AGE DATING THE AGE DATING THE EARTHEARTH

Geologic Techniques andGeologic Techniques and

The Geologic Time ScaleThe Geologic Time Scale

INTRODUCTIONINTRODUCTION• Speculations about the ‘nature’ of the Earth as well as the Age of the

Earth inspired much of the lore and legend of early civilizations. • In the 3rd century B.C., Eratosthenes depicted a spherical Earth and

even calculated its diameter and circumference– The concept of a spherical Earth was beyond the imagination of

most men at that time. – Only 500 years ago, sailors aboard the Santa Maria begged

Columbus to turn back lest they sail off the Earth's "edge."• Herodotus, ancient historian, made one of the earliest recorded

geological observations in the 5th century B.C – Fossil shells found inland in Egypt and Libya

• He suggested Mediterranean Sea had once extended farther south

– Few believed him, however, and this idea did not catch on. • Similar opinions and prejudices about the nature and age of the Earth

have waxed and waned through the centuries• Even today - traditional beliefs among certain religious groups

suggest the Earth is quite young--that its age might be measured in terms of thousands of years, but certainly not in millions.

AGE OF THE EARTHAGE OF THE EARTH

• Scientists have established the age of the Earth as 4.54 billion years old.

HOW DO GEOLOGISTS KNOW HOW DO GEOLOGISTS KNOW THE AGE OF THE EARTH?THE AGE OF THE EARTH?

• The evidence to age-date the Earth is concealed in the rocks that form the Earth's crust and surface.

• The rocks are not all the same age -- or even nearly so -- but, like the pages in a long and complicated history book, rocks record the Earth-shaping events and life of the past.

WHAT DO GEOLOGISTS USE?WHAT DO GEOLOGISTS USE?• Two time-measuring systems are used to

date past Earth-shaping events and to measure the age of the Earth – (1) RELATIVE DATING: A ‘relative time’

measuring system • based on the sequence of layering of the rocks and

the evolution of life as ‘recorded’ in the rocks

– (2) ABSOLUTE DATING: A ‘radiometric time’ measuring system

• based on the natural radioactivity of chemical elements in some of the rocks

RELATIVE DATING: NICOLA STENO’S LAWS RELATIVE DATING: NICOLA STENO’S LAWS and STRATIGRAPHY (BEDROCK LAYERS)and STRATIGRAPHY (BEDROCK LAYERS)

Principle of Original Horizontality- Bedrock layers formed from sedimentary material are deposited in a horizontal position under gravity - Any deviations from this position are due to outside forces disturbing the rocks later.

• Law of Superposition– Wherever uncontorted (no faults, no

folds) layers are exposed at the surface, the bottom layer was deposited first and is the oldest layer exposed

– Each succeeding layer, up to the topmost one, is progressively younger.

– In any rock layer, each layer represents a specific interval of geologic time.

Nicola Steno – 1638 - 1686

THE BEGINNING – JAMES HUTTON THE BEGINNING – JAMES HUTTON “FATHER” OF MODERN GEOLOGY“FATHER” OF MODERN GEOLOGY

• In the late 18th century, James Hutton, a Scottish geologist, proposed a fundamental principle in Geology:

• Uniformitarianism– A theory that natural agents

(wind, rain, etc.) at work on and within the Earth have operated with general uniformity through immensely long periods of time

– Uniformitarianism went directly against “Catastrophism”:

• Prevailing thought that all Earth’s features were formed by catastrophic events

Picture at right may have been Hutton’s first clue as to age of material with later disruption, assuming that materials on bottom were originally deposited horizontally and then uplifted much later

Charles Lyell – “Principles of Charles Lyell – “Principles of Geology”Geology”

• Charles Lyell, in the 1830s, followed up on James Hutton’s work.

• Lyell viewed the history of the Earth as vast and directionless.

• Lyell worked from the theory of “Uniformitarianism” not “Catastrophism” – Lyell found evidence that valleys were formed

through the slow process of erosion, not by catastrophic floods.

