Earth structure and plate tectonics

1
Lecture 3
Earth structure and plate tectonics
21 August 2007
2
Lecture schedule
Lecture Date Time 1. Introduction to
scientific writing 8/20 900-1020 2. How to
write a scientific paper in English 8/20 1500-16
20 3. Earth structure and plate
tectonics 8/21 900-1100 4. Circulation of the
atmosphere 8/21 1300-1500 5. Water and ocean
structure 8/22 1000-1100 6. Continental
margins and ocean basins 8/22 1300-1500 7.
Sediment 8/23 1000-1200 8. Marine
resources 8/23 1300-1400 9. Circulation of
the ocean 8/24 1100-1230
3
Earth is an Ocean World
71 of Earths surface is covered by water
 
gt97 of water at Earths surface is in oceans
www.harmsy.freeuk.com/earth.html
4
Arctic Ocean
Atlantic Ocean
Pacific Ocean
Indian Ocean
All oceans and seas together the world ocean
http//www.uwsp.edu/geO/faculty/ritter/geog101/tex
tbook/images/maps/oceans_CIA_base.jpg
5
A few statistics
Average land elevation 840 m Average ocean
depth 3,800 m
Average ocean temperature 3.9oC
About 50 of Earths population lives lt240 km
from the ocean

• In Japan, 96 of the population lives lt100 km
  from the ocean

6
Northern Hemisphere

• 61 ocean

Equator
Southern Hemisphere

• 81 ocean

http//www.uwsp.edu/geO/faculty/ritter/geog101/tex
tbook/images/maps/oceans_CIA_base.jpg

• BUT, on a planetary scale, the ocean is small
• 0.13 of Earths volume
• more water within Earths interior than in its
  ocean and atmosphere

7
Origin of the ocean
To understand the ocean, we need to know how it
formed and evolved. The formation of the ocean is
linked to the formation of Earth and of the solar
system.
8
Earth Was Formed of Material Made in Stars

• The universe began with the big bang
• about 14 billion years ago
• atoms began to form after about 1 million years
• mostly hydrogen

cooling
http//outreach.web.cern.ch/outreach/public/cern/P
icturePacks/BigBang/BigBang.JPG
9
Stars and planets are contained in galaxies
A galaxy is a huge, rotating aggregation of
stars, dust, gases and other debris.
There are perhaps 100 billion galaxies in the
universe.
Our galaxy is named the Milky Way.
http//www.atlasoftheuniverse.com/milkyway.jpg
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Milky Way

• stars are arrayed in spiral arms
• contains 200-400 billion stars
• our sun is a typical star

Spiral arms
http//www.atlasoftheuniverse.com/milkyway.jpg
11
Solar system
http//www.astroarts.co.jp/news/2006/08/16planet_d
efinition/index-j.shtml
How do stars and planets form?
12
Condensation theory
spinning nebulae (a large cloud of gas and dust)
Material concentrated near its center became the
protosun

• much of the outer material eventually became the
  planets
• formed by accretion

http//physics.uoregon.edu/jimbrau/BrauImNew/Chap
06/FG06_20.jpg
13
Formation of Earth
The young Earth was probably homogenous throughout

• Density stratification is layering by density
• the heaviest material forms deeper layers, and
  the lighter material forms layers near the
  surface.

Earth began to cool.

• The first surface formed about 4.6 billion years
  ago.
• so we say Earth is 4.6 billion years old

14
Earths Interior is Layered Inside
How can we study Earths interior?
Dig a hole?
http//www.nasa.gov/images/content/103949main_eart
h10.jpg
15

• Kola Superdeep Borehole (KSDB)
• a scientific drilling project in the USSR to
  drill into the Earth's crust
• goal 15 km
• began digging in 1970
• in 1992, reached a final depth of 12 km

• 245oC!

lt1/500 of Earths radius
16
Ocean drilling
JOIDES Resolution (1985 - )

• scientific drilling ship
• penetrated 2 km below 2.5 km of seawater

http//www-odp.tamu.edu/images/bluejr.jpg
Chikyu (2002- )
210 m
http//en.wikipedia.org/wiki/ImageDVChikyu.jpg
17

• Chikyu Hakken (????)
• Japanese for "Earth Discovery
• a mission primarily led by the Japan Agency for
  Marine-Earth Science and Technology (JAMSTEC)
• AIM be the first to drill seven kilometers
  beneath the seabed and into Earths mantle

http//www.jamstec.go.jp/chikyu/eng/index.html
18

• Chikyu Hakken (????)
• Japanese for "Earth Discovery
• a mission primarily led by the Japan Agency for
  Marine-Earth Science and Technology (JAMSTEC)
• AIM be the first to drill seven kilometers
  beneath the seabed and into Earths mantle

No one has yet sampled below Earths outermost
layer, the Crust. So how do we know Earth is
layered?
http//www.jamstec.go.jp/chikyu/eng/index.html
19

• Chikyu Hakken (????)
• Japanese for "Earth Discovery
• a mission primarily led by the Japan Agency for
  Marine-Earth Science and Technology (JAMSTEC)
• AIM be the first to drill seven kilometers
  beneath the seabed and into Earths mantle

No one has yet sampled below Earths outermost
layer, the Crust. So how do we know Earth is
layered?
Earthquakes
http//www.jamstec.go.jp/chikyu/eng/index.html
20
Earthquakes generate seismic waves.
Seismic waves travel through Earth and along its
surface.

