Transcript 10A_InternalEarrthStructTectonicsx

Internal Structure of the Earth
and Plate Tectonics
Lecture by Dr. Ken Galli, Boston College
EESC116301 Environmental Issues and
Resources
July 28, 2015
Please do not distribute beyond the EESC116301 Class.
Case History: Two Major CA Cities
• San Andreas fault: a transform plate
boundary between the North American
and the Pacific plates
• Two major cities on the opposite sides
of the fault: Los Angeles and San
Francisco
•Many major earthquakes
related to the fault system
•Loss of many lives and
billions of property damages
due to earthquakes
•New construction and
retrofitting of infrastructures
has become more expensive
•When will be the next “big
one” and what to do? How to
deal with the potential
consequence?
Internal Structure of Earth
• The Earth is layered and dynamic: Interior
differentiation and concentric layers
• Chemical model by composition and density
(heavy or light): Crust, mantle, core, and Moho
discontinuity between the crust and mantle
• Physical property model (solid or liquid, weak or
strong): Lithosphere (crust and upper rigid
mantle), asthenosphere, mesosphere, liquid
outer core, inner solid core
Theory of Plate Tectonics
• The upper mechanical layer of Earth (lithosphere) is divided into rigid
plates that move away from, toward, and along each other
• Most (!) deformation of Earth’s crust occurs at plate boundaries
Earth’s Layers
3 main layers
defined by
composition:
• Crust - Outer
• Mantle - Middle
• Core - Center
http://www.physicalgeography.net/fundamentals/10h.html
Study of Earth’s Interior Structure
• Knowledge primarily through the study of
seismology
• Seismology: Study of earthquakes and seismic
waves
• Examining the paths and speeds of seismic
waves through reflection and refraction
• Magma likely generated in the asthenosphere
• Slabs of lithosphere have apparently sunk deep
into the mantle
• Variability of lithosphere thickness reflects
changes in its age and history
Composition - How Do We Know?
Best Guess!
Whole Earth
• Meteorites - Fe, Ni (same age as Earth)
• Information from velocities of seismic waves indicate material
Crust (5-40 Km)
• Samples (mountain building helps!)
Mantle (5/40 to 2885 Km)
• Kimberlite pipes - intrusive igneous rock from the mantle
• Lava / volcanic rock
• Mountain building
Core (2885 to 6371 Km)
• Inference
– Earth’s mean density = 5.5 g/cm3
– Crust 2.5 to 3 g/cm3; mantle 3.3 g/cm3 to 5.5 g/cm3
– Density of core at least 10 to 11 g/cm3 (iron and nickel)
Seismic P Wave
• Primary or push-pull wave, travels like sound
wave
• Direction of rock particle vibration parallel to
that of wave propagation
• Fastest rates of propagation, first arrival to
the seismograph
• Body wave travels through Earth interior and
all media—solid and liquid
Seismic S-Wave
• Secondary or shear waves
• The direction of particle vibration
perpendicular to that of propagation
• Propagates slower than P waves
• Body wave, propagating through Earth’s
interior, but not its liquid layers
Seismic Waves and Internal
Structures
• Earth’s interior boundaries: Sudden
changes in the speed of seismic waves
• Different characteristics: Different rates
and paths of wave propagation
• Asthenosphere: Low velocity zone, major
source of Earth magma
• Outer Core: Liquid, no S wave transmits
through it
Model of Earth’s
Interior
Figure 2.2b
Crust
• Two types of crust:
– Continental
• 30% of crust
• Granites and Diorites - rich in silicates
and feldspars (lighter materials)
• 35-40 Km thick
• Oldest is 3.8 billion years (90% solar
system age; missing ~700 m.y.)
– Oceanic crust
• Basalt - Mg, Fe (heavier materials)
• 5-10 Km thick
• 200 Ma oldest; 100 Ma average
Our deepest hole:
9 Kilometers ….. we have a
long way to go!
Mantle
• MOHO - Mohorovicic Discontinuity
• Core mantle boundary - change in
mineralogy
• Density - getting heavier
• 3.3 - 5.5 g/cm3
• Probably material such as peridotite
(lots of heavy olivine - Fe, Mg)
• Samples from kimberlites, xenoliths
in volcanic eruptions, basalt
composition; lab experiments
Our deepest hole:
9 Kilometers ….. we have a
long way to go!
Core
• Outer core
– Molten, near solid point (does not
transmit certain seismic waves)
– Density of pure iron or nickel/iron
– Includes ~ half of diameter of Earth
– 2x density of mantle
• Inner core
– Solid (higher pressure than outer core)
– Density of pure iron or nickel/iron
– ~ Size of moon
Our deepest hole:
9 Kilometers ….. we have a
long way to go!
Earth’s Layers
3 layers defined by
mechanical
properties (strength):
• Lithosphere
• Asthenosphere
• Mesosphere
http://earthguide.ucsd.edu/mar/dec5.html
• Lithosphere
–
–
–
–
PLATES in Plate Tectonics
Upper 100 km
Crust and upper mantle
Rigid
• Asthenosphere
– 100 km to ~350 km
– Upper mantle
– Near melting point; little strength;
ductile - NOT A LIQUID!
