Unit 5: Relative dating rules
Uniformitarianism – The idea that the same processes that shape the earth’s surface
___________, are the processes that occurred in the ____________.
“THE PRESENT IS THE KEY TO THE PAST”
Cross-section A
Original Horizontality: Sedimentary rocks are deposited in flat, horizontal
layers.
Superposition: For SEDIMENTARY ROCK LAYERS, the oldest layer is on the
bottom and the youngest on top.
Cross-section B
Unconformities: An unconformity is a break in the sequence of the relative
time scale… Part of the record in the rock has been removed by erosion then
BURIED. An unconformity is a buried erosion surface.
Cross-section C
Faulting: If the rock layers were disturbed by faulting, the faulting occurred
AFTER the rocks were already there. The fault is YOUNGER than the rocks
it cuts through.
Cross-section D
Igneous Intrusions: If an intrusion of magma occurs within a group of rock
layers, the intrusion happened AFTER the other rocks were already present.
Igneous intrusions are YOUNGER than the rocks they cut through.
Contact Metamorphism: Magma intrusions are HOT and they cause the
nearby rock to BAKE and become contact metamorphic rock. These
BAKED ZONES are shown with “whiskers” around the intrusion.
Cross-section E
Intrusions Intruding Intrusions: Cross-cutting relationships;
If an intrusion cuts another, the cross-cutting intrusion is YOUNGER.
Cross-section F
Faults and Intrusions:
If an intrusion cuts through a fault, the fault happened first. If a fault
cuts through an intrusion, the intrusion was there first.
Cross-section G
Igneous Extrusions: Igneous intrusions that reach the surface are
called EXTRUSIONS. They will not bake any rocks above because
there ARE NO ROCKS….just air.
Cross-section H
Folding: Keeping original horizontality in mind…if a sedimentary
rock sequence is folded, the folding event is younger than the rock.
The folding happened AFTER the rock was there.
Cross-section I
Tilting: See “Folding” and insert “Tilting”.
Cross-section J
Put it all together.