Download Tectonics

The tectonic system
A. Internal structure of Earth
B. Plates and plate
boundaries
C. Evidence for movement of
continents
D. The Earth’s magnetism
E. Earthquakes and the
Earth’s interior
F. Direct measurement of
plate motion
1
A. Internal structure of the Earth
1. By physical properties
2. By chemical
composition
2
Divisions of the Earth by physical properties
•
•
•
•
Atmosphere
- gas
• Atmosphere
Hydrosphere - liquid
• Hydrosphere
Lithosphere
- solid rock
Asthenosphere -rock, ~100 km
• Lithosphere ~250 km
partly (5-25%) molten
•
•
•
•
Asthenosphere
• Mesosphere - solid rock
Mesosphere
Outer core
2900 km
Inner• core
Outer core - liquid, metallic
5200 km
•
Inner core - solid, metallic
6370 km
3
Divisions of the Earth by chemical composition
•
•
•
•
•
•
•
Atmosphere
Hydrosphere
Crust
Mantle
Core
Atmosphere - (nitrogen, oxygen )
Hydrosphere - (Water: oxygen, hydrogen)
•
5-70 km
•
Crust (oxygen, silicon,
aluminum, iron,
magnesium)
Mantle (oxygen,
silicon, magnesium,
iron)
2900 km
•
6370 km
Core - iron, nickel
4
Divisions of the Earth by chemical composition
• Continental crust – less dense rock with more
silicon, aluminum
• Oceanic crust – more dense rock with more
iron, magnesium
5
B. Plates and plate boundaries
•Lithosphere is divided into
plates.
•Plates are in relative motion
at speeds of a few cm per
year
•There are 3 types of plate
boundary
1. Spreading centres
2. Subduction zones
3. Transform faults
6
Plates and plate boundaries
• Map of principal
plates
7
1. Spreading centres
• Mid-Atlantic ridge
8
1. Spreading centres
Pillow lavas from the
ocean floor
9
1. Spreading centres
• Iceland
10
1. Spreading centres
• Cross-section of a spreading centre
11
1. Spreading centres: summary
• Occur beneath the oceans
• Marked by a mid-ocean
ridge several thousand km
wide, rising 2 or 3 km
above surrounding ocean
floor
• Site of submarine
volcanoes and earthquake
activity
• New lithosphere formed by
ocean-floor spreading
• Plates move apart (a few
centimetres per year)
12
2. Subduction zones
• Deep trenches around
the Pacific Ocean
P&S 1.12
13
2. Subduction zones
• Subduction zone volcanoes (Mount St. Helens - before)
14
2. Subduction zones
• Subduction zone volcanoes (Mount St. Helens – after)
15
2. Subduction zones
• Where subduction
occurs close to a
continental margin,
there is often a
mountain belt
(orogen)
• Rocks within
orogen are
crumpled
(deformed)
16
2. Subduction zones: summary
• Subduction zone or
convergent plate boundary
• Deep ocean trench (up to 11
km deep)
• Benioff zone of deep
earthquakes
• Melting in mantle produces
magma
• Volcanic arc
• One plate moves under
another (a few centimetres
per year)
• Orogens (mountain belts)
form where subduction
zones affect continental crust
17
3. Transform faults
Transform faults (transcurrent plate boundaries)
18
3. Transform faults
• San Andreas Fault
19
3. Transform faults
• Dextral or right-lateral
transform fault
P&S 1.17
20
3. Transform faults
• Right-lateral transform fault
Shallow
earthquakes
Lithosphere
Crust
Asthenosphere
Right-lateral
transform fault
21
3. Transform faults
• Left-lateral transform fault
Shallow
earthquakes
Lithosphere
Crust
Asthenosphere
Left-lateral
transform fault
22
3. Transform faults
• Many small
transform faults
occur along the
mid-ocean ridges
• Larger transform
faults cut
continental crust
• Many shallow
earthquakes
23
C. Evidence for moving continents
• Common sense tells us the Earth is solid
• Until ~1960 most scientists also believed
continents remained fixed
• Lines of evidence supporting moving plates
•
•
•
•
•
•
•
Match of geologic structures
Fossils
Glaciation and climate
Paleomagnetism
Match of continent outlines
Seismicity
Direct measurement of plate movement by GPS
24
1. Match of continent outlines
Some continents
show 'jig-saw' fit
25
2. Match of rock units between continents
Very similar rock
units are now
separated by
oceans
26
3. Fossil evidence
Fossils of very similar land animals and plants are now
separated by oceans
27
4. Glaciation and climate
• Locations of ice
sheets at 350-300
Ma - no sense on
modern map
• Can be explained
if "Gondwanaland"
is reassembled
28
D. Paleomagnetism
• Before 60's most geophysicists claimed that Earth
was too rigid to allow continental drift.
