[Q2] EarthSci - History of the Earth

• Uniformitarianism - present is the key to the past; main principles of the study of Earth's history
• Petrology - origin, composition, structure, and classification of rocks
• Stratigraphy - study of rock layers [strata] and layering [stratification]
• Paleontology - study of life that existed prior to, or sometimes at the start of, the Holocene epoch
• How Layers of Rocks (Stratified Rocks) Are Formed

  • Majority of rocks are sedimentary rocks formed from old rocks, broken down by water or by wind
  • Older rocks become sedimentary particles (gravel, sand, mud)
  • As time passes, particles accumulate, ones at the bottom become rocks
• • Gravel has become conglomerate
  • Sand has become sandstone
  • Mud is turning into shale or mudstone

  The animals or plants that have been buried with them become fossils.
• Methods to Determine the Age of Stratified Rocks

  • Relative Dating
  • Absolute Dating
• Relative Dating - method of arranging geological events based on the rock sequence; cannot provide actual numerical dates of rocks; only tells one rock is older than the other
• Nicholas Steno - studied the relative position of sedimentary rock; discovered they settled on the basis of their relative weight or fluid size; largest settle first, smallest settle last
• Beds - formations of layers of rock due to any slight change in particle size or composition
• Layering or Bedding - a distinct quality of sedimentary rocks
• Strata - layered rocks are also called this
• The relative order of geologic events can be established by applying the following basic principles:

  • Principle of Original Horizontality
  • Principle of Superposition
  • Principle of Cross-cutting Relationships
  • Principle of Inclusion
• Principle of Original Horizontality - deposited as horizontal or nearly horizontal layers; any deviation from horizontality indicates that deformation occurs after the deposition
• Principle of Superposition - in the sequence of sedimentary rocks, the layer at the bottom of the           sequence is the oldest, and the successively higher levels are successively younger
• Principle of Cross-cutting Relationships - geologic features like faults or igneous intrusions are younger than the rocks they cut across
• Principle of Inclusion – if rocks or rock fragments are included within another rock layer, the rock fragments must be older than the rock layer where they were embedded
• Absolute Dating - method that gives an actual date of the rock or period of an event; measures their radioactive decay; a radioactive rock isotope decays into a stable daughter isotope
• The older the rock is, the more daughter material there is.
• Radiocarbon dating for organic remains could date up to 60000 years.
• Radiometric Dating - accomplished by determining the absolute age of the sample on the basis of the ratio of parent material to daughter material
• Radiometric-Dating Methods based on the estimated age of an object

  • Potassium-argon Method
  • Uranium-lead Method
  • Rubidium-strontium Method
  • Carbon-14 Method
• Geological Time Scale - shows the geological time intervals based on the geological rock record, which describes the relationship between events that have occurred throughout Earth's history; sequence of events is based on radiometric data of igneous rocks associated with fossil-bearing sedimentary rocks
• Geologic Time Scale Order

  • Geologic Time Scale
  • Eons
  • Eras
  • Periods
  • Epochs
• Eon - largest unit of time; Earth is divided into 4 eons
• Era - eons may be divided into this; most recent eon is divided into 3 eras: Paleozoic, Mesozoic, Cenozoic
• Period - each era is subdivided into a number of this
• Epoch - periods of the Cenozoic, most recent era, are further divided into this
• Based on the subdivision of the geologic time scale, we are now living in the Holocene epoch, of the Quaternary period, of the Cenozoic era, of the Phanerozoic eon
• Precambrian Time - geological time begins with this; covers approximately 88% of Earth's history
• Cenozoic Era - began about 65 million years ago and continues today; sometimes called the "Age of Mammals"

  • Mammals began to increase and evolve adaptations that allowed them to live in different environments
  • Many mountain ranges formed
  • Growth of mountains cooled down climate
  • As climate changed, animals had to adapt to the rise and fall of the oceans
• • Paleozoic (Ancient) - 544 million years ago (lasted 300 million years)
  • Mesozoic (Middle) - 245 million years ago (lasted 180 million years)
  • Cenozoic (Recent) - 65 million years ago to present