extends
laterally until it tapers or
thins at its edges
Which are
the youngest
rock layers?
What is the
sequence of
formation?
Relative Age Determination
• Cross-cutting relationships - a
disrupted pattern is older than the
cause of the disruption
– Intrusions and faults are younger than the
rocks they cut through
• Baked contacts - contacts between
igneous intrusions and surrounding
rocks, where surrounding rocks have
experienced contact metamorphism
• Inclusions - fragments embedded in
host rock are older than the host rock
Which are
the youngest
rock layers?
What is the
sequence of
formation?
Unconformities
• Unconformity - a surface (or
contact) that represents a gap in
the geologic record
• Disconformity - an unconformity
in which the contact representing
missing rock layers separates beds
that are parallel to each other
• Angular unconformity - an
unconformity in which the contact
separates overlying younger layers
from eroded tilted or folder layers
Unconformities
• Nonconformity - an unconformity
in which an erosion surface on
plutonic or metamorphic rock has
been covered by younger
sedimentary or volcanic rock
– Plutonic and metamorphic rocks
exposed by large amounts of erosion
– Typically represents a large gap in
the geologic record
Which are the
youngest rock
layers?
What is the
sequence of
formation?
Where is the
unconformity?
What is the sequence of events?
Correlation
• Correlation - determining the time-equivalency of rock units
– Within a region, a continent, between continents
• Physical continuity
– Physically tracing a continuous
exposure of a rock unit
– Easily done in Grand Canyon
• Similarity of rock types
– Assumes similar sequences of rocks
formed at same time
– Can be inaccurate if very common
rock types are involved
• Correlation by fossils
– Fossil species succeed one another through
the layers in a predictable order (faunal succession)
– Similar fossil assemblages (groups of different fossil species) used
William Smith: father of stratigraphy
Smith’s Fossil Assemblages
Smith’s Fossil Assemblages
Stratigraphic Methods
􀂄 Lithostratigraphy (correlating rock layers by using rock types)
􀂄 Biostratigraphy (correlating rock layers by using fossils)
􀂄 Magnetostratigraphy (correlating rock layers by using magnetic
reversals)
􀂄 Chemostratigraphy (correlating rock layers by using chemical or
isotopic methods for correlating; e.g., oxygen isotopes or (iridium
spike at end of Cretaceous)
􀂄 Chronostratigraphy (correlating rock layers by using absolute
and/or relative time)
Geologic Time Scale
• Standard geologic time scale
– Worldwide relative time scale
– Subdivides geologic time
based on fossil assemblages
– Divided into eons, eras,
periods, and epochs
• Precambrian - vast amount
of time prior to the Paleozoic
era; few fossils preserved
• Paleozoic era - “old life”
– appearance of complex life;
many fossils
Geologic Time Scale
• Mesozoic era - "middle life"
– Dinosaurs abundant on land
– Period ended by mass extinction
• Cenozoic era - "new life"
– Mammals and birds abundant
– We are currently in the Recent
(Holocene) Epoch of the
Quaternary Period of the
Cenozoic Era
– Most recent ice ages occurred
during the Pleistocene Epoch of
the Quaternary Period
Combining Relative and
Numerical Ages
• Radiometric dating gives numerical
time brackets for events with known
relative ages
– Individual layers may be dated directly
– Radiometric dating of units above and
below brackets age of units in between
• Geologic Time Scale
– Divided into four Eons
• Hadean, Archean, Proterozoic, Phanerozoic
– Precambrian (all time prior to Phanerozoic)
represents 87% of geologic time)
Age of the Earth
• Numerical dating gives
absolute age for Earth of
about 4.56 billion years
– Oldest age obtained for
meteorites, believed to have
been unchanged since the
formation of the solar system
– Earth and rest of solar system
very likely formed at this time
• Geologic (deep) time is vast
– A long human lifetime (100
years) represents only about
0.000002% of geologic time
Why Study of Historical Geology?
• Survival of the human species may depend on
understanding how Earth’s subsystems work
and interact and exploring the past is our only
‘laboratory’ for testing hypotheses.
• Knowing what occurred in the past can help us
to understand our origins and place in both the
Earth and the Universe.
Latest Precambrian / Early
Paleozoic
Supercontinent Rodinia, centered
about the south pole, breaks apart.
North America (Laurentia), Baltica,
and Siberia moved North.
Marine Invertebrates.
North America: arc on the south.
Baltica and Siberia moved in from
the SE.
Texas (505-570 Ma): Flat plain;
remnants of eroded collisional belt
(Llano). Shallow marine seas across
much of Texas. Sandy sediment
onshore, limestone offshore.
