Time and History of Earth Why study Earth history and time? Relative dating Absolute dating The Geologic Time Scale Ages and rates of earth objects and events.

Why Study Earth History? 1. Exploration for natural resources - ie, locations of tectonic/hydrothermal activity 2. Natural evolution of earth systems - ie, climate change, past to future 3. Assessing geological hazards - ie, risk = frequency x magnitude 4. Understanding our surroundings - ie, how did the Grand Canyon form?

Geologic Time Concept of geologic time is based upon the Principle of Uniformitarianism or Actualism. This concept assumes that the same processes and natural laws that have shaped Earth’s history in the past are operative today. “The present is the key to the past”

Timing of Events Relative time Absolute time chronological sequence of events without regard to age. Absolute time dates and rates of events in years.

Relative Dating Stratigraphic relationships Fossil sequence Key beds Original horizontality Superposition Cross-cutting and inclusions Unconformities Fossil sequence Faunal secession Index fossils Key beds

Stratigraphic Principles Original horizontality Sediments are deposited in horizontal layers parallel to Earth’s surface. Superposition Younger layers are deposited on top of older layers. Cross-cutting and inclusions Feature that cuts is younger than feature being cut, and inclusions are older than host rock.

Unconformities Represent interruptions in deposition. Disconformity - sedimentary layers are parallel above and below. Angular unconformity - older beds are tilted relative to younger beds. Nonconformity - sedimentary rocks lie on igneous or metamorphic rocks.

Fossil Sequence Principle of Faunal Succession Index Fossils Fossil organisms succeeded one another through time in a definite and recognizable order. Index Fossils Geologically widespread organism (fossil) that existed for a relatively short time interval, was abundantly preserved, and is easily identified in the field.

Key Beds Thin, widespread and easily recognizable rock/sediment layer. Often layers of volcanic ash from large volcanic eruptions. Useful for both relative and absolute dating.

Correlating Rock Units Physically separated rock units can be correlated using: Principle of lateral continuity Physical similarities Similar stratigraphic sequence/position Index fossils and fossil assemblages

Absolute Dating Used to provide an age for an event or object in years. Historical records few thousand years Tree rings/glacial ice core records 10,000 years Radioactive decay age of solar nebula

Radioactive decay Decay of unstable isotopes provides an internal clock for minerals and rocks. Unstable parent isotope spontaneously decays to daughter isotope at a constant rate. The decay rate is typically reported as the half-life of the unstable isotope, where the half-life represents the amount of time (in years) required for one half of an unstable isotope to decay.

Isotope Systems Commonly Used for Dating Rocks Isotope Half-life Dating range Materials Used (parent/daughter) (years) (years) 40K/40Ar 1.25 billion 50,000-4.6 billion mica,K-feldspar 238U/206Pb 4.5 billion 10 million-4.6 billion zircon, galena 235U/207Pb 713 million 10 million-4.6 billion zircon, galena 87Rb/87Sr 49 billion 10 million-4.6 billion mica, hornblende 14C/14N 5,730 100 - 70,000 organic material, trapped CO2

Geologic Time Scale Based upon fossil assemblages (absolute ages were determined after the scale was established). Consists of Eons, Eras, Periods, and Epochs Cenozoic Era - “age of mammals” Mesozoic Era - “age of reptiles” Paleozoic Era - “age of fishes and amphibians”

Important Events in Earth History Age of Earth 4.6 billion years Oldest Terrestrial Rock 4.26 billion years First Marine Organisms 570 million years ago First Land Plants 408 million years ago Dinosaurs 245 - 66 million years ago First Mammals 200 million years ago First Flowering Plants 140 million years ago Earliest Human Ancestors 4-5 million years ago

Rates of Earth Processes Magma Ascent (Mt. St. Helens) 15->66 m/hr Basaltic lava flow 0.05 -1 m/hr Sea-floor Spreading 30-80 mm/year Uplift of Mountains 1-8 mm/year Subsidence 0.2-1 mm/year Erosion of River Valleys 0.04-0.2 mm/year Accumulation of Deep Sea Sediments 0.001-0.01 mm/year

Review - Earth History What is relative time? How do we determine relative time? What is absolute time? How do we determine absolute time? What is the Geological Time Scale, what is it based on?