– Changes to the Earth’s surface were gradual and over time, great changes could be effected: the Earth was vastly old.

• “The present is the key to the past’

• Darwin used Lyell’s “Principles of Geology” to decipher the volcanic rocks on the Canary Islands

WILLIAM ‘STRATA’ SMITH WILLIAM ‘STRATA’ SMITH – Fossil Record – Fossil Record and Relative Time Scaleand Relative Time Scale

• William "Strata" Smith, a civil engineer and surveyor in the early 19th century, collected and cataloged fossil shells from rocks

• Discovered that certain layers contained fossils unlike those in other layers – “Index Fossils”– Index Fossils existed in limited periods of

geologic time, but were widespread geographically

– They can be used as guides to age of rocks• Age-dating the fossils also provides an age-

date for the rock layer in which they were found.

PUTTING IT ALL TOGETHERPUTTING IT ALL TOGETHER• DEVELOPING A RELATIVE TIME SCALE• Studying origin of rocks (petrology),

combined with studies of rock layers (stratigraphy) and studies of the evolution of life (paleontology), allow geologists to reconstruct the Earth using four basic principles.– Original Horizontality– Superposition– Lateral continuity– Cross-cutting relationships

GEOLOGIC TIME AND DATING – 4 BASIC PRINCIPLES OF RELATIVE DATING

(1) Principle of Original HorizontalityBeds of deposited sediment form as horizontal or nearly horizontal layers.

(2) Principle of SuperpositionWithin a sequence of undisturbed sedimentary or volcanic rocks, the layers become younger going from bottom to top.

(3) Lateral ContinuityAn original sedimentary layer extends laterally until it tapers or thins at its edges

(4) Cross-cutting RelationshipsA disrupted pattern is older than the cause of the

disruption.

PRINCIPLE OF ORIGINAL HORIZONTALITY PRINCIPLE OF ORIGINAL HORIZONTALITY and SUPERPOSITIONand SUPERPOSITION

Principles of DatingPrinciples of Dating

RELATIVE DATING - How it worksRELATIVE DATING - How it works• Correlation by Physical Continuity

– Physically tracing the course of a rock unit to correlate rocks between two different places

• Correlation by Similarity of Rock Types– Correlation of two regions by assumption that similar rock types in two

regions formed at same time, under same circumstances

• Correlation by Fossils– Plants and animals that lived at the time rock formed were buried

by sediment

– If there are fossil remains preserved in the layers of sedimentary rock: fossils nearer the bottom (in older rock) are more unlike those near the top (in younger rock)

– Observations formalized into Principle of Faunal Succession –

fossil species succeed one another in a definite and recognizable order.

– Index Fossil – a fossil from a short-lived, geographically widespread species known to exist during a specific period of geologic time.

CORRELATION OF ROCK UNITSCORRELATION OF ROCK UNITS• Each column represents the

sequence of sedimentary beds at a specific locality

• The same beds are bracketed within the lines connecting the three columns.

• Adjoining beds that possess similar or related features (including fossils) are grouped into a single, more conspicuous unit called a formation

FAUNAL SUCCESSIONFAUNAL SUCCESSION

Faunal SuccessionFaunal Succession

INDEX FOSSIL CHARTINDEX FOSSIL CHART

Relative Dating: Relative Dating: “Reading” the Sedimentary Layers“Reading” the Sedimentary Layers

• When ‘reading’ the sedimentary layers, geologists not only look at the layers and the fossils within them (‘what’s there”) - geologists also look for an Unconformity (“what’s missing”): – An Unconformity is a surface representing a ‘gap’ in the

geologic record

• Types of Unconformities:– Disconformity – parallel strata missing a layer– Angular Unconformity – younger horizontal layer overlying

a folded or tilted layer– Nonconformity – a plutonic/metamorphic rock layer covered

by younger sedimentary or volcanic rock

Formation of An UnconformityFormation of An Unconformity

Discnformity

Disconformity

Angular unconformity

Angular UnconformityAngular UnconformityExamples of angular unconformities

DISCONFORMITYDISCONFORMITYDisconformity, Death Valley, California. Disconformities are unconformities in which the younger material is roughly parallel to the contact.