Trilobites, brachiopods.
http://vishnu.glg.nau.edu/rcb/globaltext.html
Precambrian Stromatolites
two kinds cyanobacteria from the Bitter Springs chert of central Australia, a site dating to the Late Proterozoic, about 850 million years
old. On the left is a colonial chroococcalean form, and on the right is the filamentous Palaeolyngbya.
Cyanobacteria – green algae
Dickinsonia is known from Vendian rocks of south Australia and north Russia. It is often
considered to be an annelid worm
Ediacaran Fauna
Kimberella,
one of the
most
fascinating
Vendian
fossils, has
received a
great deal of
attention
lately. It was
hypothesized
to be a box
jellyfish
(cubozoan)
this unusual disc-shaped
form with three-part
(triradial) symmetry.
Named Tribrachidium
heraldicum, its affinities
are still mysterious,
although distant
relationships have been
proposed with either the
Cnidaria (corals and
anemones) or
Echinodermata (urchins
and seastars).
Ediacaran Fauna
Nemiana is one of the simplest of all Vendian fossils,
and is difficult to interpret. It seems to be an
impression of a saclike body
Ediacaran Fauna
‘ The Small Shellies’
specimen is only a small part of Anomalocaris, which was a
large (up to 60cm or more) arthropod-like predator.
Burgess Shale
Marrella splendens is a small "arthropod" somewhat reminiscent of a trilobite
Tuzoia is a "bivalved" crustacean grossly similar to certain
types of modern brine shrimp.
Vauxia gracilenta has a branching morphology: a sponge
Burgess Shale
Burgess Shale
Trilobites
Trilobites
Trace Fossils
Arthropod tracks
Worm trails
Trilobite tracks
Latest Precambrian / Early
Paleozoic
Supercontinent Rodinia continues to
break apart. Pieces move north.
-Fish.
-Glaciation.
North America: Numerous plates
and continental blocks move in
from the south and east. The
Taconic arc collides, forming the
Taconic orogeny.
Texas 438-505 Ma: Shallow marine
seas across much of inland Texas.
Warm-water limestone. Corals,
brachiopods.
http://vishnu.glg.nau.edu/rcb/globaltext.html
Middle / Late Paleozoic
Remains of Rodinia (Gondwana) move
northward to collide with Laurasia -creating the super continent Pangaea
and the Tethys Ocean.
First land-plants.
Baltica collides with North America in
the Caledonian-Acadian orogeny.
Texas 403-438 Ma: Shallow marine
seas across much of west Texas limestone. Corals, brachiopods.
http://vishnu.glg.nau.edu/rcb/globaltext.html
Middle / Late Paleozoic
Most drifting Rodinia blocks assembled
into the super continent of Laurussia.
Amphibians. Fish really get
going. Ferns.
Glaciation.
North America: Caledonian-Acadian
orogeny marks assemblage of Laurussia.
Gondwana closed in from the south. An
arc formed along western North America.
Texas 360-408 Ma: shallow marine
sandstones and limestones in west Texas.
http://vishnu.glg.nau.edu/rcb/globaltext.html
Middle / Late Paleozoic
Gondwana, with a large,
developing glacier, nears southern
Laurussia.
Fern-forests.
North America: The Antler arc
collides with western North
America creating the Antler
orogeny.
Texas 320-360 Ma: shallow marine
seas inland. Shales and limestones.
http://vishnu.glg.nau.edu/rcb/globaltext.html
Middle / Late Paleozoic
Rodinia blocks of Laurussia and
Siberia collide to form Laurasia.
Reptiles.
North America: Gondwana collides
from the south. The resulting
Appalachian, Ouachita, Marathon,
Ural, Variscan, and Hercynian
orogenies formed some of the largest
mountains of all time. The Ancestral
Rockies form.
Texas 286-320 Ma: Ouachita
Mountains. Collision formed inland
basins filled by seas. Limestone,
sandstone, shale.
http://vishnu.glg.nau.edu/rcb/globaltext.html
Latest Paleozoic / Early Mesozoic
The supercontinent Pangeae dominates
the Permian Earth, lying across the
equator.
Extinctions! Trilobites go away.
North America: A new arc approaches
western North America. A new spreading
center forms as Cimmeria rifts from
Gondwana and opens the Tethyian
Ocean.
The western fringe of Pangaea lay along
the eastern margin of the Pacific "ring of
fire” subduction zone.
Texas 245-286 Ma: Shallow marine
inland of mountains. Reefs. Evaporites.
Red shales.
http://vishnu.glg.nau.edu/rcb/globaltext.html
Latest Paleozoic / Early Mesozoic
Mammals.
North America: Arc collision along
western edge forms the Sonoman
orogeny.
As the Tethys Ocean expands,
Cimmeria (Turkey, Iran, and
Afghanistan) move
northward towards Laurasia.
