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Deep Time: How Old Is Old?
Understanding Geologic Time
Geologic Time

Discovering the magnitude of the Earth’s past was a
momentous development in the history of humanity.
Geologic Time

Understanding time
permits assigning an age
to…
 Rocks.
 Fossils.
 Geologic
structures.
 Landscapes.
 Tectonic
events.
Geologic Time

Deep time – The immense span of
geologic time is so vast that it is
difficult for people to grasp.

We think of time in terms of our
lives…
The lives of our parents and
grandparents.
The lives of our children or
grandchildren.

Human history is tiny compared
to geologic time.
Geologic Time

James Hutton (1726-1797), Scottish physician.

Called “the Father of Modern Geology.”

First to articulate the “principle of uniformitarianism.”

Of the abyss of time, Hutton wrote: “…we find no
vestige of a beginning; no prospect of an end.”
Geologic Time

James Hutton’s principle of uniformitarianism.

“The present is the key to the past.”
Processes seen today are the same as those of the past.
Ancient mudcracks formed as mudcracks do today.
Geologic change is slow; large changes require large amounts
of time.
Paleozoic mudcracks:
this is solid rock.
Geologic Time

There are two ways of dating
geological materials:
 Relative
ages – Based upon
order of formation.
 Absolute
ages – Actual
number of years since an
event.
Quantitative method.
Age is assigned a
number.
Relative Age

Logical tools are useful for defining
relative age.

Principle of uniformitarianism.

Principle of original horizontality.

Principle of superposition.

Principle of original continuity.

Principle of cross-cutting relationships.

Principle of baked contacts.

Principle of inclusions.
Geologic Time

Uniformitarianism – The present is the key to the past.
 Physical
processes that we observe today operated in the
same way in the geological past.
 Modern
processes help us understand ancient events.
Relative Age

Horizontality and continuity.
 Strata
often form laterally extensive horizontal sheets.
 Subsequent
 Flat-lying
erosion dissects once continuous layers.
rock layers are unlikely to have been disturbed.
Defining Relative Age

Superposition.

In an undeformed sequence
of layered rock each bed is
older than the one above,
 Younger
strata are on top;
older strata below.
Relative Age

Cross-Cutting Relations.

Younger features truncate (cut across) older features.

Faults, dikes, erosion, etc., must be younger than the material that is
faulted, intruded, or eroded.

A volcano cannot intrude rocks that aren’t there yet.
Relative Age

Inclusions – A rock fragment within another.
 Igneous
xenoliths – Country rock that fell into magma.
 Weathering

rubble – Debris from preexisting rocks.
The inclusion is older than the material enclosing it.
Relative Age

Baked contacts.
 Thermal
metamorphism occurs when country rock is
invaded by a plutonic igneous intrusion.
 The
baked rock must have been there first (it is older).
Relative Age

Determining relative ages empowers geologists to
easily unravel complicated geologic histories.
Geologic History

Relative ages help to unravel a complicated history

Simple rules permit one to decipher this diagram.
Geologic History

Deposition of horizontal strata below sea level in
order 1, 2, 3, 4, 5, 6, 7 and 8 (oldest to youngest).
Geologic History

An igneous sill intrudes.
Geologic History

Folding, uplift, and erosion take place.
Geologic History

An igneous pluton cuts older rock.
Geologic History

Faulting cuts the strata and the pluton.
Geologic History

A dike intrudes.
Geologic History

Erosion forms the present land surface.
Fossil Succession

Fossil remnants or traces of once-living organisms are often
preserved in sedimentary rocks.

Fossil are useful for relative age determination.
 Several
 Fossils
types of fossils will occur as an assemblage.
are time markers.
Fossil Succession

Species evolve, exist for a time, and then go extinct.

First appearance, range, and extinction are used for dating.

Fossils succeed one another in a known order.

A time period is recognized by its fossil content.
Fossil Succession

Fossil range – First and last
appearance.
 Each
fossil has a unique range.
 Overlapping
ranges provide
distinctive time markers.

Permit correlation of strata.
 Locally.
 Regionally.
 Globally.
Unconformities

An unconformity is a time gap in the rock record due
to non-deposition or erosion.

There are three types of unconformity: angular
unconformity, nonconformity, and disconformity.
Unconformities

Three unconformity types:

Angular unconformity – Represents a huge gulf in time.
Horizontal marine sediments deformed by orogenesis.
High mountains are eroded away to below sea level.
Sediments deposited horizontally on the erosion surface.
Angular Unconformity


James Hutton was the first to
realize the enormous timesignificance of angular
unconformities.

Mountains created.

Mountains completely erased.

New sediments deposited.
Incomprehensible time.
Mountains form and layers
fold, then erosion removes
the highland.
Angular Unconformity

“Hutton’s Unconformity” on Siccar Point, Scotland, is a
common destination for geologists.
 Vertical
beds of Ordovician sandstone.
 Overlain
by gently dipping Devonian redbeds.
 Missing
time? 50 million years.
Unconformities

Three unconformity types:
 Nonconformity
–
Metamorphic or igneous
rocks overlain by
sedimentary strata.
Crystalline ig/met rocks
were exposed by erosion.
Sediment was deposited
on this eroded surface.
Erosion removes cover, so
basement lies exposed at the
Earth’s surface.
Nonconformity
Cambrian Sawatch sandstone.
Pre-Cambrian Pikes Peak granite
near Manitou Springs, Colorado.
Unconformities

Three unconformity types:

Disconformity – Parallel strata bracketing non-deposition.
Due to an interruption in sedimentation.
May be difficult to recognize.
Sea level drops and flat-lying
strata are eroded.
Unconformities

Earth history is recorded in
strata.