This photograph shows the rocks being parallel on the left side. However, on the right it shows the gravel cutting down intothe marble to indicate erosion.

Photo is approximately 1 meter across.

DISCONFORMITYDISCONFORMITY• The upper 2/3 of the cliff is

Redwall Limestone, whereas the lower part is Cambrian carbonate rock

• The age difference between these units is roughly 150 Ma.

• The contact between the two rock units represents a significant span of geologic time and is termed a disconformity

NON CONFORMITYNON CONFORMITY

Stratified rocks upon unstratified rocks (sedimentary rocks overlying metamorphic or plutonic rocks).

ABSOLUTE DATING – RADIOMETRICABSOLUTE DATING – RADIOMETRIC• Radiometric dating –“Absolute Dating” - based on radioactive

decay of ‘isotopes’ – An isotope is a form of an element containing different atomic mass (Carbon-12 vs Carbon-14, for example)

• Same number of protons, but different number of neutrons in ‘nucleus’’• Most isotopes are radio-active and unstable

• Radioactive decay:– any number of processes by which unstable isotopes emit

radioactive particles and eventually become stable elements• Radioactive decay rate can be quantified because it occurs at a

constant rate for each known isotope and is measured in ‘half-life’– ‘Half-life’ is the time required for a quantity of radio-active material to

decay to half of its initial value • Unstable radioactive (“Parent”) isotope → stable, non-radioactive

(“daughter” ) isotope– The half-lives of isotopes have all been measured directly

• Using a radiation detector to count the number of atoms decaying in a given amount of time from a known amount of the parent material

• Measuring the ratio of ‘parent-to-daughter’ atoms in a sample that originally consisted completely of parent atoms

• Measuring ratio of ‘parent-to-daughter’ isotopes determines absolute ages of some rocks.

RADIOMETRIC DATINGRADIOMETRIC DATING

• The decay of Radio-active atoms compares to sand grains falling in an hourglass.

• You cannot predict when the individual sand grain will fall, but you can predict from one time to the next how long the whole pile of sand takes to fall to the bottom.

• Similarly, you can predict how long it takes for all the radio-active atoms in a given amount of rock to decay to a non-radioactive form.

RADIOMETRIC DATINGRADIOMETRIC DATING

In exponential decay the amount of Parent material decreases by half during each half-life ; rapidly at first, then slowly with each succeeding half-life.

The daughter  element or isotope amount increases rapidly at first and more slowly with each succeeding half life

ABSOLUTE DATING ISOTOPESABSOLUTE DATING ISOTOPES• URANIUM–LEAD (U238→Pb206)

– Half-life: 4.5 billion years– Dating range: 10 million – 4.6 billion years

• URANIUM–LEAD (U235 →Pb207)– Half-life: 713 million years– Dating Range: 10 million – 4.6 billion years

• POTASSIUM-ARGON (K40→Ar40)– Half-life: 1.3 billion years– Dating Range: 100,000 – 4.6 billion years

• CARBON-14 (C14→N14)– Half-life: 5730 years– Dating Range: 100 – 40,000 years

Absolute Dating – Half-lifeAbsolute Dating – Half-life

Uranium half-life Radio-carbon half-life

Radio Carbon – CarbonRadio Carbon – Carbon 14 14

• All living organisms absorb radiocarbon (C14), an unstable form of carbon.

• After death and fossilization, C14 continues to decay without being replaced (half-life of about 5,730 years).

• Radiation counters are used to detect the electrons given off by decaying C14 as it turns into nitrogen (N14).

• Remaining amount of C14 is compared to the remaining amount of C12, the stable form of carbon, to determine how much radiocarbon has decayed to date the fossil.

