GEOLOGY 101CLASS 13SPRING 2014

Geologic Time Scale

• Eons, Eras, Periods,– and Epochs

• Eras - Paleo, Meso, Ceno - zoic, proterozoic, archean

• Periods – rocks and places rocks found –Perm, Jura, Cambria Pennsylvania,

• Epochs – named by Charles Lyell

Dr. Lofton declaresbeginning of Cenoconferocene 7/2012Ending Holocene in Aggieland

The Geologic Time ScaleQuaternary Latin, “fourth” 1822

Tertiary Latin, “third” 1760

Cretaceous Latin creta, “chalk” 1822

Jurassic Jura Mountains, Switzerland 1795

Triassic Latin, “three-fold” 1834

Permian Perm, Russia 1841

Carboniferous Carbon-bearing 1822

Devonian Devonshire, England 1840Silurian Silures, a pre-Roman tribe 1835

Ordovician Ordovices, a pre-Roman tribe 1879

Cambrian Latin Cambria, “Wales” 1835

Charles Lyell (1797-1875)• Was a champion of Hutton’s

work in his 1830 book on the Principles of Geology.

• Has been criticized for his naming methods for the Periods. He would support calling the current epoch in Aggieland the “Cenoconferocene”

• So we are in Cenoconferocene Epoch, Quaternary Period, Cenoziac Era of the Phanerozoic Eon. The rest of the world is still in in the Holocene (Recent) Epoch

Finding a Geologic Clock

• Lyell also lacked a Geologic Clock• 1896 – radioactivity was discovered.• By 1911, concept of a radioactive clock.• A clock provides ABSOLUTE dates as

opposed to RELATIVE dating.

How to we count back?

• How many calendars exist in the world? More than you think. It’s 2451 now in Thailand, for example.

• Mayan calendar broken• All dates for radioactive dating relate to

1950 A.D. This date is defined by international agreement as the present. So a date of 1500BP is 1500 years BEFORE 1950.

M&W4 Fig. 3.3; M&W5 Fig. 3.4

• The number of protons sets the atomic number for an element

• The number of neutrons in atoms of different elements can vary.

• Atoms of an element having different numbers of neutrons are referred to as the isotopes (of that element).

GEOLOGIC DATING:ABSOLUTE AGE DETERMINATION

What are Radioactive Isotopes?

• Nuclei of atoms: Neutrons + Protons• Neutrons no charge, weigh = proton• Protons determine properties• One isotope usually

dominates • Carbon 12 more than

99% of all Carbon.• Unstable isotopes fiss (fission) to stable ones at log. decay rates

Radioactivity• Nuclei of elements of unstable or

radioactive elements disintegrate spontaneously!

• Carbon 14 ( 14C) decays at precise rate. • By measuring the rate at which beta

particles emitted, we can determine age• 14C emits 15 betas/min/gram (456 g/lb).

Counted in a lead shielded environment.• Half-life is 5,730 years which means half

of the 14C is gone – only 7.5 betas then.

Three major forms of radioactive decay

From Tarbuck and Lugens2008

Showing the Change

M&W4 Fig. 17.24; M&W5 Fig. 17.24

C14 is an isotope of carbon that forms from Nitrogen in the atmosphere.

Living things consume this radioactive carbon.

Once dead, no new carbon is absorbed, and C14 turns back into Nitrogen.

The Half-Life of C14 is 5,730 years.

This method works best for fossils younger than 50,000 years. Why?

CARBON 14 DATING

Half – life of Isotopes used in dating rocks and fossils

Radioactive Parent Stable Daughter Half-lifePotassium-40 Argon-40 1.25 billion yrsRubidium-87 Strontium-87 48.8 billion yrsThorium-232 Lead-208 14 billion yearsUranium-235 Lead-207 704 million yearsUranium-238 Lead-206 4.47 billion yearsCarbon-14 Nitrogen-14 5730 years

Levin - http://higheredbcs.wiley.com/legacy/college/levin/0471697435/chap_tut/chaps/chapter03-04.html

Nuclear fusion or radioactive decay = energy release as unstable atomic nucleui creates new stable ones.Natural constant rate of decay = exponentialHalf-life

Radioactivity Cont’d.• In dating, you really count beta radiation

released• This declines as with each half-life, only

half as many betas are released• 14C is found in every living thing• After death, no new carbon is added and

the decay process begins.• Complications – the amount of carbon 14

produced yearly does vary. • This is good clock for the last 40,000 to

50,000 years.

For Billions of Years

• 238U (uranium) decays to 206Pb (lead) with a half-life of 4.5 x 109 years or 4,500,000,000. Hence, we have a clock that will go back to the beginning of Earth.

