Milankovitch Cycles, Sea Level Rise, & Tsunamis
Orbital Variations: Milankovitch Cycles
-eccentricity
variation in the shape of Earth’s orbit
-obliquity
axial tilt
-precession
wobble
Correlated with climate predicted by deep sea sediments
Quaternary ice ages
Predicts a cooling period
Needs land mass near the poles to support ice sheets
No human influence
Variations in Earth’s Orbit-Eccentricity
3% difference between aphelion and perihelion
20% - 30% maximum
T = ~100,000 years
Variations in Earth’s axial tilt-Obliquity
Decrease tilt = decrease temperature variation
between winter and summer
T = ~41,000 years
Variations in Earth’s axial wobble-Precession
Controls where in the orbit seasons occur
T = 23,000 - 26,000 years
Sea-Floor Sediments
Organisms at the surface
Balance between ocean surface waters and the atmosphere
As climate changes so does the composition of the surface organisms
Recorded in sediments as the organisms die
Rates of Sedimentation
• Ocean Basins: slow, pelagic sediments
– 0.5 – 1.0 cm/ 1000 years
– Average Accumulation 500 – 600 m
– Thickness depends on age
– Oldest sea floor is 200 million years
Biogenous
Siliceous (SiO2)
diatoms, radiolaria (photosynthetic)
ocean is under-saturated
dissolves quicker in warm water
Diatoms = cool regions
Radiolaria = warmer
regions
Biogenous
Calcareous (CaCO3)
cocolithophorids, pteropods, foraminifera
dominant pelagic sediment
Lysocline = calcium carbonate dissolves
CCD
= dissolution equals accumulation 4500m
Oxygen Isotope Analysis from Ice Cores
Isotope = One of two or more atoms having the same atomic
number (number of protons) but different mass numbers
(protons + neutrons)
18O 16O
ratios
Water forms with either
16O
evaporates easier
Oxygen Isotope Analysis from Ice Cores
Precipitation & glacial ice are enriched in 16O
Oceans are enriched in 18O
Ocean
18O ice ages
16O increases warmer periods
cocolithophorids, pteropods, foraminifera = record ratios in their shells
(CaCO3)
Oxygen Isotope Analysis
Temperature Variations
18O
more easily evaporated
during warm periods
Ice cores record the warm periods
Pockets of air within the crystal lattice yield
gasses (CO2 and CH4), pollen, ash, pollutants
Link between CO2 and CH4 concentrations and temperature
changes
14
Tsunamis
Disturbing
force
Restoring force
Amount of energy
in ocean surface
Type of wave
Seismic
Gravity disruption
landslides
Wind
Surface
tension
Gravity
Tide
Tsunami
Seiche
Wind wave
Capillary wave
(ripple)
2412
hr.hr.
100,000 sec 10,000 sec 1,000 sec
(1 1/4 days) (3 hr)
(17 min)
100 sec 10 sec
1 sec
1/10 sec
1/100 sec
1
10
100
Period (time, in seconds for
two successive wave crests to Frequency (waves
pass a fixed point)
per second)
Tsunami (Harbor Wave)
•
•
Displacement of a large volume of water
Shallow water waves
 Long wavelength
 Long period (few minutes to over an hr)
http://www.seed.slb.com/en/scictr/watch/living_planet/tsunami.htm
Satellite images of a coastal village in Banda Aceh,
Indonesia, before and after the December 26, 2004
tsunami.
Plates  Rigid Slabs of Rock
Seven major plates – Pacific, African, Eurasian, North American,
Antarctic, South American, Australian
Minor plates – Nazca, Indian, Arabian, Philippine, Caribbean, Cocos,
Scotia, Juan de Fuca
20
Density is a key concept for
understanding the structure of
Earth – differences in density
lead to stratification (layers).
Density measures the mass per unit
volume of a substance.