Radiocarbon DatingRadiocarbon Dating

Relative and Radiometric DatingRelative and Radiometric DatingUsing Relative and Radiometric Dating together gives the most accurate time-scale for geologic time

Absolute Dating – non-Radiometric Absolute Dating – non-Radiometric - Dendrochronology- Dendrochronology

• Annual growth of tree rings– Dating back 11,500

years – Holocene Epoch

Principles of DendrochronologyPrinciples of Dendrochronology

• The dating of past events (climatic changes) through study of tree ring growth

• A chronology (arrangement of events in time) can be made by comparing different samples

Cross-dating in DendrochronologyCross-dating in Dendrochronology

• Process of matching rings of trees in an area based on patterns of ring widths produced by regional climate.

• More accurate age than ring counting

Cross-dating techniquesCross-dating techniques

Absolute Dating -Varve ChronologyAbsolute Dating -Varve Chronology

• Parallel strata deposited in deep oceans or lakes

• Varves are a pair of sedimentary layers deposited on seasonal cycles– Winter/summer

• Date back to over 200 million years

VarvesVarves

Geologic Time ScaleGeologic Time Scale• Fossils in rock used to age date rocks• Time scale consists of periods of time broken into

smaller and smaller units: eons (100s of millions of years), eras, periods, epochs (millions to thousands of years)– Eons, eras, periods and epochs are listed with

oldest at the bottom of the scale and youngest at the top

• Names of eras, periods and epochs based on global location– PreCambrian: from rocks near Wales “Cambria”– Jurassic: from limestone found in Jura Mountains,

France

THREE MAJOR ERAS IN THREE MAJOR ERAS IN GEOLOGIC TIME SCALEGEOLOGIC TIME SCALE

• (1) Paleozoic Era – appearance of complex life– Approximately 600 million years ago to 250

million years ago)• (2) Mesozoic Era – Age of Dinosaurs

– Approximately 250 million years ago to 65 million years ago)

• (3) Cenozoic Era – Age of Mammals – Approximately 65 million years ago to

present

Red Arrows point to mass extinction dates

TERTIARY

The Great Permian ExtinctionThe Great Permian Extinction • At the end of many large time

units on the Geologic Time Scale, mass extinctions took place.– Index Fossils used for dating,

remember• The end of the Permian,

approximately 250 million years ago (also the end of the Paleozoic era), was marked by the greatest extinction of the Phanerozoic eon.

• During the Permian extinction event, whose cause(s) remain controversial, over 95% of marine species became extinct, while 70% of terrestrial taxonomic families suffered the same fate of extinction!

PERMIAN EXTINCTION – CAUSES?PERMIAN EXTINCTION – CAUSES?• (1) Climate change, possibly caused by glaciation and/or volcanic activity, has

been associated with many mass extinctions. It seems likely that climate change is a consequence of the cause of extinction rather than the root cause itself.

– The Siberian Traps triggered a massive, sudden glaciation as well as other environmental consequences of volcanic eruptions.

– The opening of the Atlantic Ocean basin as the result of sustained volcanic eruptions (the Central Atlantic Magmatic Province) led to the release of toxic fumes, greenhouse gases, and ultimately, global climate change – perhaps triggering an ice age

– Formation of Pangaea has been invoked as a cause for the extinction. • Pangaea's presence may have led to extreme environments with hotter

interior areas of the continent and colder polar areas, possibly producing glaciation.

• (2) Poisoning of the ocean has been suggested due to an apparent drop in carbon

isotope data obtained from marine sediments formed at the time of the extinction. – The cause of this apparent drop off in the photosynthetic rate in the seas has

not yet been determined

• (3) Extraterrestrial Objects:– Evidence of a large impact at the close of the Permian is not strongly

supported, although some indirect evidence suggests an impact did occur during the Permian, although possibly not at the time of the extinction crisis.