• But first measurements of movement came
geophysics: studies of Earth's magnetism.
a) The Earth's magnetic field
b) Remanent magnetization
c) Magnetic reversals and anomalies on the
ocean floor
29
1. Earth's magnetic field
• Earth behaves
approximately as if
there is a bar magnet in
the core
30
1. Earth's magnetic field
• Field at any place has an inclination (steepness) and a declination
(direction)
• Inclination indicates distance from pole
• Declination indicates direction to pole
Inclination angle
31
2. Remanent magnetism
• Some ancient rocks were
(weakly) magnetized when
formed - "Remanent
magnetism"
• "Fossil compass needles"
• If age of rocks is known,
remanent magnetism
indicates the ancient location
of the pole
“Ma” in the diagrams signifies
“Million years before present”
500 Ma
32
2. Remanent magnetism
• Some ancient rocks were
(weakly) magnetized when
formed - "Remanent
magnetism"
• "Fossil compass needles"
• If age of rocks is known,
remanent magnetism
indicates the ancient location
of the pole
• Pole appears to have
wandered through time
“Ma” in the diagrams signifies
“Million years before present”
225
100
0 Ma
270
400
500
600 Ma
33
2. Remanent magnetism
• Some ancient rocks were
(weakly) magnetized when
formed - "Remanent
magnetism"
• "Fossil compass needles"
• If age of rocks is known,
remanent magnetism
indicates the ancient location
of the pole
• Pole appears to have
wandered through time
• Apparent polar wander path
(APWP)
• Hence either the pole moved
or the continent moved
“Ma” in the diagrams signifies
“Million years before present”
225
100
0 Ma
270
400
500
600 Ma
34
2. Remanent magnetism
• Different continents
show different
APWPs
• Hence it must be the
continents that
moved
“Ma” in the diagrams signifies
“Million years before present”
North America
0 Ma
Europe
35
2. Remanent magnetism
• Other changes are recorded by remanent
magnetism
• N. and S. magnetic poles appear to have "flipped'
through time
Volcano showing magnetized lava flows
36
2. Remanent magnetism
•
N. and S. magnetic poles appear to have "flipped'
through time
Gary A Glatzmaier
University of California, Santa
Cruz
www.es.ucsc.edu/~glatz
37
2. Remanent magnetism
• Time scale of
magnetic reversals
38
3. Reversals and ocean-floor anomalies
• Magnetic anomaly:
– field slightly stronger or
weaker than normal
• Surveys in the oceans
show
– Central positive anomaly
– Symmetric pattern
39
3. Reversals and ocean-floor anomalies
• Vine-Matthews hypothesis
• Magnetic anomalies result from remanent magnetism
acquired during spreading of ocean-floor while magnetic
reversals occurred.
40
3. Reversals and ocean-floor anomalies
• Match with
reversal
history
• Measure
rates
• Map age of
ocean floor
• New ocean
floor is
found along
mid-ocean
ridges
41
E. Earthquakes and seismicity
1.
2.
3.
4.
5.
6.
Intensity and magnitude
Seismic waves
Origin of earthquakes
Locating earthquakes
Earthquakes at plate boundaries
Interior of the Earth
42
1. Intensity and magnitude
• Effect of earthquake in Japan
P&S 18.18
43
1. Intensity and magnitude
• Seismographs &
seismometers
Ancient
seismic
detector
Seismograph
Seismometer
44
1. Intensity and magnitude
• Intensity: Strength of
ground shaking at a
point.
• Intensity depends
on many factors e.g.
distance from the
focus.
45
1. Intensity and magnitude
• Magnitude: a measure of total energy
released
• Charles Richter
• Ground movement at standardized distance
• Log scale
• Modern scale based on Richter's
• Each step on scale multiplies energy by
√1000.
