Introduction to geology- chapter notes

Geological Questions about Our Planet and Oceans

• How and when did our planet and its ocean originate?ocean originate?

• How do they function?

• How do they change?

• How and when will they end?

Page 1Chapter 2

Time line of Earth history: 4.6 billion

years

Page 2Chapter 2

From Garrison, 5th ed.

Not completely to scale.

Time line of Earth history

• Earth forms: ~4.6 Ga.– Within 50 Ma,

gravitational settling of iron to form the core;

– Impact that formed the pMoon (or maybe 4.1 Ga).

• Ocean forms: 4.2 Ga 1, “snowball” Earth.

• Heavy bombardment, 3.9‐3.85 Ga.

• Oldest dated rocks: 3.8 Ga, but zircons 4.4 Ga (from W. Australia) imply granitic, continental type

Page 3Chapter 2

g , yprocks.

• First evidence of life: 3.65‐3.85 Ga

• Mag. field: ~3.2 Ga• Oxygenation: begins ~

2.0 Ga and is complete by 0.8 Ga. 1 Ga = giga-annum,

billion years ago


Page 4Chapter 2

Ma = mega-annum, millions of years ago.


• The future: Events that could stop life:

– Stop convection of the core: magnetic field stops

Ga

Page 5Chapter 2

field stops.

– Stop convection of the mantle: C‐cycle halts.

Ga

How do we know this?!

• Read: (on the ftp site)

– Valley, John W., Early Earth: Elements, y yv. 2, no. 4, p. 201‐204.

– You’re invited to read the whole issue, which is also on the FTP site and was open access at the time I downloaded it.

• This course will focus on the nature of ocean basins and how they fit into the functioning of the planet though it will

Page 6Chapter 2

functioning of the planet, though it will, by necessity, include some whole‐Earth issues.

How does our planet function?

Clues from astronomy about composition of the interior.

Clues from seismology about the structure of the interior.

Clues from gravity about the distribution of mass.

gravity

the geoid

isostasy

Page 7Chapter 2

Clues from the magnetic field about internal processes.

Clues from nature of Archeozoic(>2.5 Ga)1 crust, which will be discussed later in course.

1 Ga = giga-annum, billion years ago.

Clues from astronomy ‐composition

D it f th h l l t• Density of the whole planet

– In 1798, Lord Henry Cavendish* measured Newton’s universal G and calculated Earth’s density, 5.52 g/cm3.

– Crustal rocks are only 2.7 to 3.3 g/cm3.

– So, interior rock must be denser!

• What is the interior stuff made of?

Page 8Chapter 2

– Is it dense simply due to compression of crust‐like rock? Calculations show this doesn’t provide enough mass.

– Look at the composition of meteors.

*http://www.physicsclassroom.com/class/circles/u6l3d.cfm

Clues from astronomy ‐composition

– Two types of meteors in our region of the solar system:

• Stony types: chondrites and carbonaceous chondrites (average density ~3.3‐3.4 g/cm3)*

• An iron‐rich type (7‐8 g/cm3).

– An iron core adds enough mass (assume density = 8 g/cm3) to account for Earth’s average density.

Page 9Chapter 2

Iron

Chondrite

CarbonaceousChondrite

How does our planet function?

Clues from astronomy about composition of the interior.

Cl f i l b h

Clues from seismology about the structure of the interior.

Clues from gravity about the distribution of mass.

gravity

the geoid

isostasy

Cl f th ti fi ld b t

Page 10Chapter 2

Clues from the magnetic field about internal processes.

Clues from nature of Archeozoic(>2.5Ga)1 crust, which will be discussed later in course, not now.

1 Ga = giga-annum, billion years ago.

Clues from Seismology about Earth’s structure

• Earthquakes produce seismic waves at a q ppoint in the brittle crust where a fault has ruptured and moved.

– The hypocenter is the point in the crust where it has ruptured.

– The epicenter is the projection of the hypocenter onto the surface.

Epicenter

Page 11Chapter 2

Hypocenter

p

Clues from Seismology ‐structure (cont.)

