Lesson2 - Earth

Steno’s Laws of Stratigraphy

Historical Perspective of Stratigraphy

• First advances were made in the mid to late 1700s. This was due to advances in mining and canal building.

Historical Perspective of Stratigraphy

• Abraham Werner, mining geologist • In later 1700s he noted that the same strata

could be found in the same order at widely separated locations.

• Implied that local strata could hold clues as to how the global Earth had changed with time.

• Coined the term “Neptunism” which refers to a now obsolete theory of geo-stratification.

Historical Perspective of Stratigraphy

• William “Strata” Smith, English canal surveyor and consulting engineer (until 1799) and Father of Geology.

• Carefully examined strata along canals, roads, railway cuttings and quarries while crisscrossing the English countryside.

• Found that “the same strata were found always in the same order and contained the same fossils.”

William Smith (1815)

First Geologic Map of Great

Britain

Historical Perspective of Stratigraphy

• James Hutton (Founder of Modern Geology; 1726 -1797)

• Examining the sea coast in England he realized that strata are laid down by deposition of sediment in water.

• The sediment came from erosion of the continent.• Internal forces on the Earth later raised the strata

above sea level.• The cycle can repeat over and over.• Time to form a single strata layer from deposition is

many thousands of years.

Stratigraphy (Steno 1669)

• Law of Original Horizontality-infers that sedimentary rock layers were originally deposited as flat-lying (horizontal) layers.

Stratigraphy (Steno 1669)

• Law of Original Horizontality-infers that sedimentary rock layers were originally deposited as flat-lying (horizontal) layers.

• Law of Lateral Continuity-states that sedimentary rock layers are deposited over large areas

Stratigraphy (Steno 1669)

• Law of Original Horizontality-infers that sedimentary rock layers were originally deposited as flat-lying (horizontal) layers.

• Law of Lateral Continuity-states that sedimentary rock layers are deposited over large areas

• Law of Superposition-states that, in a cross-section view, rock layers are oldest at the bottom and become progressively younger upwards.

Stratigraphy (Steno 1669)• Law of Original Horizontality-infers that sedimentary rock

layers were originally deposited as flat-lying (horizontal) layers.• Law of Lateral Continuity-states that sedimentary rock layers

are deposited over large areas• Law of Superposition-states that, in a cross-section view, rock

layers are oldest at the bottom and become progressively younger upwards.

• Law of Cross-Cutting Relations-infers that a rock body (e.g. igneous dike) cutting through another rock body (sandstone beds) is younger than the layers it intrudes; that is, the igneous dike would be younger than the sandstone beds.

Original Horizontality

Lateral Continuity

Superposition

Younger Strata

Older Strata

Even Older Strata

Cross-cutting relations

Fracture is younger than strata because it cuts through

the strata.

Let’s recap

• The story in the rocks.

Something easy. Rank the layers from oldest to youngest

D

A

B

C

Something easy. Rank the layers from oldest to youngest

D, C, B, A from superposition

D

A

B

C

Little harder. Rank the layers from oldest to youngest

D

A

C

E

B

Igneous intrusion

Little harder. Rank the layers from oldest to youngestD, C, E, B, A from cross-cut relation and

superposition

Igneous intrusion

D

A

C

E

B

Little harder. Rank the layers from oldest to youngest

Igneous intrusion

D

A

C

E

B F

Fault

Rank the layers from oldest to youngest

D, C, E, B, A, F

Igneous intrusion

D

A

C

E

B F

Fault

What is the youngest feature?

D

A

B

C

ERiver Valley

What is the youngest feature?E from cross-cutting relation

D

A

B

C

ERiver Valley

Which feature is the youngest?

Which is the youngest?

0 of 150

1 2 3 4

0% 0%0%0%

1. E2. D3. B4. F

Which is older, D or A

Relative Age Date the Features from Oldest to Youngest

A

B

D

E

C

Relative Age Date the Features from Oldest to Youngest

C, E, B, D, A

A

B

D

E

C