Texas 208-245 Ma: shales and
sandstones in NW. Start opening the
GOM - red sandstone, shale,
evaporites.
http://vishnu.glg.nau.edu/rcb/globaltext.html
Middle Mesozoic
Pangaea rotates; different components at
different rates / in different directions -rifts form.
Birds.
North America: Southern North Atlantic
Ocean opens, continuing west into the
Gulf of Mexico.
The Cordilleran arc develops along
Pacific margin.
Arc forms on western side. Nevadan
orogeny begins. Cimmeria begins
collision with Laurasia - Cimmerian
orogeny.
Texas 144-208 Ma: Change in sediment
direction. Shallow water deposition /
evaporites in GOM.
http://vishnu.glg.nau.edu/rcb/globaltext.html
Middle Mesozoic
The Atlantic continues to expand as
Pangaea breaks up.
The Cimmerian orogeny continues.
North America: Arcs and micro
continents slam into western region.
Laramide orogeny in Rockies.
Texas 66-144 Ma: Influx of sediment
from Rockies. Shallow Cretaceous sea
way across Texas. Shallow liestones,
shales.
http://vishnu.glg.nau.edu/rcb/globaltext.html
Late Cretaceous / Present
Rifts separate Africa and South
America and then India, Australia,
Antarctica. North America rifts from
Europe.
Old Gondwana lands(Africa, India,
Australia) move north toward
Eurasia, closing the Tethys Ocean
and forming the Alpine-Himalayan
mountains.
The Atlantic lengthens / widens, the
Sevier orogeny continues, and the
Caribbean arc forms.
Texas 65-144 Ma: continuing
shallow limestone and shale
deposition to the southeast (from
Rockies).
http://vishnu.glg.nau.edu/rcb/globaltext.html
Paleocene / Eocene
Himalayan Orogeny. Alps and
Pyrenees form.
The modern patterns of Planet Earth
appear.
Atlantic continues to open. Rocky
Mountains grow.
Texas 65 - 35 Ma: shale and sandstone
in southeast region prograde shoreline
(from the Rockies). Volcanic activity
in Panhandle.
http://vishnu.glg.nau.edu/rcb/globaltext.html
Oligocene and Miocene
Orogeny continues in
the Mediterranean region and India
nears its junction with southern Asia.
Antarctica isolated.
Southwestern North America
intercepts the East Pacific Rise and a
great extensional event, the Basin and
Range orogeny begins.
Texas 35-5 Ma: continued
sandstone/shale deposition and
progradation of shoreline (erosion of
Rockies)
http://vishnu.glg.nau.edu/rcb/globaltext.html
Present
Note:
Best data set available.
http://vishnu.glg.nau.edu/rcb/globaltext.html
Fossils
• Fossils are the remains or traces of prehistoric organisms
– Any evidence of past life
• Most common in sedimentary rocks
– and in some accumulations of pyroclastic materials, especially ash
• They are extremely useful for determining relative ages of
strata
– geologists also use them to ascertain environments of deposition
• Fossils provide some of the evidence for organic evolution
– many fossils are of organisms now extinct
How do Fossils Form?
• Remains of organisms are called body fossils
– mostly durable skeletal elements such as bones, teeth and shells
– rarely we might find entire animals
preserved by freezing or
mummification
Trace Fossils
• Indications of organic activity including tracks, trails,
burrows, and nests are called trace fossils
• A coprolite is a type of trace fossil consisting of
fossilized feces that may provide information about
the size and diet of the animal that produced it
Trace Fossils
• A land-dwelling beaver,
Paleocastor, made this
spiral burrow in
Nebraska
Trace Fossils
• Fossilized feces (coprolite) of a carnivorous mammal
– specimen measures about 5 cm long and contains small
fragments of bones
Body Fossil Formation
• The most favorable conditions for preservation of body fossils
occurs when the organism
– possesses a durable skeleton of some kind
– and lives in an area where burial is likely
• Body fossils may be preserved as
– unaltered remains, meaning they retain their original composition and
structure,by freezing, mummification, in amber, in tar
– altered remains, with some change in composition or structure by being
permineralized, recrystallized, replaced, carbonized
Unaltered Remains
• Insects in
amber
• Preservation in
tar
Unaltered Remains
• 40,000-yearold frozen baby
mammoth
found in
Siberia in 1971
– hair around
the feet is still
visible
Altered Remains
• Petrified tree stump
in Florissant Fossil
Beds National
Monument,
Colorado
Altered Remains
Carbon film of a palm frond
Carbon film of an insect
Fossil Record
• The fossil record is the record of ancient life preserved as
fossils in rocks
• The fossil record is very incomplete because of:
–
–
–
–
bacterial decay
physical processes
scavenging
metamorphism
• In spite of this, fossils are quite common