• Potassium (The Big K) 40 – Argon 40 method very useful for dating igneous and metamorphic minerals. Calculate K to Ar ratio to determine age. 1.25 billion year half-life

Objectives Chapter 7• Explain the principles of:

• Original Horizontality • Superposition• Cross cutting relationships• Inclusions • Lateral continuity

• Use these to relatively date rock strata• Define angular unconformities,

disconformities, and nonconformities and what they represent

• Explain the methods used to absolutely date rocks.

What type of minerals and rocks make up the

magma in both the mantle and in the core?

The magma is composed of rocks that are melted in the area around the heat source. The core is thought to be made of solid metal on the inside (inner core) and also liquid around that solid inner core.

S

MAGMA INGNORANCE

How does magma actually come up to the surface specifically?

Thanks to Chris Hoffman, Juanita Yanez, Forrest Norton, Chelsea Allen-McDaniel, Brad Bevilacqua

Objectives Chapter 7• Explain the principles of:

• Original Horizontality • Superposition• Cross cutting relationships• Inclusions • Lateral continuity

• Use these to relatively date rock strata• Define angular unconformities,

disconformities, and nonconformities and what they represent

• Explain the methods used to absolutely date rocks.

Objectives - Chapter 8

• Explain how scientists use earthquake energy to learn about the interior of the earth.

• Describe the characteristics of the five major layers of the Earth’s interior.

• Explain the origins of the heat that keeps the Earth warm.

• Describe why a heat generating Earth is different from a cold planet

Fatso Earth•  

How do we learn about Earth’s Interior structure?

An answer:Seismic Wave reflection and refraction

What are Seismic Waves?

• Seismic waves are the vibrations from earthquakes (or other transmissions of energy) that travel through the Earth

• They are the waves of energy suddenly created by rock fracture within the earth or an explosion.

• They are the energy that travels through the earth and is recorded on seismographs

Seismic Waves• Most of the damage and the shaking

people feel during an earthquake is from the seismic waves.

–Earthquake vibrations or seismic waves are of two kinds: body waves and surface waves.

• Body waves travel through Earth

• Surface waves travel along or just below the surface, like ocean waves.

Seismic Waves• Body waves

• Body waves are divisible into two types: • P-waves are compressional waves and travel faster than S-waves.

P for pressure

• S-waves are shear waves that cannot

travel through liquidsS for shear.

Fig. 8.7, p. 196

P-Wave(Body Wave) Primary or compressional (P) waves

The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave.

The P wave can move through solid rock and fluids, like water or the liquid layers of the earth.

It pushes and pulls the rock it moves through just like sound waves push and pull the air.

Highest velocity (6 km/sec in the crust)

Secondary Wave (S Wave)

Secondary or shear (S) waves The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake.

An S wave is slower than a P wave and can only move through solid rock. (3.6 km/sec in the crust)

This wave moves rock up and down, or side-to-side.

Fig. 8-7, p. 196

Undisturbed material

Focus

Surface

Primary wave (P-wave)

Com

pres

sion

Com

pres

sion

Com

pres

sion

Expa

nsio

n

Expa

nsio

n

Undisturbedmaterial

Direction of wave movement

Stepped ArtSecondary wave (S-wave)Wavelength

Seismic Waves

• Surface waves

– Surface waves are divisible into two types, Rayleigh and Love waves.

Fig. 8.8, p. 197

L-WaveLove Waves• The first kind of surface wave is called a

Love wave, named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911.

• It's the fastest surface wave and moves the ground from side-to-side.

L-Wave

Particle motion

Particle motion consists of alternating transverse motions. Particle motion is horizontal and perpendicular to the direction of propagation (transverse). Particle motion is purely horizontal, focus on the Y axis (black lines) as the wave propagates through it. Amplitude decreases with depth (yellow lines). Material returns to its original shape after wave passes.

Deformation propagates

Rayleigh Waves• Named for John William Strutt, Lord Rayleigh, who

mathematically predicted the existence of this kind of wave in 1885.

• A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean.

• Rolls the ground, moving it up and down, and side-to-side in the direction of wave travel moving.

• Cause of most of the shaking felt from an earthquake

• Often larger than other waves.

Locating an Earthquake• First measure the

amplitude on the seismograph.

• Then plot on a time-distance graph the arrival times of the P- and S-waves.

Fig. 8.9a-b, p. 198

Locating an Earthquake• Finally plot the distance from

each receiving station.

• Three seismograph stations are required.