• Seismic waves include both S waves and P waves.

S ( d h• S waves (= secondary or shear waves) have an oscillatory lateral motion perpendicular to the direction of propagation.

Page 12Chapter 2Shear waves


– P waves (= primary or pressure waves) propagate by alternatewaves) propagate by alternate compression and dilation of the medium in the direction of motion of the wave.

Page 13Chapter 2

Pressure waves

Clues from Seismology—structure (cont.)

• Properties of S and P waves

– P waves can pass through liquids, but shear waves can not.

W li d– P waves are faster than S waves.

• These characteristics provide clues about Earth’s

Wave amplitude

Time

Page 14Chapter 2

interior.

P-wave arrival

S-wave arrival

Seismic record – arrivals of earthquake wavesat a seismograph.

P and S Wave Arrivals• Algerian earthquake 21 May 2003, stations from southern Europe to Greenland.Greenland.

• Note the lag in slower S‐wave arrivals with distance.

S. Eu.

PS

TIME.

Page 15Chapter 2Van Eck et al., 2004, EOS 85(13).

Green-land

lag


• What controls the speed of waves in rock?i ock?

– Compressibility = volume change due to pressure.

– Speed 1/(compressibility xdensity)

• Speed slows when (density xcompressibility) gets large, an

Page 16Chapter 2

p y) g g ,inverse relationship.

• Speed increases when (compressibility x density) of rock gets small.


• What controls the speed of waves in rock? (cont.)

– Consider this: both wave speed and density increase with depth in Earth. What must be happening?

• Compressibility decreases faster than density increases, making (com. x den.) decrease. This results in the faster wave speeds.

Page 17Chapter 2

• What affects compressibility? (

–More melting ,

–Higher temperature,

–Certain mineralogic changes decrease/increase compressibility.


• Surface ‐ horizon of a discrete change in physical properties like compressibility, density, change in state (solid, liquid, partial melts).

• Reflection – a wave bounces off a surface where properties change and the wave returns to the surface

Page 18Chapter 2

surface.

• Refraction – a wave passes through the surface where properties change, but the wave changes direction and speed.

Reflection and Refraction

SourceSea surface

i c Totalreflection

Source

Low velocity medium

Interface

Reflection

Page 19Chapter 2

rr

= 90o

i = r = 0 High velocity medium

Interface

Refrac-tion

r = reflection, i = incidence, c = critical


• How can this be applied to study of Earth’s interior?

– Waves change speed (and direction) when they encounter a big change in physical properties that affect density and/or compressibility (reflection and refraction).

– Remember that P waves are faster than S waves.

– Remember that P waves pass through li id b S d

Page 20Chapter 2

liquid, but S waves do not.

– These wave properties allow us to make inferences about the layering, state, and density of Earth’s interior.


• Seismographic stations have been installed all over the world to monitor seismic a es generated by ery largeseismic waves generated by very large earthquakes in order to infer the structure of Earth’s interior.

• Here is what can be inferred about internal structure from S waves and P waves from very big earthquakes.

Waves from P

Page 21Chapter 2

Waves from an earthquake reflect and refract off internal boundaries.

P waves


• Boundaries of layers in the interior, like the core/mantle boundary.

• State of layers, like the outer and yinner core.

– S waves do not penetrate the core.

S waves

Page 22Chapter 2


– In contrast, P waves do penetrate the core and slow down (Oldham, 1906).

• At least the outer part must be liquid.

– Waves through the center are faster th t d (L h 1936 Bi h

gthan expected (Lehman, 1936; Birch, 1940).

• Apparently they speed up, which would happen if the inner core were solid.

P waves

Page 23Chapter 2

See K. C. Creager, 2008, News and Views, Nature 454, 833-835, for more on proof of a solid core.


• Three major concentric layers:

– Crust, mantle, and core.

– Recognized as surfaces where seismic waves change speed and are strongly reflected and refracted.

Page 24Chapter 2

Documents

Introduction to geology- chapter notes