Geologic time the present is the key to the past

Unifor m itarianism
“The Present is the Key to the Past”
Laws of nature DON’T change with time
Past events explained & estimated by moder n
processes


Uniformitarianism


Original Horizontality
sedimentary rocks form in horiz ontal layers
parallel to Earth’s surface and will remain that
way unless disturbed
SILT
SAND
CLAY
SILT
CLAY


Horizontal Layers of the Grand Canyon


• Relative Time – determination of the age
of a rock or geologic event by comparing
with other rocks/geologic events

Can you infer who is older?

He is

3 Ways to Determine
Relative Time………?
Coming Right Up!!


Law of Superposition
In undisturbedsedimentary
sedimentary rock
or lava
flowsflows:
In undisturbed
rock
or lava

Top = youngest layer

Bottom = oldest layer


Law of Cross-Cutting
Relationships
Any feature that cuts across a rock or layer of
sediment is y ounger than the rock or layer it cuts


Example of CrossCutting

Intrusion

Intrusion- igneous
- igneousrock
rockthat
thatforms
formswhen
whenmagma
magma
squeezes
squeezesbetween
betweenexisting
existingrock
rockand
andhardens
hardens


Examples of Cross-Cutting

Example of Cross-cutting
Extrusion - igneous rock that forms when lava
flows on Earth’s surface and hardens


Example
Exampleof
ofCross-cutting
Cross-cutting
Inclusion - pieces of older rock trapped within
younger rock

Examples of Cross-Cutting
Faults, joints, tilts, and veins also follow the
law of cross--cutting relationships


Law of Included Fragments
Sediments are older than the cement
and rock they are part of
The pebble is
older than the
conglomerate rock
it is found in.


However, there
are exceptions to
these laws…….


Exceptions to Law of
Superposition
Rock layers can be overturned, older
layers pushed on top of younger layers

THEREFORE, geologists use these clues
to find the original position of rock
layers…

Exceptions to Law of
Superposition
Graded Bedding - coarse, heavy particles - b


Exceptions to Law of
Superposition
Ripple-Marks - tops of ripple marks poin


Exceptions to Law of
Superposition
Cross-Beds - curved at bottom layer, cut o


Unconformity

Gaps or missing layers in the rock record du

Angular
unconformit
y


Formation of an Unconformity


Formation of an Unconformity

Formation of an Unconformity


Formation of an Unconformity


Formation of an Unconformity


Picture of Unconformity