Dating RocksDating Rocks

Ways to tell the age of a rock

Ways to tell the age of a rock

2 Ways to Date Rocks:2 Ways to Date Rocks:

Relative Dating: Places events in geologic history in the

proper order. The basis for the geologic time scale Mainly Sedimentary Rocks Does not provide a true “age”

Relative Dating: Places events in geologic history in the

proper order. The basis for the geologic time scale Mainly Sedimentary Rocks Does not provide a true “age”

2 Ways to Date Rocks:2 Ways to Date Rocks:

Absolute Dating: All you need is a tiny sample of material

(mineral, bone) no larger than a grain of rice. Gives us the true “age” of a fossil or rock Mainly organic tissue or igneous crystals Measure the amount of unstable isotopes that

have “decayed” to figure out age

Absolute Dating: All you need is a tiny sample of material

(mineral, bone) no larger than a grain of rice. Gives us the true “age” of a fossil or rock Mainly organic tissue or igneous crystals Measure the amount of unstable isotopes that

have “decayed” to figure out age

Principles for Relative Dating:

• Superposition - The youngest rocks are on the top, oldest at the bottom.

Relative Dating Principles• Superposition • Cross-cutting relationships - Geologic

features that cut through and across rocks are younger than those rocks.

• Mostly Faults and Igneous intrusions

Cross-cutting Igneous Rock

Cross Cutting Relationships - Geologic features that cut through and across rocks are younger than those rocks.

Relative Dating Principles• Superposition• Cross-cutting relationships• 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.

Relative Dating Principles• Superposition• Cross-cutting relationships• Law of Inclusions• Law of Original Horizontality (and

Lateral continuity) – layers of sediments are originally deposited horizontally

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Relative Dating Principles• Superposition• Cross-cutting relationships• Law of Inclusions• Law of Original Horizontality (and

Lateral continuity)• Law of Unconformities

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.

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 2: Label the layers from:

A The Oldest

T The Youngest

Activity 2: Label the layers from:

A The Oldest

T The Youngest