29.2 RELATIVE TIME

PAST MISCONCEPTIONS

Before the 1700’s, scientist thought the Earth was ~ 6000 y.o. Given geologic processes take much longer to take shape they were able to get a beter idea of how old Earth was.

RELATIVE DATING

What is relative dating?

Relative Dating – The process of placing events in the sequence in which they occurred. Does not give exact dates, just ballpark figures.

RELATIVE DATING

The Principle of Superposition

Remember the Law of Superposition? Explain. Youngest Top, Oldest

bottom Intrusions can be

youngest depending on their penetration.

Embedded fragments are older than the rock that surrounds it. (ex. A pebble in conglomerate is older than the conglomerate itself.)

GAPS IN RELATIVE TIME

Unconformity Indicates

whether layers of rock are missing from strata (rock layers) sequence.

GAPS IN RELATIVE TIME

Unconformity Angular

unconformity Strata is tilted

during uplift then young flat strata is then deposited over older tilted strata.

GAPS IN RELATIVE TIME

Disconformity When layers are

eroded away and younger layers replace them causing gaps in the time sequence. (Optional writing) Ex.

Layers in a sequence is eroded then covered by a body of water. The sediments in the water settle to form new layers. Although there is a sequence, some of the “pages” are missing.

GAPS IN RELATIVE TIME

Nonconformity When sediments are

deposited on top of uplifted, eroded igneous rock.

ROCK LAYER CORRELATION

Can you define correlation?

Matching of rock layers from one area to another.

ROCK LAYER CORRELATION

Walking the Outcrop An outcrop is a

part of the rock layer that can be seen at Earth’s surface. Compare visible

rock of one area to another

Can be difficult in areas of high vegetation and dense soil.

ROCK LAYER CORRELATION

Matching Rock Characteristics

Simply compares rock characteristics such as color, appearance and composition

ROCK LAYER CORRELATION

Using Index FossilsIndex fossils are organisms that died during a

certain time period. Fossils in one layer of sediment will be different

(age wise) than one in another layer of sediment.

Four characteristics of index fossils Unique (Easily Identifiable) Abundant Widespread Existed for only a brief period

ROCK LAYER CORRELATION

Fossils as environmental indicators

Can indicate an areas past climate. (Optional) Ex. Coral

only forms in shallow, warm water b/t 30 N and 30 S. Coral discovered in an area that doesn’t normally have coral would indicate that an area would have been covered by warm, shallow water.

ROCK LAYER CORRELATION

Matching Key Beds Key beds are like

index fossils Have to be unique Widespread Easily recognizable

Ex. Volcanic eruptions spread across great distance

STRATIGRAPHIC MATCHING

This is simply matching multiple layers within a strata.