Chapter 8, Section 1 Notes
Determining Relative Age
1. Uniformitarianism – states that current geologic processes, such as erosion and
volcanism, are the forces and processes that shape the Earth now, as well as in the past.
-based on his observations of how slowly these forces worked on the landscape
Hutton hypothesized that Earth was millions, if not billions of years old.
2. Relative age – refers to the general order in which rock layers are laid down. The
younger rock is on top of the older rock, but does not give a specific age.
3. Law of Superposition – refers specifically to sedimentary rock. The older sedimentary
rock layer are at the bottom, followed by a younger rock layer over those, and then the
youngest layer at the top. Where two distinct layers of rock meet is called a bedding
plane.
4. Principle of original horizontality – it is known that sedimentary rock forms
horizontally and if left undisturbed will remain horizontal. In some cases tectonic forces
change the position of the sedimentary rock by tilting it or flipping it over. In those cases
scientists must look for other clues as to the original position of this sedimentary rock.
-Graded bedding – refers to the heaviest and largest particles settling out at the
bottom of a layer of rock.
-Cross-beds – as sandy sediments form rock, the top of the layer will be eroded
before the next layer can be started and this leads to this distinct pattern.
-Ripple Marks – are small waves that form due to wind or water action and then
are preserved as the sedimentary rock hardens.
5. Unconformities – when there is a break in the geologic record when examining rock
layers.
Nonconformity – igneous or metamorphic rock is uplifted to the surface, then
eroded some before sedimentary rock form over it.
Angular unconformity – sedimentary rock is tilted, then eroded some before more
sedimentary rock forms over it.
Disconformity – when older sedimentary rock layers are exposed to erosion for
some time before sedimentation resumes. Even though all layers are horizontal
it is obvious that there is a bedding plane between the older and younger layers
of sedimentary rock.
-Cross-cutting Relationship – when rock layers have been disturbed by faults or
igneous intrusions.