The Fundamental Principles of Geology (1.3.2)

Question 1

Methods of dating geological materials: Relative Ages

Answer 1

• Brought together concepts developed hundreds of years ago.
• Based on order of formation of features.
• Qualitative method.
• Assigns a relative order to events.

Question 2

Methods of dating geological materials: Absolute Ages

Answer 2

• Quantitative methods developed recently.
• Age is given as a number.
• Assign absolute dates to events.

Question 3

What is superposition?

Answer 3

• Rock layers laid down on top of another.
• Oldest layers at bottom, youngest at top.
• Determine relative ages of rock strata.

Question 4

What is origin horizontality?

Answer 4

• Sediments settle out of fluid by gravity.
• Causes sediment to accumulate horizontally.
• Sediment accumulation not favoured on slopes.
• Tilted sedimentary rocks likely to have been deformed.

Question 5

What is lateral continuity?

Answer 5

• Strata often form laterally extensive horizontal sheets.
• Spatially isolated.

Question 6

What are cross-cutting relations?

Answer 6

• Younger features truncate (cut across) older features.
• Faults, dykes, erosion etc must be younger than material that is faulting, intruded, or eroded.
• Volcano can’t intrude rocks that aren’t there.

Question 7

What are baked contacts?

Answer 7

• Igneous intrusions cooks invaded country rock.
• Baked rock must have been there first.
• Chilled margin formed (in younger, intrusive rock) at the contact, from rapid cooling.

Question 8

What are inclusions (rock fragment with another)?

Answer 8

• Weathering rubble must have come from older rock.
• Fragments (xenoliths) and igneous intrusions are older.
• Inclusions always older than enclosing material.

Question 9

What are uncomformities?

Answer 9

• Represent a time gap in rock record as result of non-deposition and erosion.
• Three types: Unconformity, nonconformity, disconformity.

Question 10

What are nonconformities?

Answer 10

• Igneous/metamorphic capped by sedimentary rocks.
• Crystalline igneous/metamorphic rocks exposed by erosion, sediment was deposited eroded on surface.

Question 11

What are disconformities?

Answer 11

• Parallel strata bounding non-deposition due to an interruption in sedimentation (pause in deposition, sea level falls, rises, erosion).
• May be hard to detect, sometimes detects from fossils.

Question 12

What is fossil succession?

Answer 12

• First appearance, range, and extinction used for dating.
• Fossils succeed one another in known order.
• Index fossils are diagnostic of particular geological time period.
• Fossils correlate strata locally, regionally, and globally.