Density = _Mass_
Volume
Water has a density of 1 g/cm3
Granite Rock is 2.7 g/cm3
21
Waves associated with earthquakes:
P waves – Primary, compressional, arrive
first, pass through solid, liquid and gas
S waves – Secondary, ‘side-to-side’ or
shear waves, arrive second, cannot pass
through liquid, pass through solid
Seismograph
P
S
Mantle
Focus
0
10
P
Core
S
Minutes
20
30
40
50
Surface
waves
22
Earth’s Crust: cold, brittle
thin layer, 0.4% of Earth’s mass and 1% of its
volume
Continental Crust:
•Primarily granitic type rock (Na, K, Al, SiO2)
•40 km thick on average
•Relatively light, 2.7 g/cm3
Oceanic Crust
•Primarily basaltic (Fe, Mg, Ca, low SiO2)
•7 km thick
•Relatively dense, 2.9 g/cm3
cool, solid crust and upper (rigid) mantle “float”
and move over hotter, deformable lower mantle
23
Lithosphere & Asthenosphere: More detailed
description of Earth’s layered structure according
to mechanical behavior of rocks, which ranges from
very rigid to deformable
1. lithosphere: rigid surface
shell that includes upper
mantle and crust (here is
where ‘plate tectonics’
work), cool layer
2. asthenosphere: layer
below lithosphere, part of
the mantle, weak and
deformable (ductile,
deforms as plates move),
partial melting of material
happens here, hotter layer
(100 – 200 km)
(200 – 400 km)
24
Evidence for Seafloor Spreading
•
•
•
•
Earthquake epicenters
Heat flow
Ocean Sediments
Radiometric dating of rocks of ocean and
continental crust
• Magnetism
25
Synthesis of Continental Drift and Seafloor
Spreading --> Theory of Plate Tectonics
Main points of theory (Wilson, 1965):
Earth’s outer layer is divided into lithospheric plate
Earth’s plates float on the asthenosphere
Plate movement is powered by convection currents in the
asthenosphere seafloor spreading, and the downward
pull of a descending plate’s leading edge.
Hess and Dietz in 1960 proposed a model to
explain features of ocean floor and of continental
motion powered by heat  mantle convection
26
Model of Mantle Convection
spreading centers – where new sea floor and oceanic lithosphere form
subduction zones – where old oceanic lithosphere descends
27
Improved Mapping, WWII
Convergent plate
boundary marked by
trench
Asthenosphere
Africa
Divergent plate boundary
marked by mid-ocean ridge Transform fault
(spreading center)
Oceanic lithosphere
Subduction fueling
volcanoes
Asia
Descending plate
pulled down by
gravity
Philippine Trench
Mantle
upwelling
Superplume
Outer core
Mariana Trench
Mantle
Mid-Atlantic
Ridge
Inner
core
Hot
South America
Cold
Possible
convection cells
Rapid
convection
at hot
spots
Peru–Chile
Trench
Hawaii
East
Pacific Rise
28
Age and thickness of sea floor sediment – heat flow
29
Earthquake Epicenters
Shallow epicenters –
crustal movement
(less than 100 km)
Mid-deep epicenters
subduction
(greater than 100 km)
30
The Earth’s Magnetic Field
• Rocks record the direction of magnetic field
(Magnetite)
• Magnetic field direction changes through geologic time
– polar reversals recorded in rocks
* 560 °C = rock solidifies (Curie Point)
* Captures magnetic signature
• Particles of Magnetite align with the direction of
Earth’s magnetic field at the time of rock formation
31
The patterns of paleomagnetism support
plate tectonic theory. The molten rocks
at the spreading center take on the
polarity of the planet while they are
cooling. When Earth’s polarity reverses,
the polarity of newly formed rock
changes.
(a) When scientists conducted a
magnetic survey of a spreading center,
the Mid-Atlantic Ridge, they found bands
of weaker and stronger magnetic fields
frozen in the rocks.
(b) The molten rocks forming at the
spreading center take on the polarity of
the planet when they are cooling and
then move slowly in both directions from
the center. When Earth’s magnetic field
reverses, the polarity of new-formed
rocks changes, creating symmetrical
32
bands of opposite polarity
As plates float on the deformable
aesthenosphere, they interact among
each other. The result of these
interactions is the existence of 3
types of boundaries:
(a) Divergent: plates move away from each
other, examples:
* Divergent oceanic crust:
the Mid-Atlantic Ridge
* Divergent continental crust:
the Rift Valley of East Africa
(b) Convergent: plates move toward each
other.
Three possible combinations:
continent-ocean, ocean-ocean,
continent-continent
(c) Transform:
neither (a) nor (b), plates slide
past one another – transform faults.
* Example: San Andreas fault
33
34
Oceans are created along divergent boundaries
Recall that seafloor
spreading was an idea
proposed in 1960 to explain
the features of the ocean
floor. It explained the
development of the seafloor
at the Mid-Atlantic Ridge.
Convection currents in the
mantle were proposed as the
force that caused the ocean
to grow and the continents
to move.