• Seismic waves form in two types
• surface waves
• sometimes seen as an wavelike motion in the
  ground
• cause most of the property damage in an
  earthquake
• 2) body waves (P and S waves)
• useful for analyzing Earths interior structure

http//ja.wikipedia.org/wiki/E794BBE5838FPs
waves.jpg
21
Body waves 1) P (primary) wave

• a compressional wave
• similar to a sound wave
• pass through liquids and solids

http//www.met.gov.pk/Subpage4/waves_files/pwave_w
eb.jpg
2) S (secondary) wave

• a shear wave
• like a rope shaken side to side
• pass through solids only
• about 1/2 the speed of P waves

http//www.geo.uib.no/jordskjelv/img/s-wave.jpg
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seismograph
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P waves are faster than S waves
About 3.5 km/sec
About 6 km/sec
24
Sumatra-Andaman earthquake (26 December 2004)
http//zog.typepad.com/malaysia/epicenter.jpg
How do we locate the epicenter of an earthquake?
25
http//www.theimage.com/geology/notes15/index.html
http//ja.wikibooks.org/wiki/E998B2E781BD_E
59CB0E99C87
http//www.calstatela.edu/faculty/acolvil/earthqua
kes.html
26
If the Earth were homogenous, the paths of
seismic waves would be straight lines

• Richard Oldham
• British geologist

http//cires.colorado.edu/bilham/oldham.gif
27
If the Earth were homogenous, the paths of
seismic waves would be straight lines

• Richard Oldham
• British geologist

http//cires.colorado.edu/bilham/oldham.gif
Found that seismic waves arrived earlier than
expected at seismographs far from the quake
waves traveled faster as they went into Earth
28
Earthquake wave shadow zones confirmed the
presence of Earths core.
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refraction
30
Data from an earthquake confirmed the model of
Earth layering.
Anchorage, Alaska 27 March 1964 9.2 on Richter
scale

• P and S waves were very large and easily detected
  at great distances
• helped confirm the models of Earth layering.

http//www.greatdreams.com/alaska/alaska-collapse-
1964.jpg
31
Question Time!
1. What are the two kinds of seismic waves?
32
Earths Inner Structure was Gradually Revealed.

• Researchers have not collected samples from below
  the outermost layer of Earth.
• but they have indirect evidence about
• chemical composition
• density
• temperature
• thickness

33
Earths layers can be classified based on
composition.

• Crust
• 1 of Earths volume
• oceanic crust
• mostly basalt
• continental crust
• mostly granite

• Mantle
• 83 of Earths volume
• mainly silicon and oxygen

• Core
• 16 of Earths volume
• mainly iron (90) and nickel

http//www.llnl.gov/str/JulAug04/gifs/Fried1.jpg
34
Earths layers can also be classified by physical
properties.

• Lithosphere
• cool, rigid

• Athenosphere
• hot, partially melted

Lower mantle

• Core
• outer core
• liquid
• inner core
• solid
• 6,600oC!

35
Isostatic equilibrium supports continents above
sea level
The lithosphere floats on the asthenosphere
buoyancy
Iceberg
Isostatic equilibrium
36
What happens when a mountain erodes?

• Isotatic uplift
• now occurring in Canada and United States
• caused by melting of ice sheets
• one cause of earthquakes

37
How do mountains form?
Himalayas
http//upload.wikimedia.org/wikipedia/commons/thum
b/7/79/Himalayas.jpg/350px-Himalayas.jpg
38
How do mountains form?
Himalayas
http//upload.wikimedia.org/wikipedia/commons/thum
b/7/79/Himalayas.jpg/350px-Himalayas.jpg
In 1800s, geologists thought that the Earth
shrunk as it cooled
http//www.grape.sk/grape_2006.jpg
http//en.wikipedia.org/wiki/ImageSingle_raisin.j
pg
39
Earths interior is heated by the decay of
radioactive elements

• Some elements are unstable
• give off heat when their nuclei beak apart

Some of this heat moves to the surface by
conduction
Some also rises by convection
This heat - builds mountains and volcanoes -
causes earthquakes - moves continents
40
The age of Earth was controversial and not easily
determined.
Earth is about 4.6 billion years old.
But, in late 1700s, most European scientists
thought the year was only 6,000 years old!