• They will intersect at the epicenter of the earthquake. Fig. 8.9b, p. 198

Fig. 8.10, p. 199

HistorySeismology - the Study of Earthquakes and

Seismic Waves Dates back almost 2000 years

Earthquake Effects - Ground Shaking

Kobe, Japan 1995

Earth's Mantle• The Mantle’s Structure, Density and

Composition

• Peridotite is thought to represent the main composition in the mantle. – Experiments indicate that peridotite has the

physical properties and density to account for seismic wave velocity in the mantle.

– Peridotite makes up the lower parts of ophiolite sequences that represent oceanic crust and upper mantle.

– Peridotite is also found as inclusions in kimberlite pipes that came from depths of 100 to 300 km.

What is Earth’s Interior Like?– The concentric layers of Earth, from its

surface to interior, are :

– Oceanic / Continental crust– Rocky mantle– Iron-rich core

• liquid outer core• solid inner core

Fig. 8.21, p. 214

What is Earth’s Interior Like?

Geologists study the bending or refraction and reflection of P- and S-waves to help

understand Earth's interior.

– This indicates boundaries

between layers of different densities called discontinuities. Fig. 8.22 c, p. 214

Anatomy of Earth

Layering based on different criteria

1. Density (crust, mantle, core)

2. Chemical composition (consistent with density)

3. Mechanical behavior of materials (lithosphere, asthenosphere, mantle, core)

Physiology of ‘solid ‘ Earth – driving mechanism for plate tectonics

Plate Tectonics is the surface expression of the mechanism by which heat escapes the Earth’s interior

Origin of heat in the Earth’s interior

1. radioactive decay

2. residual heat from Earth’s formation, mostly in mantle, higher % in crust and to a lesser extent, heat contribution from the growth of the inner core which drives the convection in the outer core

The Core• The P- and S-waves both refract and reflect as they cross discontinuities.

• This results in ’shadow zones’. These shadow zones reveal the presence of concentric layers within the Earth, recognized by changes in seismic wave velocities at discontinuities.

• P-wave discontinuities indicate a decrease in P-wave velocity at the core-mantle boundary at about 2900 km.

• S-wave blocked from passing thru liquids, thus indicating that the outer core is liquid.

Fig. 8.24, p. 215

The Core• Density and Composition of the Core

• The density and composition of the concentric layers have been determined by the behavior of P-waves and S-waves.

• Compositionally, the inner core is thought to be iron and nickel, the outer core iron with 10 to 20% other, lighter substances, and

• The mantle probably peridotite.

Fig. 8.23, p. 215

Earth’s Mantle– The boundary between the crust and mantle

is known as the Mohorovičić Discontinuity. – It was discovered when it was noticed that

seismic stations received two sets of P- and S-waves. This meant that the set below the discontinuity traveled deeper but more quickly than the shallower waves.

Fig. 8.25, p. 216

Earth's Mantle• The Mantle’s Structure, Density and

Composition - Several discontinuities exist within the mantle.

– The velocity of P- and S-waves decrease markedly from 100 to 250km depth, which corresponds to the upper asthenosphere.

– The asthenosphere is an important zone in the mantle because this is where magma is generated.

– Decreased elasticity accounts for decreased seismic wave velocity in the low-velocity asthenosphere. This decreased elasticity allows the asthenosphere to flow plastically.

Fig. 8.26, p. 217

Seismic Tomography• Tomography - a technique for developing better

models of the Earth’s interior.

• Similar to a CAT-scan for producing 3-D images, tomography uses seismic waves to map out changes in velocity within the mantle.

Earth's Mantle Structure, Density and Composition

• Peridotite is thought to represent the main composition in the mantle. – Experiments indicate that peridotite has

the physical properties and density to account for seismic wave velocity in the mantle.

– Peridotite makes up the lower parts of sequences that represent oceanic crust and upper mantle.

– Peridotite is also found as inclusions in that came from depths of 100 to 300 km.

Exposed section of Mantle in Oman

Earth's Internal Heat • Geothermal gradient – measures the increase in temperature

with depth in the earth. Most of Earth's internal heat is generated by radioactive isotope decay in the mantle.

• The upper-most crust has a high geothermal gradient of 25° C/km.

• This must be much less in the mantle and core, probably about 1° C/km.

• The center of the inner core has a temperature estimated at 6,500° C.

Earth's Crust • Continental crust is mostly granitic

and low in density

• It has an average density of 2.7 gm/cm3 and a velocity of about 6.75 km/sec

• It averages about 35 kilometers thick, being much thicker beneath the shields and mountain ranges of the continents.

Objectives - Chapter 8

• Explain how scientists use earthquake energy to learn about the interior of the earth.

• Describe the characteristics of the five major layers of the Earth’s interior.

• Explain the origins of the heat that keeps the Earth warm.

• Describe why a heat generating Earth is different from a cold planet