HISTORY OF THE EARTH

• What is the principle of uniformitarianism?


  The present is the key to the past, based on the idea that natural laws have remained the same throughout time.
• How do different types of rocks provide hints about Earth's past?

  Different rocks record various geological activities and environmental conditions.
• What are the three main types of rocks and their significance?

  • Igneous: Records volcanic activities and specific ages.
  • Metamorphic: Indicates plate movements and continental drift.
  • Sedimentary: Contains climate records, previous environmental conditions, and fossils.
• What is the composition of most rocks exposed at the Earth's surface?

  Most rocks are sedimentary, formed from particles of older rocks broken apart by water or wind.
• What does gravel become when it solidifies?

  Conglomerate.
• What does sand become when it solidifies?

  Sandstone.
• What does mud become when it solidifies?

  Mudstone or shale.
• What are the two main methods to determine the age of stratified rocks?


  • Relative Dating: Determines the sequence of events without specific ages.
  • Absolute Dating: Provides actual dates or date ranges in years.
• What is relative dating?

  Relative dating determines if one rock or geologic event is older or younger than another without knowing their specific ages.
• What are rock layers called in geology?

  Strata.
• What is stratigraphy?


  Stratigraphy is the science of strata, dealing with the characteristics of layered rocks and their relation to time.
• Who is Nicholas Steno and what did he study?

  Nicholas Steno studied the relative positions of sedimentary rocks and how solid particles settle from a fluid.
• What does Steno's Law of Original Horizontality state?

  Most sediments were originally laid down horizontally, although many layered rocks are no longer horizontal.
• How can the Law of Superposition be described?


  In a sequence of layered rocks, a given bed must be older than any bed on top of it.
• What does the Principle of Crosscutting Relations state?


  Any rock or fault that cuts across other rocks is younger than those it cuts across.
• What is the Law of Lateral Continuity?

  All rock layers are laterally continuous and may be broken up or displaced by later events.
• What does the Principle of Inclusion state?

  Any inclusion found in rock layers is older than the rock that contains it.
• What is an unconformity in geology?

  An unconformity is a surface that reflects a time of nondeposition or erosion, indicating a missing record of time.
• What does the Law of Faunal Succession state?

  Fossils found in rock layers occur in a definite sequence and in a predictable manner across different locations.
• Why are fossils important for relative dating?

  Fossils help determine the relative ages of sedimentary rocks based on the organisms that left their remains.
• What is biostratigraphy?


  • Study of the order in which fossils appeared and disappeared through time.
  • Helps in understanding the relative ages of rocks.
• What is correlation in geology?

  • A matching process where fossils help match rocks of the same age.
  • Useful even when rocks are found far apart.
• What are index fossils?

  Index fossils are any animal or plant preserved in the rock record that is characteristic of a particular span of geologic time or environment.
• What is the most abundant index fossil?

  Trilobites.
• How did nineteenth-century geologists and paleontologists view the age of Earth?

  They believed Earth was quite old but had crude ways of estimating its age.
• What discovery allowed for the assignment of ages of rocks in thousands, millions, and billions of years?

  The discovery of radioactivity.
• What is absolute dating?

  • Absolute dating methods provide actual dates or date ranges in years.
  • Different from relative dating, which only orders geological events.
• How do scientists determine the absolute ages of rocks and fossils?

  Scientists analyze isotopes of radioactive elements.
• What are isotopes?


  Isotopes are atoms that have the same number of protons but differ in the number of neutrons in the nucleus.
• What are radioisotopes?


  Radioisotopes are radioactive isotopes of an element with an unstable combination of neutrons and protons.
• How can the relative amounts of stable and unstable isotopes determine age?

  By analyzing the ratio of parent isotopes to daughter isotopes during radioactive decay.
• What is a parent isotope?

  An unstable radioactive isotope that breaks down into a stable daughter isotope.
• What is a daughter isotope?

  A stable radioactive isotope that results from the decay of a parent isotope.
• What is half-life?


  Half-life is the time needed for half of a sample of a radioactive substance to undergo radioactive decay.
• What are the half-lives of common isotopes?

  • Uranium-235: 704 million years (Lead-207)
  • Potassium-40: 1.25 billion years (Argon-40)
  • Uranium-238: 4.5 billion years (Lead-206)
  • Thorium-232: 14.0 billion years (Lead-208)
  • Lutetium-176: 35.9 billion years
  • Rubidium-87: 48.8 billion years (Strontium-87)
  • Samarium-147: 106 billion years (Neodymium-143)
• If there are 60 grams of Np-240 present, how much Np-240 will remain after 4 hours if its half-life is 1 hour?


  1. 5 grams will remain after 4 hours.
• If a sample of Cf-251 originally contained 100g, how much will remain after 800 years?

  1. 25 grams will remain after 800 years.
• If a sample of Ac-225 originally contained 8.0 µg and has a half-life of 10 days, how much remains after 720 hours?

  0.5 µg will remain after 720 hours.
• If a sample of Rubidium-87 originally contained 235 g, how much will remain after 36,000 minutes if its half-life is 5 days?

  Approximately 29.375 g will remain after 36,000 minutes.
• How long will it take for 18.0 grams of Ra-226 to decay to 2.25 grams if its half-life is 1600 years?

  4800 years will be needed for the decay.