41
The age of Earth was controversial and not easily
determined.
Earth is about 4.6 billion years old.
But, in late 1700s, most European scientists
thought the year was only 6,000 years old!

42

• James Hutton (1726-1797)
• Scottish geologist
• the father of modern geology

• he argued that Earth is much older than 6,000
  years

43
In mid-late 1700s, a strong debate between
Catastrophism
vs.
Uniformitarianism
Gradually evidence began to support
uniformitarianism
44
The fit between the edges of continents suggested
that they might have drifted.
jigsaw-puzzle fit
45

• Alfred Wegener (1880-1930)
• German meteorologist and polar explorer
• in 1912, proposed the theory of continental
  drift

• Pangaea
• landmass

• Panthalassa
• surrounding ocean

(http//physics.fortlewis.edu/Astronomy/astronomy
20today/CHAISSON/AT307/IMAGES/AT07FG21.JPG)
46
Evidence from
fossils
(http//hyperphysics.phy-astr.gsu.edu/hbase/geophy
s/platevid.html)
glacial deposits
http//www.indiana.edu/g103/G103/week11.htm
47
Evidence from
fossils
(http//hyperphysics.phy-astr.gsu.edu/hbase/geophy
s/platevid.html)
glacial deposits
But how?
http//www.indiana.edu/g103/G103/week11.htm
48
The idea of continental drift evolved as evidence
accumulated
????
Kiyoo Wadachi
(1902-1995)
http//en.wikipedia.org/wiki/Pacific_Ring_of_Fire
http//www.japan-acad.go.jp/image/common/wadati.jp
g

• speculated that this could be associated with
  continental drift

49
More evidence

• Radiometric dating
• oldest rocks on ocean floor lt200 million years
  old

• Echo sounders
• showed a large mountain range in the middle of
  the Atlantic

Mid-Atlantic Ridge
50
A synthesis of continental drift and seafloor
spreading produced the theory of plate tectonics.
Harry Hess (1906-1969)
Robert Dietz (1914-1995)
http//scilib.ucsd.edu/sio/archives/photos/dietz.j
pg

• proposed the theory of seafloor spreading

51
Theory of seafloor spreading

• new seafloor develops at ocean ridges and
  spreads outward

http//oceansjsu.com/images/exp5_divergent.GIF

• motion powered by convection currents

52
subduction
subduction zones
http//www.le.ac.uk/geology/art/gl209/lecture5/ima
ge68.gif
continental drift

plate tectonics
seafloor spreading
John T. Wilson
53
Plate tectonics

• the Earths outer layer consists of about 14
  lithospheric plates

Thailand on Eurasian plate
http//en.wikipedia.org/wiki/ImagePlates_tect2_en
.svg
54
A tectonic system powered by heat
Convection currents
Plate movement powered by gravity
55
Plate motion based on The Global Positioning
System (GPS)
56
Question Time!
Why is the inside of Earth so hot?
57
Most tectonic activity occurs at plate
boundaries.
58
Earthquakes
Plates
59

• There are three types of plate boundaries
• divergent boundaries
• convergent boundaries
• transform boundaries

http//courses.science.fau.edu/rjordan/phy1931/WE
GENER/VG22.gif
60

• 1. Divergent plate boundaries
• new crust is created as two or more plates pull
  away from each other
• oceans are born and grow wider where plates
  diverge or pull apart

magma
lava
magma
rift valley
61
Mid-Atlantic Ridge
http//images.suite101.com/files/articles/102000/1
02057/midAtlantic_Ridge.jpg
62
Iceland
http//www.norvol.hi.is/amy/mar.jpg
Hot springs
Fracture
http//news.bbc.co.uk/olmedia/1725000/images/_1727
312_geo300.jpg
63

• 2. Convergent boundaries
• crust is destroyed and recycled back into the
  Earth
• known as subduction zones
• three types

(1) Oceanic-Continental convergence (2)
Oceanic-Oceanic convergence (3)
Continental-Continental convergence
64

• Oceanic-Continental convergence
• the continental plate is lifted up, and a
  mountain range is created

lithosphere
athenosphere
65

• 2) Oceanic-Oceanic convergence
• when two oceanic plates converge one is usually
  subducted under the other
• forms a deep oceanic trench
• also forms undersea volcanoes
• volcanoes form island chains called island arcs

66
(No Transcript)
67

• 3) Continental-Continental convergence
• when two continents meet, neither is subducted
• the crust is pushed upwards or sideways

68
Formation of Himalayas
http//www.monanneeaucollege.com/4.svt.chap8_fichi
ers/himalaya2.b.jpg
http//upload.wikimedia.org/wikipedia/kn/thumb/2/2
f/Himalaya-formation.gif/180px-Himalaya-formation.
gif
69

