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How do we know what happened first?
Historical Developments
James Hutton (1726-1797) “Father of Modern Geology”
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native of Edinburgh, Scotland
educated as a medical doctor in Leiden (1749)
passionate about scientific inquiry
“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….
Relative time
order 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
Absolute time
age of events or objects expressed numerically
she is twenty-one and he is nineteen
study of timing of geologic events and processes is geochronology
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 horizontality
all beds originally deposited in water formed in horizontal layers
sediments will settle
to bottom
and blanket
the 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 than
the cause of the disruption
e.g. an intrusion is younger
than the rocks it intrudes
inclusions
fragments of other rocks contained
in a body of rock
must be older than the
host rock
e.g.
1) xenoliths in granite are older
than granite and
2) pieces of rock in
conglomerate 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
conformity
from: http://www.elohi.com/photo/grandcanyon
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
from: http://www.uakron.edu/envstudies/parks/rmgcan2.html
angular unconformity
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 by
sedimentary rocks
• large gap in
geologic record
nonconformity
Unconformity Types Using Grand Canyon as Example
How is this done?
faunal succession (correlation by fossils)
fossil species succeed one another through the layers
in a predictable order
index fossil
short-lived organism;
points to narrow range
of geologic time
fossil assemblage
group of fossils
associated
together