Concept Review – Geologic Time
- Geologists are now able to assign fairly accurate dates to
events in Earth history. There are two main types of
geologic time: relative time and absolute time.
- Relative dating means that dates are placed
in their proper sequence or order without
knowing their specific age in years (like
knowing your mom is older than you without
knowing her age).
- Absolute dating is when you can attribute a
specific time to an event or specimen (like
knowing the exact date you were born helps
us know how old you are).
Concept Review: Relative Time
- At first, geologists didn’t know what kind of timeline
they were working with, so they created logical rules to
help date things relatively.
- Principle of Superposition:
In layers of sedimentary
rocks or lava flows, the
youngest layer is on top
and the oldest is on the
bottom (assuming nothing
has disturbed it since it
was deposited).
Concept Review: Relative Time
- Principle of Original Horizontality: Layers of sediment
are generally deposited in a horizontal position, then
moved or upturned.
Concept Review: Relative Time
-Principle of CrossCutting Relationships:
When a fault cuts
through other rocks,
or when magma
intrudes and
crystallizes, we can
assume the
intrusion/fault is
younger than the
rocks affected.
Concept Review: Relative Time
- Principle of Fossil Succession: Fossil organisms
succeed one another in a definite and
predictable order, so any time period can be
recognized by its fossil content.
Concept Review: Relative Time
- Principle of Inclusions:
An inclusion is older than
the rock that contains it.
Concept Review: Relative Time
-Principle of Lateral Continuity:
Sediments and lava flows are generally
laterally continuous unless something
breaks them or they taper off into
something else.
Unconformities
Something
happened here!
But what??
An unconformity is a surface that represents a break in the geologic
record. The rock unit(s) immediately above the break is/are much
younger than that/those below. Usually represent buried erosional
surfaces – parcel of geology is missing!
Formation of an
unconformity
Unconformities
Three types:
1) Disconformity: unconformity is parallel to layering, but there
is a gap in the geologic record. (Typically erosional and hard
to spot.) Look for weathering surfaces, boulders/pebbles of
older rock in younger.
2) Angular Unconformity: Younger strata overly older rocks that
were tilted/folded.
3) Nonconformity: A contact in which an erosional surface on a
plutonic or metamorphic rock has been covered by younger
sediments or volcanic rocks (i.e., unconformity separates
different rock types).
Disconformity
Angular Unconformity
Nonconformity