THE HISTORY OF THE EARTH

• One of the Geology’s objectives is to reconstruct the Earth’s history.

• Geological events generate changes.

• Those changes leave prints or evidence in the rocks (Earth’s history is written in its rocks)

TILLITAS CRÁTER DE IMPACTO

PLIEGUES

RIPPLES DE OSCILACIÓN CAPA DE CENIZAS VOLCÁNICAS FALLA NORMAL

Geologic processes and natural laws now operating to modify

the Earth's crust have acted in the same regular manner and

with essentially the same intensity throughout geologic time,

and that past geologic events can be explained by phenomena

and forces observable today; the classical concept that 'the

present is the key to the past'."

Uniformitarianism Principle (principio

del actualismo o uniformismo)

The doctrine of Uniformitarianism was significantly advanced by James Hutton (1726-1797) in his publication, Theory

of the Earth (1785). Hutton influenced Sir Charles Lyell (1797-1875), who is acclaimed as the father of modern geology

with his work, Principles of Geology (1830-1833, a three volume work). Lyell, in turn, influenced Charles Darwin, who

later wrote The Origin of Species (1859). Lyell is responsible for the general acceptance of Uniformitarianism among

geologists for the past 150 years.

(James Hutton and Sir Charles Lyell)

• What for? To reconstruct a history we should chronologycally order the events.

• 2 strategies:

– To say what happened before and what happened afterwards = Relative dating.

– To directly measure the age of the events/rocks/fossils = Absolute dating

Dating (Estimation of the age of an event or object by placing it in a specific

time period)

Relative dating principles

1) Principle of superposition = sediments are deposited in horizontal layers (strata). A stratum is newer than those below it, and older than those above it.

(Nicolas Steno, 1669)

Sedimentary basin

strata

• A stratum has the same age all along its lenght (principle of lateral continuity)

A geologist following the lenght of a stratum

STRATIGRAPHY (the science that studies strata)

• We can easily order strata if no changes have happened from its deposition.

THE OLDEST

THE NEWEST

But how can we order them if changes have happened?

2) Principle of cross-cutting: Any geological event is younger than the rocks it affects, and older than the rocks it does not affect.

Order chronologycally the strata (A, B, C), the volcano, the erosion and

the magma column.

C

A

B

Erosion

Magma column

Volcano

Relative dating principles

DISCONFORMITY (= discordancia): strata D and E are discordant.

What happened between them? An episode of erosion.

How many discordancies are there?

Between the sedimentation of the oldest and the newest strata,

there has been a folding phase (orogeny) and a subsequent

erosion phase, which surface is forming an unconformity with the

newer strata.

Fossils present in an stratum where formed at the same age

that the stratum they are in.

3) Faunistic sucession principle: two strata that contain

the same type of fossils are the same age.

Relative dating principles: STRATIGRAPHY CORRELATION

Guide fossils (=fósiles guía): Belong to species that lived for short

periods of time, but colonised large areas.

Have a look at the table in page 60.

4) Principle of lateral continuity: we can compare stratigraphic

columns that are relatively close together and look for recognisable

strata (same type of rocks) that would be the same age. It is based

in the idea that a stratum has the same age all along its lenght.

Relative dating principles: STRATIGRAPHY CORRELATION

ABSOLUTE DATING: RADIOACTIVE DATING

Unstable radioactive isotopes (parent isotopes) transform into their

stable isotopes (daughter isotopes) at a constant speed, in a process

called radioactive disintegration,.

Each type of parent isotope has a different half-life, which is the time

needed to have half of the mass of the parent isotope disintegrated.

RADIACTIVA

• When we know the half-life of a parent isotope (unstable one), we can measure the quantity of it and of its daughter isotope (stable one) in a given mass of rock, and calculate the age of the rock.

• Activity 30.

ABSOLUTE DATING: RADIOACTIVE DATING

The age of the Earth has been calculated using the isotope

pair uranium-lead, which half life is closed to the Earth’s age

(around 4600 millions of years)

El tiempo geológico se divide en tramos cuya separación se

realiza utilizando grandes sucesos biológicos o geológicos.