Geologic Time

James Hutton – The Father of Geology 18th Century Scottish Geologist, questioned belief that the Earth was several thousand years old. Believed in Uniformitarianism – natural processes that operate today must be the same everywhere else and in the past. “The present is the key to the past”. Geologic change is slow, so the Earth must be very old.

Catastrophism Violent changes shape the Earths surface Can be responsible for mass extinctions (Ex: End of Mesozoic, meteor that killed the dinosaurs) Origins in religion, but evidence exists to support the idea.

Formation of the Solar system Cloud of gas and dust collapses and rotates 4.6 billion years ago. Nuclear fusion begins forming the sun. Planets form from dust grains and gases that are left.

Early Earth Heavy bombardment – remaining debris from solar nebula impacts Earth. Heavy volcanic activity releasing gases to form atmosphere. Life not likely.

Young Earth Begins to Cool Crust begins to thicken. Meteor impacts and volcanic activity decrease. Water begins to collect on surface forming the oceans. Simple life may have started between 3 and 4 billion years ago.

Division of Geologic Time The Earth is approximately 4.6 billion years old (4,600,000,000) Earth’s history divided into periods of time based on life that was present: Eon  Era  Period  Epoch Longest Shortest

Division of Geologic Time Eons – Precambrian (Before complex life, about 4 billion years long) and Phanerozoic (Period of life up to current day) Phanerozoic divided into Eras: - Paleozoic (ancient life, fish and amphibians, insects) - Mesozoic (middle life, dinosaurs) - Cenozoic (modern life, rise of mammals) Eras divided into smaller units of time (periods and epochs)

Geologic Time Scale

Dating the Earth: Relative age Relative Age compares the position of rock layers against other rock layers (relative to other layers, older or younger). Absolute Age uses techniques that assigns a numerical value on the age of a rock layer or geologic feature.

Relative Age Laws Original Horizontality: Rock Layers will always be laid horizontal unless disturbed by geologic processes. Superposition: Rock Layers that are below other rock layers must have been deposited first. Crosscutting Relationship: Rock layers that cut vertically through horizontal layers of rock must have happened after the horizontal layers were deposited.

Superposition: Road had to exist before lava flow. Original Horizontality: Lava flow is layered horizontally (would be same if sediment was deposited).

Igneous Intrusion Dike – Vertical Igneous Intrusion Sill – Horizontal Igneous Intrusion. Laccolith – Dome shaped intrusion that works in between existing horizontal layers. Shows Crosscutting or Original Horizontality.

Crosscutting Relationship: Igneous Intrusion must be younger than surrounding rock

Faults – Show Crosscutting Relationships

What are Fossils? Fossils are any evidence of life that has been preserved in rock. Usually found in sedimentary rock.

Imprint Fossil A carbon residue is left as the material starts to decompose. The imprint displays the surface features of the organism

Types of Fossils: Molds and Casts Material is filled in with sediment after the original material decomposes. Mold – right image Cast – left image

Other Types of Fossils Amber - hardened tree sap, preserves DNA Freezing – flash freezes organism Tar Seep – animal falls in and is preserved.

Other types of fossils (continued) Petrification - Organic material that has been covered and preserved quickly by sediment is replaced by minerals in ground water. Mummification – Preserved in very dry places, slows bacteria.

Trace Fossils Trace Fossils are fossils that indirectly indicate that an organism was alive. Footprints are an example. No main parts of the organism are preserved. Dinosaur State Park, Rocky Hill, CT is pictured to the right.

Trace Fossil: Coprolites Yes, its fossilized feces.

Index Fossils Paleontologists use fossils to help determine the relative age of a rock layer. A geologic column can be constructed using index fossils in different layers around a region to determine the relative age of rock.

Absolute Age Determining the absolute age of a rock layer or fossil can be done by using one of the following methods: Erosion or Deposition Rates of Sediment Counting Varves (Lines of sediment made by melting glaciers) Radiometric Dating – Using radioactive decay to determine age.

Radiometric Dating Uses radioactive decay in certain rocks to date the rock’s age. Percentage of parent to daughter element determines age of rock. Overview of radioactive decay

Half-Life Half Life is the time it takes for half the original sample of a radioactive element to decay into another element. Ex: U-235 has a half life of 704 million years. In 704 million years, 10 grams of U-235 (parent) will become 5 grams of Th-231 (daughter). 5 grams of U235 will remain.