CRETACEOUS-TERTIARY EXTINCTIONCRETACEOUS-TERTIARY EXTINCTION

• Impact Theory:– 1980: L.W. Alvarez and

colleagues published a paper proposing that, approximately 65 million years ago, the earth was struck by an asteroid-sized object on Yucatan Peninsula – Chicxulub, Mexico

– Evidence:• Boundary Clay with high

levels of iridium – A very rare mineral in

terrestrial rocks– More common in

extraterrestrial rock samples• Microtektites: hollow,

microscopic, glass-like spheres that form when a violent explosive event occurs in association with molten rock

• "Shocked" quartz grains, where the regular, crystalline structure has been distorted by the application of large forces

The disappearance of ‘dinosaurs’ from the fossil record ~ 65 million years ago

ma

Shocked QuartzShocked Quartz• To visualize this type of deformation,

imagine a perfectly vertically stacked deck of playing cards.

• Now slant the stack by pushing the upper part of the deck a little to the side. This is a rough analogy of what happens when quartz goes through a lattice offset.

• As the compression wave from the blast passes through the sand grains, planes of atoms in the quartz get "shifted" slightly to the side relative to adjacent planes of atoms.

• These latice offsets create zones of optical interference in the sand grain which, under a microscope, show up as two or more groups of dark lines that intersect each other

MicrotektitesMicrotektitesHow do they form?1) A comet or asteroid impacts the Earth, probably at an oblique angle.2) Terrestrial (Earth) rock is melted and ejected into the upper atmosphere at hyper-velocities.3) Tektites rain down.

Iridium – Boundary Layer ClayIridium – Boundary Layer Clay

• The asteroid hit a geologically unique, sulfur-rich region of the Yucatan Peninsula and kicked up billions of tons of sulfur and other materials into the atmosphere.

• Darkness prevailed for about half a year after the collision.

• This caused global temperatures to plunge near freezing

Chicxulub Crater, Yucatan PeninsulaChicxulub Crater, Yucatan Peninsula

ALTERNATE THEORY FOR ALTERNATE THEORY FOR CRETACEOUS-TERTIARY EXTINCTIONCRETACEOUS-TERTIARY EXTINCTION

• VOLCANISM:• CAMP – Central Atlantic Magmatic Province

– Massive basaltic eruptions that broke up the super-continent Pangaea and opened up the Atlantic Ocean Basin.

• The eruption of the Deccan Traps approximately 65-64 million years ago is the largest volcanic event since the Permian-Triassic event at 245 Ma

• The impact at Chicxulub, Mexico predates Dinosaur extinction by 300,000 years.

• Selective extinction: only dinosaurs

Central Atlantic Magmatic ProvinceCentral Atlantic Magmatic Province

• Volcanic events flooded the center of the former supercontinent of Pangaea with molten rock

• The area—which today stretches around the Atlantic, across parts of Canada, the eastern US, Europe, South America and Africa—is referred to as the CAMP, or Central Atlantic Magmatic Province.

CENTRAL ATLANTIC MAGMATIC PROVINCE

Deccan Volcanic Province in Deccan Volcanic Province in IndiaIndia

• The Deccan volcanic province in India today covers an area the size of France or Texas.

• The original size is estimated twice this size, but was reduced by erosion.

• Arrows show the direction of the largest lava flows 1500 km across India and into the Gulf of Bengal.

EARTH STRUCTURESEARTH STRUCTURES• The Earth is composed of four major

layers:– Inner Core– Outer Core– Mantle– Crust

EVIDENCE OF EARTH’S LAYERSEVIDENCE OF EARTH’S LAYERS• DIRECT EVIDENCE OF LAYERING

– Mantle rock brought up to surface through volcanism– Intrusion and erosion of diamond-bearing kimberlite pipes– Lower layers of oceanic lithosphere brought to surface at

subduction zones

• Ophiolite Suite: a sequence of rocks that appears to represent a section through oceanic crust

• INDIRECT EVIDENCE OF LAYERING– Seismic reflection: return of energy from seismic waves

