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