Chapter 2 Early Geologists Tackle Historys Mysteries

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Chapter 2 Early Geologists Tackle Historys
Mysteries

• History of the Earth, that is
• These folks investigated sedimentary rocks
• Such rocks form in layers called beds or strata
• Each layer is the result of some natural event

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• Fundamental rules
• These were formulated during after the 1600s
• Most are common-sense now, but several were
  hotly debated at the time
• Many are attributed to specific investigators
  dont dwell on their names, their ideas are
  whats important

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• Nicholaus Steno (1600s)
• A Danish physician, later lived in Italy
• His employer allowed him freedom to visit
  quarries examine strata
• (back then, the more natural sciences you were
  involved in, the wiser you were thought to be)
• Came up with 3 basic principles, important in
  deciphering Earths history

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• Principle of superposition
• Basically, in a sequence of undisturbed layers,
  the oldest layer is at the bottom, and
  (successively) younger layers are (successively)
  higher
• Start pouring bags of sand gravel into your
  fish tank (with or without water)
• Material from the first bag will be at the bottom
  of the tank
• Each successive bags material will be
  successively higher (itll be very difficult to
  pour a flat layer underneath the material already
  there)
• Geologically, not always simple to decipher

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• Principle of Original Horizontality
• Sediments are deposited in layers that are
  originally horizontal
• Again, pour that bag of sand into your
  aquariumyou will get a (more or less) horizontal
  layer of sand
• Inclined layers suggest some kind of disturbance
  after deposition

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Horizontal layers
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Inclined layers
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• Principle of Original Lateral Continuity
• A layer of sediment (rock) extends continuously
  in all directions until it thins out or
  encounters a barrier
• Looking more closely at that sand you poured into
  the aquarium.
• Its thickest beneath where you pour it in, and
  thins out away from that point
• It stops at the outer glass of the aquarium
• (concept of geologic cross-sections)

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• Some other ideas creep in
• Neptunism
• All rocks were originally deposited/precipitated
  in a great ocean (even the continents were
  underwater)
• Where did all that water go?
• Also stated that lava flows formed from the water
• Plutonism
• Heat was the key to the origin of the primitive
  igneous-type rocks
• Only had problems with the crystalline rocks
  those we now call igneous or metamorphic

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• James Hutton (again, dont fret about his name)
• A Scottish physician, farmer, and geologist
  (1700s)
• Uniformitarianism
• Has to do with the physical chemical laws
  governing nature
• You can observe events today assume that those
  same phenomena have operated the same throughout
  geologic history
• Simply put the present is the key to the past
• Also came up with the concept of unconformities
  (more on these later)
• Published Theory of the Earth important
  publication, but was hard for most to follow.
  Suggested the immensity of geologic time

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• William Smith
• English surveyor engineer located routes of
  canals, designed marsh drainages, restored
  springs
• Noted that stratified rocks occurred in a
  definite order, and could identify them by their
  rock type, the soils formed, and fossils they
  contained
• Noted the precise locations of fossils the rock
  units they were found in
• Allowed him to trace strata from place to place
• Principle of Fossil (Biologic) Succession
• Fossils occur in a consistent vertical order in
  sedimentary rocks worldwide
• Has nothing to do with concept of evolution (50
  yrs later)

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• Others also looked at fossils in the rocks
• Large groups of strata separated by
  unconformities
• The fossils in the overlying strata looked
  dramatically different from those in the
  underlying ones
• Must have been great calamities
• Viewpoint of catastrophism great catastrophes
  happened
• Nothing uniform ever happened
• Current thoughts both uniform catastrophic
  events occur.just that, the uniform ones prevail

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• Sir Charles Lyell (early 1800s)
• Published Principles of Geology
• Expanded on work of Hutton
• Several ideas
• Principle of cross-cutting relationships
• A rock body, or a geologic structure (fault,
  unconformity) that penetrates/cuts an existing
  rock is younger than the rock cut
• Or, the feature that is cut is older than the
  feature that does the cutting

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• Principle of components/inclusions
• An inclusion in a rock is older than the rock it
  is included in
• Pieces of an eroded older rock included in a
  younger rock are called clasts
• Pieces of country rock included in an igneous
  intrusion are called xenoliths

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clast
xenolith
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Xenoliths in Stone Mountain Granite
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• Charles Darwin
• Unpaid member of expedition of H.M.S. Beagle
  around the world
• Had an hypothesis of change in organisms by
  natural selection
• Collected much data returned to England in 1836
• Came up with idea of natural selection
  (evolution), but did not publish until 1858,
  after someone else sent him a similar manuscript
  to review
• The idea was controversial even then
• And, he did not ever say that Man evolved from
  monkeys

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• Many others contributed to the knowledge-base
• Louis Agassiz Swiss glacial investigations
• James Hall strata in New York total thickness
  there was some 40,000 ft also noted the rocks
  had fossils indicating shallow water
• Geologists of the American West
• Those that dug dinosaur fossils
• Many others outlined in the text/online notes
• Dont worry about em too much