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How do we know what happened first?
James Hutton (1726-1797) “Father of Modern Geology”
native of Edinburgh, Scotland educated as a medical doctor in Leiden (1749) passionate about scientific inquiry
Historical Developments
“Theory of the Earth” -- processes are slow; take a long time
Charles Lyell (1795-1875)• Scotsman who attended Oxford University • father was an avid naturalist • rebelled against prevailing thought of
“catastrophism”/”Neptunism”. “Principles of Geology” -- popularized Hutton’s views
idea of “uniformitarianism” -- same processes operating today occurred in the past
….the present is the key to the past….
study of timing of geologic events and processes is geochronology
Relative timeorder of events or objects from first (oldest) to last (youngest)she is older than he is; she was born first and he was born last
age of events or objects expressed numericallyshe is twenty-one and he is nineteen
Absolute time
Relative Time- “this rock is older than that”
Principles Used to Determine Relative Age Unconformities Correlation The Standard Geologic Time Scale Index Fossils
Absolute Time- “this rock is 28 million years old”
Principles of radioactive decay
apply simple concepts to determine…
• original horizontality
• superposition
• lateral continuity
• cross-cutting relationships
• inclusions
• unconformities
original horizontalityall beds originally deposited in water formed in horizontal layers
sediments will settleto bottomand blanketthe sea floor
Superposition: within a sequence of undisturbed sedimentary or volcanic rocks, layers become younger, upward
Lateral Continuity: original sedimentary layers extend laterally until it thins out at edges
rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous.
cross-cutting relationships
a disrupted pattern is older thanthe cause of the disruptione.g. an intrusion is youngerthan the rocks it intrudes
inclusions
fragments of other rocks contained in a body of rockmust be older than the host rock
e.g. 1) xenoliths in granite are olderthan granite and2) pieces of rock inconglomerate are older than conglomerate
unconformities
A gap in the geologic record -- “gap” may be an amount of time or amount of missing section
conformity
• relatively continuous deposition
• deposition of a sequence of parallel layers
• contacts between formations do not represent significant gaps in time
from: http://www.elohi.com/photo/grandcanyon
conformity
different types of unconformities
1. angular unconformity
• contact separates overlying younger layers from tilted older layers• sequence of layers is not parallel
• contacts between formations may represent significant amounts of time
angular unconformity
angular unconformityfrom: http://www.uakron.edu/envstudies/parks/rmgcan2.html
angular unconformity
different types of unconformities
2. disconformity
• contact separates beds (formations) that are parallel
• sequence of layers is parallel
• contacts between formations may represent significant amounts of time• missing time is difficult to recognize
different types of unconformities
3. nonconformity
• strata deposited on older crystalline (metamorphic/igneous) rock• erosion surface on igneous/metamorphic rock covered bysedimentary rocks
• large gap in geologic record
nonconformity
Unconformity Types Using Grand Canyon as Example
faunal succession (correlation by fossils)
How is this done?
index fossil short-lived organism; points to narrow range of geologic time
fossil species succeed one another through the layers in a predictable order
fossil assemblage group of fossilsassociatedtogether