Day 1 Vocabulary Review Background terms: Principle of Uniformitarianism Doctrine of Catastrophism

Day 1 Vocabulary Review Relative Dating principles: Original horizontality Superposition Cross-Cutting Relationships Intrusion: body of igneous rock forced between or through existing formations, without reaching the surface example: sill—an intrusion between parallel layers of rock Faults: a fracture in Earth’s crust that makes rock layers appear discontinuous

Day 1 Vocabulary Review Inclusions Unconformities: Disconformity: sed. layer above sed. layer Nonconformity: sed. layer above non-sed. layer Angular unconformity

Practice: Sequence the layers!

E, G, L, C, H, M, D, J, A, N, K, B, F

Calculating the exact numerical age of samples Absolute Dating Calculating the exact numerical age of samples

Use of Radioactive Isotopes Radioactive atoms, like the isotope uranium-238, emit particles from their nuclei that we detect as radiation. Ultimately, the radioactive decay process produces atoms that are stable and no longer radioactive. Example: uranium-238  lead-206 The amount of time this process takes depends on the isotope.

Use of Radioactive Isotopes Radiometric dating: determining the age of a sample by measuring the percentage of radioactive isotopes remaining within it

Half-life: the length of time it takes for one-half of the original parent isotope to change to the daughter isotope.

Other Ways to Determine Age Tree Rings The age of a tree can be determined by counting the number of annual tree rings in a cross section of the tree. The width of tree rings are directly related to the climatic conditions during growth periods (spring and winter rings equal one year) Dendrochronology is the science of comparing annual growth rings to date events and changes in past environments

Other Ways to Determine Age Seasonal Climatic Changes Varves are bands of alternating light and dark colored sediments of sand, clay, and silt. In the summer months, deposits are light colored and thick… in the winter they are darker and thinner Varves are similar to tree rings as they show evidence of cyclic events Varves from different lakes can be compared to determine the ages of glacial lake sediments

Other Ways to Determine Age Distinctive Sediment Layers When layers are formed by instantaneous or short-lived events, geologists may be able to determine the time of the event through radiometric dating. This layer is called a key bed, and can correlate rock layers across large areas

Types of Fossils Original Preservation Altered Hard Parts Index Fossils Molds and Casts Trace Fossils

Original Preservation Fossils with original preservation have not undergone any significant change since the organism’s death Contain soft and hard parts Uncommon because their preservation requires extraordinary environmental conditions (e.g., permanent freezing, desiccation, or anaerobic conditions) Habitats that favor original preservation Tundra Deserts Bogs Tar pits Amber

Original Preservation

Altered Hard Parts When all the organic material has been removed and the hard parts of the organism have been changed either by mineral replacement or by recrystallization, fossils are referred to as altered hard parts. Permineralization: process by which pore spaces are filled in with mineral substances

Altered hard parts Petrified wood (silica fills in spaces between cells walls)

Altered hard parts Silicified marine fossils

Index Fossils Any fossil that can be used to correlate rock layers over large geographic areas or to date a particular rock layer Particularly useful for relative dating Characteristics of index fossils Easily recognized Abundant Widely distributed geographically Must have lived during a short period of time Provide clues to the environments in which the rocks formed

Index Fossils

Molds and Casts Mold: hollow impression left when the original shell parts of an organism within a sedimentary rock are weathered and eroded away Cast: occurs when minerals or sediment fill in the mold

Trace fossils Indirect evidence of previous organisms Examples: worm trails, burrows, footprints Provide information about how an organism lived, how it moved, and how it obtained food

Correlation Correlation is the matching of rock layers of one geographic region to another. Can be useful for the discovery of oil or other valuable minerals unique rock or mineral features distinctive fossils

Correlation is often done by comparison of fossils

Volcanic Time Markers and Correlation Violent volcanic eruption can deposit a thin layer of volcanic ash over the entire Earth. This layer is very distinguishable and is similar to index fossils (cover entire earth and represent a very short period of time) http://www.youtube.com/watch?v=QW9DVoa-BJM

Why Study Fossils? (Paleontology) Provides evidence of the past existence of a wide variety of life-forms ~99.9% of all species in earth’s history have gone extinct and can be known only from fossils. Shows evidence of ancient environmental conditions Provides evidence that a population has undergone changes (evolution) in response to its environment Allows scientists to interpret and describe Earth’s history Helps scientists find patterns and cycles that can be used to predict future phenomena