‘bouncing’ off rock boundaries.• Similar to light off a mirror, rock boundaries of differing densities

set up a reflection of seismic waves

– Seismic refraction: bending of seismic waves as they pass through rock layers of differing densities

• Seismic waves change speed or direction passing through different rock boundaries

DIRECT EVIDENCE:VOLCANOESDIRECT EVIDENCE:VOLCANOES

Magma ejected by a volcano may have its origin in the Mantle layer of the Earth

Mantle minerals include pyroxenes, amphiboles, biotites and plagioclase

KIMBERLITE PIPE DIAGRAMKIMBERLITE PIPE DIAGRAMThe complex volcanic magmas that solidify into kimberlite and lamproite are not the source of diamonds, only the ‘elevators’ that bring them with other minerals and mantle rocks to Earth's surface

DIRECT EVIDENCE: DIRECT EVIDENCE: OPHIOLITE SUITE (SEA FLOOR)OPHIOLITE SUITE (SEA FLOOR)

The sequence of ‘layers’ of rock found on all ocean floors.

An idealized ophiolite sequence shows an upper layer consisting of deep sea sediments (limestones, cherts, and shales), overlying a layer of pillow basalts. 

Pillow basalts overly the sheeted dikes of basalt material.

Beneath the sheeted dike complex are gabbros that likely represent the magma chambers for the basalts. 

The marine sediments are typically from animal and terrestrial material settling at the bottom of the ocean

The Pillow lavas, dykes, gabbros and peridotite are typical mantle materials

INDIRECT EVIDENCE: SEISMIC INDIRECT EVIDENCE: SEISMIC WAVESWAVES

CORE: INNER AND OUTER CORE: INNER AND OUTER • INNER AND OUTER CORE –

represent approximately 31% of Earth’s mass

• The intense heat of the core is derived from decay of radio-active isotopes – Inner core

• The inner core is under such extreme pressure that it remains solid

– Composed mostly of iron (Fe) and some Nickel (Ni)

– Temperatures over 7,0000 F (4,3000 C)

– ~2200 Km across– Outer core

• The outer core is under less pressure and is molten

– Composed mostly of iron (Fe), Sulphur (S), and Nickel (Ni)

– Temperatures of 6,700-7,7000F (3,700 – 4,3000C)

– ~3000 Km across

MANTLE AND CRUSTMANTLE AND CRUST– MANTLE– represents 68% of Earth’s

mass -~2900 km thick• At over 1000 degrees C, the

mantle is solid but can deform slowly in a plastic manner

– Composed of iron (Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O) and a number of other minerals

– LITHOSPHERE • Upper mantle more

rigid, bonded to Crust:• 100 km thick

– ASTHENOSPHERE • Mantle below

Lithosphere more plastic, weaker and more molten:

• 100-200 km thick– CRUST – represents 1% of Earth’s

mass• The crust thinnest of the layers:

5-70 km thick– Continental crust: Granitic– Oceanic crust: Basaltic

EARTH’S CRUSTEARTH’S CRUST• The Earth’s crust is composed of almost all of the basic elements.

• Listed below (in order of abundance) are the eight (8) basic elements that compose approximately 99% of the crust:

– Oxygen (O) Silicon (Si) Aluminum (Al) Iron (Fe) – Calcium (Ca) Potassium (K) Sodium (Na) Magnesium (Mg)

• Continental Crust is composed mainly of a “granitic” rock type– High silica content (a combination of Oxygen and Silicon)– Lower density (2.7 grams per cubic centimeter)– Thicker (20-70 km thick)– Underlies most continents

• Oceanic Crust is composed mainly of a “basaltic” rock type– Low silica content– Higher density (3.0 grams per cubic centimeter)– Thinner (5-10 km thick)– Underlies most ocean basins

TECTONIC PLATESTECTONIC PLATES• Earth’s Lithosphere

(crust and upper mantle) is broken into large moving slabs: Tectonic (or Lithospheric) Plates

• Interaction between plates drives mountain building:– Volcanic mountains– Folded mountains– Faulted mountains