Missing strata = missing
history.

The Grand Canyon:

Thick layers of strata.

Numerous gaps.

A partial record of
geological history.
Stratigraphic Correlation

Stratigraphic columns depict
strata in a region.
 Drawn
to scale to accurately
portray relative thicknesses.
 Rock
types are depicted by
graphical fill patterns.
 Divided
into formations
Mapable rock units.
 Formations
are
separated by contacts.
Stratigraphic Correlation

In 1793, William “Strata” Smith was the first to note that strata
could be matched across distances.
 Similar
 Rock

rock types in a similar order.
layers contained the same distinctive fossils.
After years of work, he made the 1st geologic map.
Stratigraphic Correlation


Lithologic correlation is based on rock type.

Sequence – The relative order in which the rocks occur.

Limited to correlation between nearby regions.
Fossil correlation – Based on fossils within rocks.

Applicable to much broader areas.
Stratigraphic Correlation

National Parks of Arizona and Utah.
 Formations
 Overlap
can be traced long distances.
is seen in the sequences of rock types.
 Overlapping
composite.
rock columns are used to build a
The Geologic Column

A composite stratigraphic column can be
constructed.

Assembled from incomplete sections across
the globe.

It brackets almost the entirety of Earth’s
history.
Geologic Time

The composite column is divided into time blocks.

This is the geologic time scale, Earth’s “calendar.”

Eons – The largest subdivision of time (100s to 1000s Ma).

Eras – Subdivisions of an eon (65 to 100s Ma).

Periods – Subdivisions of an era (2 to 70 Ma).

Epochs – Subdivisions of a period (0.011 to 22 Ma).
Geologic Time and Life

Life first appears on Earth ~ 3.8
Ga.

Early life consisted of anaerobic
single-celled organisms.

O2 from cyanobacteria built
up in atmosphere by 2 Ga.

~ 700 Ma, multicellular
life evolved.

~ 542 Ma marks the
1st appearance
of hard shells.
Origin of Life
Complex life appears
Cambrian E
Age of dinosaurs
Age of mammals
Numerical Age

Many relative ages can now be assigned actual dates.

Based on radioactive decay of atoms in minerals.


Radioactive decay proceeds at a known, fixed rate.

Radioactive elements act as internal clocks.
Numerical dating is also called geochronology.
Radioactive Decay

Isotopes – Elements that have varying #s of neutrons.

Isotopes have the same atomic number but different mass
numbers.
 Stable
– Isotopes that never change (i.e. 13C).
 Radioactive
14C).
– Isotopes that spontaneously decay (i.e.
Radioactive Decay

Radioactive decay progresses along a decay chain.
 Decay
creates new unstable elements that also decay.
 Decay
proceeds to a stable element endpoint.

Parent isotope – The isotope that undergoes decay.

Daughter isotope – The product of this decay.
Radioactive Decay

Half-life (t½) – Time for ½ unstable nuclei to decay.
 t½

is a characteristic of each isotope.

After one t½, one half of the original parent remains.

After three t½, one eighth of the original parent remains.
As the parent disappears, the daughter “grows in”.
Radiometric Dating

The age of a mineral can be determined by…

Measuring the ratio of parent isotopes to daughter isotopes.

Calculating the amount of time by using the known t½.

Must pick the right mineral and the right isotope.

Geochronology requires analytical precision.
What is a Radiometric Date?

Radiometric dates give the time a mineral began to preserve all atoms
of parent and daughter isotopes.

Requires cooling below a “closure temperature.”

If rock is reheated, the radiometric clock can be reset.

Ig / Met rocks are best for geochronologic work.

Sedimentary rocks cannot be directly dated.
Other Numerical Ages

Numerical ages are possible without isotopes.
 Growth
rings – Annual layers from trees or shells.
 Rhythmic
ice.
layering – Annual layers in sediments or
Dating the Geologic Column

Geochronology is less useful for
sedimentary deposits.

It can, however, constrain these
deposits.

Sediments can be bracketed by
numerical dates.

Yields age ranges that narrow
as data accumulates.

Defines major boundaries in the
geologic column.
The Geologic Time
Scale

Names of the Eons.

Phanerozoic “Visible life” (542 Ma to the present).
Started 542 Ma at the Precambrian / Cambrian boundary.
Marks the 1st appearance of hard shells.
Life diversified rapidly afterwards.

Proterozoic – “Before life” (2.5 to 0.542 Ga).
Development of tectonic plates like those of today.
Buildup of atmospheric O2; multicellular life appears.

Archean – “Ancient” (3.8 to 2.5 Ga).
Birth of continents.
Appearance of the earliest life forms.

Hadean – “Hell” (4.6 to 3.8 Ga).
Internal differentiation.
Formation of the oceans and secondary atmosphere.
The Geologic Time Scale

Names of the Eras.

Cenozoic – “Recent life.”
65.5 Ma to present.
The “Age of Mammals.”

Mesozoic – “Middle Life.”
251 to 65.5 Ma.
The “Age of Dinosaurs.”

Paleozoic – “Ancient Life.”
542 to 251 Ma.
Life diversified rapidly.
The Age of the Earth

Before radiometric dating, age
estimates varied widely.
 20
Ma – From Earth cooling.
 90
Ma – Ocean salinization.
If oceans started as fresh.
Unchanging mass of
dissolved material added by
rivers.
 Uniformitarianism
and evolution
indicated an Earth much older
than ~100 Ma.
The Age of the Earth

The oldest rocks on Earth’s surface date to 3.96 Ga.

Zircons in ancient sandstones date to between 4.1and
4.2 Ga.

Age of Earth is 4.57 Ga based on correlation with…

Meteorites.

Moon rocks.