Dating RocksDating Rocks
Ways to tell the age of a rock
Ways to tell the age of a rock
What are relative and absolute Locations?
What are relative and absolute Locations?
2 Ways to Date Rocks:2 Ways to Date Rocks: Absolute Dating: Gives us the true “age” of a fossil or rock All you need is a tiny sample of material
(mineral, bone) no larger than a grain of rice. Mainly organic tissue or igneous crystals Measures the amount of unstable isotopes that
have “decayed” to figure out age
Absolute Dating: Gives us the true “age” of a fossil or rock All you need is a tiny sample of material
(mineral, bone) no larger than a grain of rice. Mainly organic tissue or igneous crystals Measures the amount of unstable isotopes that
have “decayed” to figure out age
2 Ways to Date Rocks:2 Ways to Date Rocks: Relative Dating: Places events in geologic history in the
proper order relative to one another. The basis for the geologic time scale Does not provide a true “age”
Relative Dating: Places events in geologic history in the
proper order relative to one another. The basis for the geologic time scale Does not provide a true “age”
Principles for Relative Dating:
1. Law of Superposition :
• In any undisturbed sequence of strata,
• the oldest layer is at the bottom
• the youngest layer is at the top.
Newest
Oldest
Principles for Relative Dating:
Law of Superposition :
• In any undisturbed sequence of strata,
• the oldest layer is at the bottom
• the youngest layer is at the top.
Relative Dating Principles
2. The Cross-cutting Law
• Any feature that cuts across a body of sediment or rock is younger than the body of sediment or rock that it cuts across.
Cross-cutting Igneous Rock
Cross Cutting Relationships - Geologic features that cut through and across rocks are younger than those rocks.
Relative Dating Principles1. Law of Superposition
2. Cross-cutting Law
3. Law of Inclusions - Rocks embedded in other rocks are older than those rocks they are embedded in.
http://www.earth.ox.ac.uk/~oesis/field/medium/xenolith-1365.jpg
Law of Inclusions - Rocks embedded in other rocks are older than those rocks they are embedded in.
Do Class 12: Relative Dating Exercise 1
Relative Dating Principles1. Superposition Law
2. Cross-cutting Law
3. Law of Inclusions
4. Law of Original Horizontality (and Lateral continuity) – layers of sediments are originally deposited horizontally
Relative Dating Principles
4. Law of Original Horizontality (and Lateral continuity) …. But stuff can
happen!
• Erosion• Earthquakes• Faults• Deposition
Relative Dating Principles
4. Law of Original Horizontality (and Lateral continuity) …. But stuff can
happen!
• Erosion• Earthquakes• Faults• Deposition
Relative Dating Principles
4. Law of Original Horizontality (and Lateral continuity) …. But stuff can
happen!
• Erosion• Earthquakes• Faults• Deposition
Types of Discomformities• Angular conformity: younger sediments rest
upon the eroded surface of tilted or folded older rocks.
• Disconformity: contact between younger and older beds is marked by a visible, irregular or uneven erosional surface.
• Paraconformity: beds above and below the unconformity are parallel and no erosional surface is evident; but can be recognized based on the gap in the rock record.
• Nonconformity: develops between sedimentary rock and older igneous or metamorphic rock that has been exposed to erosion.
A DISCONFORMITY is a boundary between two layers of non-continuous ages. This boundary is usually marked by an erosional surface and is often irregular.5.
M&W4 Fig. 17.8; M&W5 Fig. 17.8
An ANGULAR UNCONFORMITY is a disconformity between layers of different angles.
The underlying layers are first tilted, then erosion scours away a new, horizontal surface.
New, horizontal layers form on top
An NONCONFORMITY is a disconformity between different rock types, one of them sedimentary.
Activity 3: Relative Dating Exercise
A The Oldest
T The Youngest
Activity 2: Label the layers from:
A The Oldest
T The Youngest
KEY