Geologic Time (History as told by Rocks)

The Rock Record Geological Time Scale Dividing the Earth’s history into sections based on the fossils and rock evidence.

How do we know? Principle of UNIFORMITARIANISM means that the natural processes in place today have been occurring since Earth formed. Processes in the past happened according to the same laws as processes do today.

Dividing It Up Eon Era Period Epoch Longest time unit measured in BILLIONS of years. Era next longest measured in HUNDREDS of MILLIONS of years Period measured in TENS of MILLIONS of years Epoch measured in MILLIONS of years

Historical Events Bacteria Form Trilobites are abundant, First fish appear, Appalachians begin to form First Land Plants and Insects appear Reptiles appear Appalachians finish forming, glaciers retreat

Important Events Pangaea breaks up, Mammals appear Dinosaurs dominate, mountain building in Western U.S. Flowering plants appear, Rockies form, Dinosaurs become extinct Alps and Himalayan Mountains form, mammals dominate 0.01 million years ago – Homo Sapiens evolves.

Fish Trilobite Insects Land Plants Reptiles

Geological Principles Original horizontality sedimentary rocks were formed in HORIZONTAL LAYERS Superposition oldest rock layers are on the BOTTOM Cross-cutting relationships Intrusions or Faults that cut across a layer are YOUNGER than the layer they cut through. Unconformity missing layer of eroded rock

Original Horizontality Cross-cutting Relationships Superposition Cross-cutting Relationships Unconformity

Bending and Breaking Layers Rock layers are disturbed by geological processes FOLDING: Compression forces BEND the layers TILTING: Lifting forces lift one end of a layer of rock FAULTING: Shear forces shift some layers of the rock

Identifying the RELATIVE AGE of Rock We can use the patterns in the rock layers to determine RELATIVE AGE of the layers Place the layers in order from Oldest  Youngest Bottom Top

Absolute-Age Dating In addition to learning about the relative age, scientists also have methods of determining the absolute-age. Absolute age = The rock is 2 million years old Relative age = The rock is older than the other rock.

Radiometric Dating Scientists can use UNSTABLE atoms to determine the age of some rocks and fossils. Unstable atoms decay (release energy) at a predictable rate over time. The atoms change as they decay. Comparing the number of unchanged atoms (parent) and changed atoms (daughter), the age can be determined The rate these atoms decay is called the half-life.

Radioactive Atoms and Their Use Carbon-14 is used for organic material less than 75,000 years old. The half-life of Carbon-14 is 5,730 years Carbon-14 has 6 protons and 8 neutrons and it decays to Nitrogen-14 with 7 protons and 7 neutrons Organisms continually take in new carbon while they are alive but stop when they die, so the carbon-14 “clock” doesn’t start until after death.

Other Radiometric Dating Uranium-235 – half-life of 0.7 billion years Decays to Lead-207 Uranium-238 – half-life of 4.6 billion years Decays to Lead-208 Potassium-40 – half-life of 1.3 billion years Decays to Argon-40 These decay processes also have intermediate steps so we can use them to date rocks as young as 50,000 years to the oldest rocks in our solar system.

Dendrochronology We can examine the annual growth of trees (tree rings) to determine an absolute age of tree. Using tree rings to determine age is called dendrochronology. The width of the rings also can tell us if the climate was wet or dry and if any major forest fires occurred during the life of the tree.

Other Age Indicators We can also determine age by looking for indicators of climate change Varves are bands of deposited sediments in sedimentary rocks. Thick bands mean more sediment was carried – so more water. Key beds are layers of distinct sediments formed by unique events – such as asteroid impacts

Fossil Record Fossil – evidence or remains of once living organisms If you know the type of organism, it can tell you about the environment If you find a fossil shell in the desert, it was probably a marine environment when the fossil formed. If you have actual genetic material, you can use the DNA record and carbon dating to determine age.

How do fossils form? Fossils usually only form in sedimentary rock. Sediments have to accumulate over the organism in order to preserve it. volcanic eruption, mudslide, tree sap or slow sediment accumulation on the bottom of a lake Most fossils are marine in origin, or are land animals that fell into a water environment. This is because sediments easily accumulate in water environments (lakes, streams, oceans).

Fossil Record Trace fossils – fossils left by animal or plant ACTIVITY (footprints, worm burrows) Mold – Rock formed around an organism that later decays out, leaving a empty space in the shape of the organism Cast – Molds that get filled up with later rock Permineralization – minerals from ground water seep into the organism and slowly fill in the pores in the animal Original Preservation – remains that have not changed since the organisms death (often frozen!)

CAST FOSSIL This ammonite cast was discovered in the United Kingdom. Petrified Fossils MOLD FOSSIL This mold, or imprint, is of an extinct mollusk called an ammonite. CAST FOSSIL This ammonite cast was discovered in the United Kingdom. Trace Fossils Preserved Remains Carbon Films

Index Fossil Index fossils are remains of organisms that can be used to relate rock layers that spread over a large geographic area. Index fossils must be Abundant Spread out geographically Lived only for a limited time frame If you know when that organism was alive, you can determine when the rock formed.

Which two rock layers formed at the same time?