• E.g., M 8 releases 1000 times more energy
than M 6.
46
1. Intensity and magnitude
P&S 18.11
47
1. Intensity and magnitude
http://wwwneic.cr.usgs.gov/neis/bulletin/
48
2. Seismic waves
• Body waves and surface waves
Epicentre
49
2. Seismic waves
•
•
•
•
•
•
Body waves: Primary or P-waves
3-7 km/s in the crust
Similar to sound waves
Compression and expansion ('dilation')
Vibration direction parallel to propagation
Pass through solid, liquid or gas.
50
2. Seismic waves
•
•
•
•
Body waves: Secondary or S-waves
1.5- 5 km/s in the crust
Shear waves
Vibration direction perpendicular to
propagation
• Solids only
51
2. Seismic waves
• Surface
waves on
land
– Surface
waves form
when body
waves reach
the surface
– Slower but
larger than
body waves
– Cause most
damage
Rayleigh waves
Love waves
52
2. Seismic waves
• Tsunami: surface
waves on ocean
– Low on open
ocean (~ 1 m)
– 600 km/hr +
– In shallow water,
slow down, get
higher (>10 m)
– Devastate coastal
communities
Effect of 1929 tsunami on
Burin Peninsula,
Newfoundland
53
3. Origin of earthquakes
• Most earthquakes originate < 70 km deep.
• Result from
– Elastic strain
– Brittle fracture (or brittle failure).
• These processes occur in cold rocks, typically
near Earth's surface
54
3. Origin of earthquakes
55
4. Locating earthquakes
• Distance of focus is found from interval between P
and S arrival
56
4. Locating earthquakes
Example
• Station A
1500 km
• Station B
5600 km
• Station C
8600 km
B
5600 km
C
1500 km
8600 km
A
Epicentre
57
5. Earthquakes at plate boundaries
• Epicentre is point on Earth’s surface directly above
focus
• Map of epicentres: Earthquakes are concentrated
at plate boundaries
58
5. Earthquakes at plate boundaries
• All deep (> 100 km) events are at subduction zones.
• Why?
– Only cold rocks display brittle fracture
– In Benioff zone cold rocks are found deep.
59
6. Interior of the Earth
• Body waves tell us about Earth's interior
– Reflection
– Refraction
60
6. Interior of the Earth
• S-waves cannot pass through liquid
61
6. Interior of the Earth
• Evidence for
core
• P waves
from major
earthquake
8
6
4
2
Focus
10
12
105°
142°
20
62
6. Interior of the Earth
• Evidence for
core:
• S waves from
major
earthquake
14
10
7
Focus
17
20
105°
63
6. Interior of the Earth
• S-waves are
blocked by liquid
core
64
6. Interior of the Earth
• P-waves are
refracted by core
• Acts as lens
65
Seismicity: summary
• Earthquakes are a major hazard when located close
to population centres
– Intensity: amount of shaking at a point
– Magnitude: total energy released at focus
• Seismic waves:
– Surface waves
• L-waves, R-waves, Tsunami waves
– Body waves
• P-waves, S-waves
• Arrivals of P and S waves at locations distant from
the epicentre can be used
– To locate earthquakes
– To recognize plate boundaries
– To identify major features of Earth’s interior
66
F. Direct measurement of plate movement
• Global Positioning
System (GPS) is a
network of satellites,
used to provide very
accurate locations on
Earth’s surface
• By reoccupying sites
over a period of years it
is possible to measure
plate movement directly
http://www.unavco.org/pubs_reports/brochures/1998_UNAVCO/1998_UNAVCO.html
67
F. Direct measurement of plate movement
• Global Positioning
System (GPS) is a
network of satellites,
used to provide very
accurate locations on
Earth’s surface
• By reoccupying sites
over a period of years it
is possible to measure
plate movement directly
http://www.unavco.org/pubs_reports/brochures/1998_UNAVCO/1998_UNAVCO.html
68
F. Direct measurement of plate movement
• Global Positioning
System (GPS) is a
network of satellites,
used to provide very
accurate locations on
Earth’s surface
• By reoccupying sites
over a period of years it
is possible to measure
plate movement directly
http://www.unavco.org/pubs_reports/brochures/1998_UNAVCO/1998_UNAVCO.html
69