Earth(age, fossils, and time)

• As geologists continue to study the Earth, they have found that it is older than previously thought
• Some 300 years ago, geologists thought that the Earth was about 6 000 years old
• Later, they discovered rocks that were formed 40 million years ago
• Today, scientists claim that the Earth is between 4 and 5 billion years old
• How scientists determined the Earth's age


  1. Study of rocks

  2. Changes in rocks occur slowly

  3. Can tell time involved in formation of rocks

  4. Can tell how long it has taken rocks to be worn away by erosion

  5. Size and position of rock layers tell about changes that have occurred
• Law of Superposition


  In undisturbed sedimentary rocks, each layer is younger than the layer below it and older than the layer above it
• Presence of igneous or volcanic rocks makes it more difficult to determine the age of rocks
• Radioactive dating


  Method to determine the Earth's age by measuring the amounts of uranium and lead present in rock samples
• Some rocks have been found to be 5 billion years old using radioactive dating
• Fossils


  Hardened remains or imprints of organisms that lived a very long time ago
• Fossils


  • They provide records of the past
  • They show when glaciers or seas covered the land, when volcanoes erupted, when mountains were pushed up, and the kinds of living things that existed millions of years ago
• Types of fossils


  • Footprints
  • Impressions of leaves or plants
  • Petrified or changed into rock
  • Whole bodies or skeletons of animals
• Ice and amber are materials that can prevent decay and preserve fossils
• Fossils reveal the sizes, shapes, appearances, habits, and habitats of living things from long ago
• Geologic time scale


  The story of living creatures divided into chapters showing the dominant families of living creatures that existed
• Eras of the geologic time scale

  • Precambrian
  • Paleozoic
  • Mesozoic
  • Cenozoic
• Precambrian era


  • Lasted from the beginning of the Earth to about 600 million years ago
  • Rocks were almost devoid of fossils
  • Primitive organisms like algae, sponges, and worms did exist
• Paleozoic era


  • Many fossils of marine invertebrates like trilobites and brachiopods
  • Development of shells, limestone, and coral reefs
  • First animals to adapt to breathe air were amphibians
  • Land plants and animals developed
• Mesozoic era


  • Formation of several continents
  • Great changes in plant and animal life
  • Dinosaurs were the largest creatures, some were carnivores and some were herbivores
• Cenozoic era


  • Warm-blooded animals like marsupials and primitive mammals appeared
  • Development of modern life forms like horses, birds, and deciduous trees
  • Start of the Pleistocene Ice Age
• Cold-blooded animals


  Animals not capable of regulating their body temperature
• Warm-blooded animals


  Animals capable of maintaining a nearly constant body temperature