Geologic Time

Overview: Explain the techniques for determining the age and composition of the Earth and the universe. Objectives: ▫Compare age of earth using radiometric aging, and expanding universe measurements ▫Identify techniques for evaluating the composition of objects in space ▫Explain geologic time scale and the major events in each era. ▫Examine radioactive dating, and carbon-14 dating ▫Explain fossil succession ▫Explain relationship between plate tectonics and continent formation

Rocks and the History of Earth Rocks record geological events and changing life forms of the past. Rocks have shown the earth is older than originally thought. Same geological process that formed the earth are still happening today. ▫Uniformitarianism – forces and process we observe today have been at work for a very long time. (James Hutton 1700s) ▫Changes can take hundreds, thousands, or even millions of years

Geological Time Scale Revolutionized the way people thought about time and how our planet was perceived. Was developed to show sequence, or order, of events based on principals of relative dating ▫Relative dating – tells us the sequence in which events occurred, now how long ago they occurred. ▫Nicholas Steno, a Danish anatomist,geologist, and priest is credited with describing a set of geologic observations that are the basis of relative dating

Geological Time Scale Principals governing the geologic time scale ▫Law of superposition ▫Principal of original horizontality ▫Principal of cross-cutting relationships ▫Inclusion (sometimes)

Geological Time Scale Law of Superposition - ▫In a underformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it

Geological Time Scale Principal of Original Horizontality ▫Layers of sediment are generally deposited in a horizontal position as in the deposition of sediments within lakes, rivers, and oceans.

Geological Time Scale Processes such as tilting, folding, faulting, and intrusions of igneous rocks can distort the original strata. Tilted geologic layers were originally horizontal. Sediments will fill in uneven geologic layers. Geologists observe the tilted layer and try to visually reposition it horizontally.

The sedimentary layers in this large roadcut near Denver, CO. can be clearly recognized by the variation in color. These layers can be recognized as having been deformed because they have been tilted so they are dipping to the east (the left side of photo). This deformation was related to the uplift of the Rocky Mountains.

Geological Time Scale Cross-Cutting Relationships ▫Faults cut through layers or magma intrudes other rock and crystallizes, it is assumed the fault or intrusion is younger than the rocks affected.

Geological Time Scale Inclusion – ▫Pieces of one rock unit that are contained within another. ▫the rock unit from which the pieces came have to be older in order to provide the fragment. Inclusion of metamorphic rock in lava

Geological Time Scale Unconformities – ▫Breaks in rock record ▫Long periods which deposition stops, and erosion removes previously formed rock, then deposition resumed.  Uplift and erosion are followed by subsidence and renewed sedimentation  Each represents significant geologic events in history

Geological Time Scale ▫3 types of unconformities  Angular  easily recognized  Disconformities  More common  Nonconformities  Separates rock

Geological Time Scale Correlation of rock layers – ▫Rocks of similar age located in different regions match up ▫Allows a more complete view of the geological history of a region ▫Use to trace rock formations over a relatively short distance

Fossils Fossil – remains or traces of prehistoric life. ▫Important time indicators (help correlate rocks) ▫Interpret and describe ancient environments including temperature ▫Must have rapid burial and posses hard parts Types of fossils ▫Unaltered remains ▫Altered remains ▫Trace remains ( indirect )

Fossils ▫Unaltered – may not have been altered over time.  Less common to find  Teeth, bones, shells, entire animals, like insects or plant parts trapped in amber

Fossils Altered – remains that have likely been altered ▫Petrified – “turned to stone” mineral-rich water soaks into dead organism and replaces the cell walls or other solid material ▫Molds/Casts –  Molds - organism is buried in sediment and then dissolved by underground water leaving an accurate representation of the organism’s shape.  No internal information  Casts – minerals fill hollow spaces in the molds

Fossils

▫Carbonization – preserving leaves and delicate animal animals / buried under fine sediment  Thin layer of Carbon residue is left after pressure squeezes out all the gas and liquid components.  Very rare

Fossils ▫Trace / Indirect Evidence – indirect evidence of prehistoric life  Made in soft sediment  Later filled with mineral water and preserved  Some of the oldest known fossils  Provide information regarding food habits  Worm burrows, coprolites (dung and stomach contents)

Fossils and Time Correlations William Smith (18 th Century) – fossils were not randomly distributed. ▫Fossils show a connection between rock layers and times ▫Fossils are distinct Principle of Fossil Succession ▫Fossils succeed one another in a definite and determinable order. Therefore, any time period can be recognized by its fossil content.

Fossils and Time Correlations Principle of Fossil Succession: Order ▫Age of Trilobites (early arthropods) ▫Age of Fish ▫Age of Coal Swamps ▫Age of Reptiles ▫Age of Mammals  “ages” correspond to a particular time period

Fossils and Time Correlations Obtaining numerical dates for geological past: ▫Radioactivity – unstable nuclei spontaneously break apart or decay  Decay continues until a stable or non-radioactive isotope is formed ▫Half-Life – necessary amount of time for half of the nuclei to reach a stable isotope

Fossils and Time Correlations ▫Radiometric Dating – calculating the ages of rocks and minerals that contain certain radioactive isotopes – it has provided 1000’s of dates for events in Earth’s history  Decay occurs at a constant rate  Decay accumulation occurs at a constant rate  Only accurate if mineral accumulation remains in a closed system (no loss of isotopes) ▫Carbon-14 - dates more recent events  All organisms contain a small amount of carbon-14  When organisms die amount of carbon-14 decreases  Carbon-12 is compared to Carbon-14 to determine age

Fossils and Time Correlations Geological Time Scale – Earth’s 4.5 billion year history divided into specific amounts of time ▫Eons – greatest expansion of time ▫Eras – division of eons  Paleozoic – “ancient life”  Mesozoic – “middle life”  Cenozoic – “recent life” ▫Periods – subdivision of eras ▫Epochs – smallest division of time

Rocks Rock – any solid mass of mineral or mineral-like matter that occurs naturally as part of our planet Three Types of Rock ▫Igneous ▫Sedimentary ▫Metamorphic

Rocks Igneous Rock – formed from hardening lava ▫Can be formed above or below the Earth’s surface ▫Classified based on texture and composition ▫ClassificationsTextures graniticcourse-grained basalticfine-grained andesiticglassy ultramaficporphyritic

Rocks Sedimentary Rock – solids settle out of a fluid such as water or air and eventually become cemented Two classifications ▫Clastic ▫Chemical

Rocks Metamorphic Rock – when existing rocks are changed by heat and pressure Classifications: ▫Foliated Metamorphic  Banded appearance ▫Nonfoliated Metamorphic  Not banded

Rocks How rocks come to be – The Rock Cycle - interactions between water, air, and land can cause rocks to change from one type to another. - driven by heat from Earth’s interior ▫1- magma forms beneath the Earth’s surface ▫2 – magma cools and solidifies to form igneous rock ▫3 – surface rocks are broken down into sediments ▫4 – sediments are compacted and cemented to form sedimentary rock ▫5 – sedimentary rock changes to metamorphic rock under extreme pressure and temperature conditions.

Minerals Must have the following to be a mineral: ▫Naturally Occurring ▫Solid Substance ▫Orderly Crystalline ▫Definite Chemical Composition ▫Inorganic

Minerals How minerals form: ▫4 major processes  Crystallization from magma  Minerals rich in iron, calcium and magnesium  Precipitation  Ex: halite, calcite  Pressure and Temperature  Ex: talc, muscovite  Hydrothermal Solution  Ex: quartz, pyrite