The breakdown of Pangea
showing spreading centers
and mid-ocean ridges
2 kinds of
plate
divergences
35
Mid Atlantic Ridge
South Indian Ridge
36
Modern divergence
East African Rift System
37
East African Rift System
38
Wilson Cycles
39
Supercontinent
Pangaea
Pannotia/Gondwanaland
Rodinia
Nuna
Formation (Ma)
350
650
900
1800
Breakup (Ma)
250
550
760
1500
Mode Ref.
Atlantic (4)
Pacific (4, 5)
Pacific (4, 6, 39)
Atlantic? (4)
40
Island Arcs Form, Continents Collide, and Crust
Recycles at Convergent Plate Boundaries
Convergent Plate
Boundaries - Regions
where plates are
pushing together can be
further classified as:
• Oceanic crust toward
continental crust the west coast of
South America.
• Oceanic crust toward
oceanic crust occurring in the
northern Pacific.
• Continental crust
toward continental
crust – one example is
the Himalayas.
3 kinds of plate convergences
41
Convergent Plate Boundaries
• Continent – Ocean
• Ocean – Ocean
• Continent – Continent
42
Continent – Ocean
West Coast of South America
43
• Continent – Ocean
• Mount St. Helens
44
Island Arcs Form, Continents Collide, and Crust
Recycles at Convergent Plate Boundaries
The formation of an
island arc along a
trench as two oceanic
plates converge. The
volcanic islands form
as masses of magma
reach the seafloor.
The Japanese islands
were formed in this
way.




Motion of the plates:
Mechanisms – not fully understood
Rates: average 5 cm/year
Mid-Atlantic Ridge = 2.5 – 3.0 cm/yr
East-Pacific Rise = 8.0 – 13.0 cm/yr
45
Convergent Plate Boundaries
Ocean-Ocean
Aleutian Islands, Alaska
46
Ocean – Ocean
Caribbean Islands
47
Plate Movement above Mantle Plumes and Hot
Spots Provides Evidence of Plate Tectonics
(See also Figure 3.33 on
page 89 of textbook)
Formation of a volcanic island chain as an oceanic plate moves over a
stationary mantle plume and hot spot. In this example, showing the
formation of the Hawai’ian Islands, Loihi is such a newly forming island.
48
Chapter 3: Summary
Keep in mind that the important points in this chapter are:
1.
2.
3.
4.
5.
6.
7.
8.
Internal Layers: inner core, outer core, mantle, crust (continental
and oceanic).
P and S waves – used to study Earth’s layered structure
Lithosphere and Asthenosphere – defined according to mechanical
behavior of rocks
Isostasy – pressure balance between overlying crust and
astheosphere deformation
Continental drift – plates/continents moving about surface;
deduced from definitive evidence: ridges, rise, trench system, seafloor spreading, spreading centers, subduction zones
Evidence of crustal motion: earthquakes epicenter, heat flow,
radiometric dating, magnetism
Plate Tectonics – 7-8 major plates, 3 types of plate boundaries
Convergent Plate Boundaries – ocean-continent, ocean-ocean,
continent-continent
49
Chapter 3: Key Concepts
Some seismic waves–energy associated with earthquakes–can pass
through Earth. Analysis of how these waves are changed, and the
time required for their passage, has told researchers much about
conditions inside Earth.
Earth is composed of concentric spherical layers, with the least dense
layer on the outside and the most dense as the core. The
lithosphere, the outermost solid shell that includes the crust,
floats on the hot, deformable asthenosphere. The mantle is the
largest of the layers.
Large regions of Earth’s continents are held above sea level by
isostatic equilibrium, a process analogous to a ship floating in
water.
Plate motion is driven by slow convection (heat-generated) currents
flowing in the mantle. Most of the heat that drives the plates is
generated by the decay of radioactive elements within Earth.
50
Chapter 3: Key Concepts
Plate tectonics theory suggests that Earth’s surface is not a static
arrangement of continents and ocean, but a dynamic mosaic of
jostling segments called lithospheric plates. The plates have
collided, moved apart, and slipped past one another since Earth’s
crust first solidified.
The confirmation of plate tectonics rests on diverse scientific studies
from many disciplines. Among the most convincing is the study of
paleomagnetism, the orientation of Earth’s magnetic field frozen
into rock as it solidifies.
Most of the large-scale features seen at Earth’s surface may be
explained by the interactions of plate tectonics. Plate tectonics
also explains why our ancient planet has surprisingly young
seafloors, the oldest of which is only as old as the dinosaurs –
that is, about 1/23 of the age of Earth.
51