1. UNIFORMITARIANISM:
-The processes that shape the Earth today are the same processes that
occurred in the geologic past
“The present is the key to the past”
2. ORIGINAL HORIZONTALITY
-Sedimentary rock layers are ALWAYS laid down in horizontal layers, until some other
process alters them
Original Horizontality
sedimentary rocks form in horizontal layers
parallel to Earth’s surface and will remain that
way unless disturbed
SILT
SAND
CLAY
SILT
CLAY
Horizontal Layers of the Grand Canyon
3. SUPERPOSITION
-The bottom layer of horizontal sedimentary rock layers is always the oldest,
UNLESS the layers have been overturned or have had older rock formed on
top of it
*The lower layers must first be in place
before the next layer can be deposited
YOUNGEST
OLDEST
For undisturbed rocks, the oldest layer is on
the bottom and the youngest is on top
Supai is oldest – WHY?
Law of Superposition
In undisturbedsedimentary
sedimentary rock
or lava
flows flows:
In undisturbed
rock
or lava
Top = youngest layer
Bottom = oldest layer
Unconformity
Gaps or missing layers in the rock record due to erosio
Angular
unconformity
1
2
3
Extrusion:
When molten rock flows on the Earth’s surface forming an igneous
rock
Intrusion:
Occurs when magma squeezes between layers of pre-existing rock beneath
the Earth’s surface
-Causes surrounding layers to metamorphose
(Contact metamorphism)
Igneous intrusions/extrusions that “cut across” or enter other rocks are
younger than the rocks they cross
Law of Cross-Cutting
Relationships
Any feature that cuts across a rock or layer of
sediment is younger 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
(extrusive)
Extrusion - igneous rock that forms when lava
flows on Earth’s surface and hardens
North shore of Lake Superior,
Ontario Canada
A
dark-colored
dike has
intruded into
older light
colored granite.
• The dike is
younger than the
granite.
Templin Highway,
Castaic, California
A
small fault
displaces
tilted beds.
• The fault is
younger than
the beds.
Key to Rock Types
Fossils of different Plants and Animals are
found in sedimentary rock layers. When
different fossils are found in the same layer,
they existed during the same time period.
Rock particles/fragments that are found in rock layers are older than the rock
layer it is found in.
Igneous intrusions that “cut across” or enter other rocks are younger than the
rocks they cross.
FOLDS AND FAULTS
Fold:
Folds and Faults are always
younger than the original
rock layers
-bends in rock layers produced by movements of the earth’s crust
Faults:
-breaks in the rock where shifting of rock layers has occurred, often
associated with earthquakes
Principles of Geology: Big Ideas
(you should know)
7
Laws of Geology
 Relative aging of rock using
Superposition
 Definitions of intrusion, geologic dike,
unconformity
 Matching rock layers using fossils
 Definitions: folding, fault, uplifting