The Rock Record

Chapter 8, Section 1 - Determining Relative Age                                                                                                                                               

Uniformitarianism - The idea that geologic processes that occurred in the past can be explained by current geologic processes.
                                  The same processes take place on the Earth now, as did in the past.

Relative Age - The age of an object in relation to the ages of other objects.  (Indicates older/young, not years.)

Law of Superposition - In sedimentary rock layers, a layer is younger than the rocks below it, older than the rocks above it.
                                        This can change if the rock layers have been disturbed.

Principle of Original Horizontality - Sedimentary rock generally forms in horizontal layers.
                                                            Thus, if layers are not horizontal, they were probably deformed by crustal movements after they were formed.                                                            If left undisturbed, the layers will remain horizontal.

If rock layers have been folded, geologists look for clues as to their original position...
  • Graded Bedding - When sediment is deposited, large, heavy particles sink to the bottom.  If they are on top, the layers may have been overturned.
  • Cross-Beds - Sandy sediments form beds at an angle to the bedding plane.  These often have eroded tops and curved bottoms.
  • Ripple Marks - Small wave formed in sand from wind/water.  The crests point up, if the rock was undisturbed.

Unconformity - A break in the geologic record formed when rock layers are eroded or when sediments are not deposited for long periods of time.

In the diagram below...
A - Angular Unconformity - Rock layers are folded/tilted and then eroded.  New layers then form on top.
B - Disconformity - Rock layers are uplifted without being tilted, then eroded.  New layers then form on top.
C - Nonconformity - Unstratified igneous/metamorphic rock is uplifted and then eroded.  Sedimentary layers then form on top.
File:Block diagrams stratigraphic relations.jpg

Law of Crosscutting Relationships - The principle that a fault or body of rock is younger than any other body of rock that it cuts through.

Chapter 8, Section 2 - Determining Absolute Age                                                                                                                                              

Absolute Age - The numeric age of an object or event.

Rates of Erosion
  • Can be used to estimate absolute age.
  • Only practical for up to about 20,000 years.
    • ex. - Niagara Falls
  • Less practical for older features (i.e. - Grand Canyon), because erosion rates vary.
Rates of Deposition
  • Can be used to estimate absolute age.
  • On average, 30cm of sedimentary rock are deposited in 1,000 years.
  • Deposition doesn't always occur at the avg. rate.
    • ex. - floods, landslides
Varve Count
  • Varve - a pair of sedimentary layers (light, course layer & dark, fine layer) deposited in an annual cycle (like tree growth rings).
  • Common in glacial lakes.
  • Summer - lots of snow/ice melt, deposits lots of course sediment.
  • Winter - as water freezes, fine silt settles out.

Radiometric Dating
  • A method of determining the absolute age of an object by comparing the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope.
  • Isotopes - Atoms of the same element with different numbers of neutrons.
  • Radioactive isotopes - emit particles and energy at a constant rate as they change into more stable isotopes.
  • The amount of time it takes for half of a sample of radioactive material to break down by radioactive decay.
  • Different substances have different half-lifes.
Radiometric Dating Methods
Radiocarbon Dating            carbon-14 => nitrogen-14        half-life: 5,730 years            effective dating range: <70,000 years
Argon-Argon Dating            argon-39 => argon-40             half-life: 1.25 billion years    effective dating range: 10,000 to 4.6 billion years
Potassium-Argon Dating     potassium-40 => argon-40      half-life:1.25 billion years    effective dating range: 50,000 to 4.6 billion years


Carbon Dating
  • Carbon-12 is converted to Carbon-14 high in the atmosphere (due to solar wind/radiation).
  • Both types incorporated into CO2, which is taken in by plants during photosynthesis.
  • While alive, plants & animals have 12C and 14C in the same ratio as they exist in the atmosphere.
  • When they die, they no longer take in carbon.  Over time, 14C decays into 14N.
  • Scientists compare the ratio of 12C and 14C in the fossil to that in the atmosphere to see how much 14C has decayed.
  • They can use this ratio, plus half-life, to calculate how much time has passed.
  • Young rocks can be dated indirectly using carbon dating on organic material in the rock.
  • Effective on materials under 70,000 years old.

Chapter 8, Section 3 - The Fossil Record                                                                                                                                                         

Fossil - The preserved trace or remains of an organism that lived long ago.
              Fossils are usually preserved in sedimentary rock.

Paleontology - The scientific study of fossils.

Ways that fossils may form from remains include...
  • Mummification
    • Occurs in dry environments where most bacteria (which cause decay) would not survive.
    • Bodies desiccate with little rotting.
  • Amber
    • Fossilized tree sap.
    • Insects, feathers, leaves, etc.
  • Tar Seeps / Tar Pits
    • Thick petroleum seeps to the surface, often covered with water.
    • Animals that come to drink the water get trapped, eventually sink in and form fossils.
    • High ratio of predatory animals (drawn by the struggles of trapped prey animals).
  • Ice / Freezing
    • Ice and frozen soil (permafrost) protect and preserve organisms - bacteria doesn't survive the low temps, or becomes dormant.
    • Mammoths, early humans, etc.
  • Burial
    • Burial slows down the decay process, as it limits the amount of oxygen that can get to the dead organism.
    • Often, the hard bits (bones, shells) are preserved.
  • Petrification
    • Groundwater dissolves away organic bits from buried organisms, replacing them with minerals.
    • Creates a near-perfect replica out of minerals, such as silica, calcite or pyrite.
    • Petrified forest.
Types of fossils...
  • Carbon Films
    • When organic material partially decays, it leaves behind a think film of carbon which preserves the surface features.
    • Plants (leaves, stems, flowers) and fish.
  • Molds & Casts
    • Mold - An empty place left behind when a buried organisms dissolves away.
    • Cast - A replica of the original organism; formed when sediments fill the mold and harden.
    • Shells.
  • Coprolites
    • Fossilized dung/waste.
    • Can be cut into thin sections and observed under a microscope.  Tells what the animals were eating.
  • Gastroliths
    • Stomach stones.  Birds and some dinosaurs swallowed them to help grind up food in their stomachs.
  • Trace Fossils
    • A fossilized mark that formed in sedimentary rock by the movement of the animal in or on soft sediment.
    • Tracks, footprints, borings, burrows.

Index Fossil - A fossil that is used to establish the age of rock layers because it is distinct, abundant, widespread, and existed only for a short span of geological time.

Key Bed - A rock layer that is easy to identify and found over a large area, such as the K-T Boundary.


Chapter Assignment:
  • Read Chapter 8.
  • Pg. 222-223 #6-22, 24-25, 34-36.