Download Reading the Rock Record

Reading the Rock Record
 During the 17th and 18th centuries the doctrine of how the earth’s landscapes developed was called
castrophism.
 This was the belief that features such as mountains and canyons developed by catastrophic events such
as earthquakes
Hutton
 In late 1700’s James Hutton proposed a new view called uniformitarianism.
 It states that the physical, chemical and biological laws that operate today have also operated in the past
 Commonly expressed as “The present is key to the past”
 Hutton is known as the founder of Modern Geology.
Reading the rock record
 The ages of past events can be indicated in two ways. Relative time and absolute time.
 Absolute time- Provides a specific date for the occurrence of an event.
Examples of absolute time:
 Half Life
 Varves
 Rate of sedimentation
 Rate of erosion
 Tree Rings
Half Life
 The time it takes for ½ of the atoms of an element to change into another element.
 This is know as radioactive decay.
 Original element=parent material
 New element= daughter material
 Relative time- Places events in a sequence, but not identifying exact years.
 Relative age is often used in reading the rock record. It is determined by using three laws.
Laws of Position
 Law of Superposition
 Scientist know that sedimentary rocks usually form in horizontal layers. (stratification)
 This law states that in a sequence of undisturbed sedimentary rocks:
 the oldest layer is the one on the bottom and the youngest will be on top.
Laws of crosscutting relationships
 This law states that a fault or igneous intrusion is always younger than the rock layers it cuts through.
 Fault- a break or crack in the earth’s crust along which rocks shift position.
 Intrusion- a mass of igneous rock formed when magma is injected into overlying rocks then cools and
solidifies
Law of included fragments
 The pieces of one rock found in another rock must be older than the rock in which they are found.
Unconformity
 A place in the rock record where layers of rock are missing.
 These are gaps in geologic time
The fossil record
 Paleontologist- scientists who study fossils
 Paleontology- study of fossils
 Fossil- remains or traces of animals or plants
Fossilization
 It is a rare event because:
 dead organisms are usually completely devoured by predators
 dead organisms are usually decomposed by bacteria
 when organisms are eaten they are usually drug in different directions and may be difficult to identify
 may be crushed by overlying rocks.
Destroyed by the abrasion of wind, water, or sand
Buried in remote locations
Destroyed by natural disasters
Lack bones or hard skeletons
Kinds of Fossils
 Fossils may form in many ways
 usually only hard parts such as bones, shells and teeth are preserved
 In rare cases, an entire organism is preserved
 Fossils may also be replicas of the original organism or other evidence that life existed
 Original remains- actual remains of an organism
 Mummification- preservation by drying. Often found in deserts or caves.
 In mummification the organism does not decay because it can not survive without water.
 Amber- Hardened tree sap. Often preserves insects.
 Tar beds- formed by thick petroleum. Often covered with water. Animals come to drink and become
trapped. Ex. La Brea Tar Pits
 Freezing- Allows for the entire organism to be preserved.
Ex. Cave men, woolly mammoth.
Other fossils
 Replacement- example Petrifaction- original material is removed and replaced with minerals like calcite
and silica ex. Petrified wood
 Mold- reflects only the shape and surface markings of of the organism, not internal structure ( it is a
hollow cavity)
 Casts- hollow spaces of a mold are filled in
 Carbonization/impressions- fine sediment encases remains of an organism.
 As time passes, pressure squeezes out the liquid and leaves
a thin residue of carbon.
Fossils
Trace Fossils
 Evidence other than actual remains, may include tracks, trails, burrows or borings
 Coprolites- fossilized dung or waste material
 Artifacts- evidence of humans.
Index fossils
 Some fossils are typical at a particular time segment.
 Useful in identifying the relative age of the rock in which they occur.
 Four characteristics
 1. Must be easily recognized
 2. Widespread in occurrence
 3. Limited in time- existed for a short period of time
4. Abundant
 Key bed- a single rock layer with the same characteristics as an index fossil ex. Volcanic eruption