• 3. Transform plate boundaries
• where two plates slide horizontally past one
  another
• most occur on the ocean floor
• commonly offset actively spreading ridges
• generally defined by shallow earthquakes

San Andreas Fault 5 cm/year
http//www.platetectonics.com/book/page_5.asp
70
San Andreas Fault

• 1300 km long

http//nationalatlas.gov/articles/geology/features
/images/sanandreas.jpg
71
Summary of plate interactions
1. Oceanic
Divergent
2. Continental
1. Oceanic-Oceanic
Convergent
2. Oceanic-Continental
3. Continental-Continental
Transform
72
Facts Combine to Confirm the Theory of Plate
Tectonics.
Earth has a magnetic field
iron fillings
http//www.scifun.ed.ac.uk/card/images/left/earth-
magfield.jpg
73
flip
magnetic reversal
During the last 100 million years, the earth's
magnetic field has reversed about 170 times.
74
In basaltic magma, there are small particles of a
magnetic mineral called magnetite
When magma erupts at mid-ocean ridges, it cools
to form rock.
Magnetite acts like a small compass needle.

• so we know the direction of Earths magnetic
  field when the rock became solid
• paleomagnetism

75

• Magnetometer
• measures the amount and direction of magnetism
  in rock

http//www.mbari.org/expeditions/EasterMicroplate/
images/Magnetometer2_640.JPG

• in 1950s, they were towed just above the ocean
  floor

• revealed interesting patterns

76
normal
reversed

zebra patterns
http//volcano.und.edu/vwdocs/vwlessons/plate_tect
onics/part9.html
77
Vine and Matthews hypothesis (1963)

http//volcano.und.edu/vwdocs/vwlessons/plate_tect
onics/part9.html
The pattern of alternating weak and strong
magnetic fields were symmetrical because freshly
magnetized rocks born at the ridge are spread
apart and carried away from the ridge by plate
movement.
http//www.calstatela.edu/faculty/acolvil/plates/s
eafloor_formation.jpg
78
Plate movement above hot spots provides evidence
of plate tectonics
Hot spots a location on the Earth's surface that
has experienced active volcanism for a long
period of time
(http//en.wikipedia.org/wiki/ImageProminent_hots
pots.png)
79
Kauai 5.1 million years 510???
Niihau 4.9 million years 490???
Maui 0.75-1.3 million years 75-130???
Oahu 2.6-3.7 million years 260-370???
Hawaii lt0.5 million years lt 50???
http//www.csc.noaa.gov/crs/lca/images/hi_post_lg.
jpg
80
http//volcano.und.edu/vwdocs/vwlessons/hotspots.h
tml
81
http//www.clas.ufl.edu/users/chmonty/HIS3931crisi
s/plates.gif
82
A new island is now forming
Loihi Seamount

• depth 969 meters
• will reach surface in about 30,000 years

http//hvo.wr.usgs.gov/volcanoes/loihi/maploihi.gi
f
83
Meiji Seamount
70 million years
assembly-line chain of islands
Emperor Seamounts
50 million years
40 million years
20 million years
Kauai 5 million years
Hawaii lt1 million years
Hawaiian Ridge
http//www.cliffshade.com/colorado/images/hawaii-e
mperor.gif
84
(No Transcript)
85
Meiji Seamount
Emperor Seamounts
Hawaiian Ridge
http//www.cliffshade.com/colorado/images/hawaii-e
mperor.gif
86
Meiji Seamount
Emperor Seamounts
40 million years ago
Hawaiian Ridge
http//www.cliffshade.com/colorado/images/hawaii-e
mperor.gif
87
Sediment
If the ocean is unchanging where would the
thickest sediment be?
88
Sediment
http//www.ngdc.noaa.gov/mgg/image/sedthick9.jpg
89
Age of ocean floor
90
Key points
1. Earth was formed of material made in the
stars. 2. Earth, Ocean, and atmosphere
accumulated in layers sorted by density. 3.
Earths interior is layered inside. 4. The
study of earthquakes provides evidence for
layering. 5. Earths inner structure was
gradually revealed. 6. The new understanding of
Earth evolved slowly. 7. Most tectonic activity
occurs at plate boundaries 8. Facts combine to
confirm the theory of plate tectonics
91
Lecture schedule
Lecture Date Time 1. Introduction to
scientific writing 8/20 900-1020 2. How to
write a scientific paper in English 8/20 1500-16
20 3. Earth structure and plate
tectonics 8/21 900-1100 4. Circulation of the
atmosphere 8/21 1300-1500 5. Water and ocean
structure 8/22 1000-1100 6. Continental
margins and ocean basins 8/22 1300-1500 7.
Sediment 8/23 1000-1200 8. Marine
resources 8/23 1300-1400 9. Circulation of
the ocean 8/24 1100-1230