– Plates moving on this
– Magma generation
• Mesosphere
– Extends to core
– Also hot; strong due to pressure
Internal Dynamics of Earth
• Evidence
– Earth’s landscape
– Dynamic phenomena: earthquakes,
volcanoes
• Plate Tectonics: Hypothesis and Theory
– Continental drift
– Seafloor spreading
– Plate tectonics – a unifying theory
Dynamic Earth—Evidence
• Mountain belts (continental mountain
ranges and oceanic ridges)
• Earthquake distribution: Concentrated
zones
• Earthquake occurrences over time
• Volcanism in space: Concentrated zones
• Volcanism over time
Why Do the Plates Move?
Got Heat?
Earth - 3 Heat Sources:
• Loss of original heat of
formation (geothermal /
core is cooling)
• Radioactive decay of
elements in Earth’s
materials
• The Sun - external; not
important to plate tectonics
Convection: Driving Force of Plate Tectonics
• Interior of Earth has
sluggish convection in
some regions
• Heat from core rises,
creates convection cells
in the mantle
NOT LIQUID!
http://pubs.usgs.gov/publications/text/unanswered.html
• Rising hot material at mid-ocean ridges and midocean volcanic islands
• Descending cooler material at trenches
• Lithospheric plates “carried” with the convection cells
Convection in the Mantle
http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html and http://www.seismology.harvard.edu/Projects.html
Blue blobs show where colder, denser material is sinking into the mantle. Near the
surface, most of the colder material is in the ancient roots of continents. Subducting
slabs of oceanic lithosphere appear, recycled into the mantle from oceanic trenches.
Convection in the Mantle
http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html and http://www.seismology.harvard.edu/Projects.html
Red blobs are warmer plumes of less dense material, rising principally into the oceanridge spreading centers. A huge plume seems to be feeding spreading at the East
Pacific Rise directly from the core. Most of the heat being released from the earth's
interior emerges at the fast-spreading East Pacific Rise.
More than Convection?
• In addition to thermal convection cells, some
geologists think that movement may be aided by
– “slab-pull” - slab is
cold and dense and
pulls the plate
– “ridge-push” rising magma
pushes the ridges
up and gravity
pushes the ocean
floor toward the
trench
Plate Tectonics as the Unifying
Concept of Earth Science
Accumulation of
Observations Evidence
 Patterns of continents
 Paleontology
 Geology
 Patterns of sea floor ages
 Patterns of seafloor depth
 Patterns of volcanoes
 Patterns of earthquakes
Alfred Wegener
• 1912 Continental Drift
• Observations
–
–
–
–
•
•
Fit of Continents
Geology
Paleontology
Climate belts
Pangaea 200 Ma
Breakup 180 Ma
• Rigid bodies moving through
yielding seafloor – could not
provide mechanism of
movement
Continental Drift
• Same fossils across both sides of the Atlantic Ocean
Figure 2.18
Continental
Drift
• Matching Mountain
Ranges (Rock
distribution)
Paleozoic
Glaciations
Figure 2.19
Seafloor Spreading
• Lack of mechanism for continental drift
• 1950s and early 1960s, ocean expedition
increased knowledge of oceanography
• In 1960s, Harry Hess proposed seafloor
spreading
– Seafloor not a single static piece
– Mid-oceanic ridges, or spreading centers
where new crust is formed and seafloor
spreads
Harry Hess and Seafloor Spreading
• New ocean crust at midocean ridges
• Ocean crust dragged down
at trenches; mountains
form here
• Continental crust too light;
remains at surface
• Earthquakes occur where
crust descends
“It explains everything….”
Seafloor Spreading
• Paleomagnetic data
– Dipolar magnetic field
– Magnetic field recorded by iron-bearing
igneous rocks
– Striking symmetrical magnetic anomaly
stripes
• Age of seafloor rocks: Progressively
younger toward the mid-oceanic ridge
• Thickness of seafloor sediments:
Progressively thinner toward the ridge
Seafloor Spreading
Figure 2.15
Seafloor Spreading - Observations
• Fit of continents - new material pushes them apart
• Topography of ocean floors - hot ridges, trenches
• Volcanism at ridge axes - hot mantle material
• Seismic zones near margins - descending plates
Magnetism – The Final Piece
• Earth has
magnetic field
• Similar to a giant
dipole magnet
– magnetic poles
essentially coincide
with the geographic
poles
– may result from
different rotation of
outer core and
mantle
Magnetic Reversals
• Earth’s present magnetic field is called normal
– magnetic north near the north geographic pole
– magnetic south near the south geographic pole
• At various times in the past, Earth’s magnetic
field has completely reversed
– magnetic south near the north geographic pole
– magnetic north near the south geographic pole
When magma cools, takes on signature of
Earth’s prevailing magnetic field
magnetic iron-bearing minerals align with Earth’s magnetic field
Vine and Matthews (1960’s)
“MOR”
Evidence for Hess’s seafloor spreading idea
What does this have to do with seafloor spreading?
Mid-Atlantic Ridge
Oceanic Crust Is Young
• Seafloor spreading theory indicates that
– oceanic crust is geologically young
– forms during spreading
– destroyed during subduction
• Radiometric dating confirms young age
– youngest oceanic crust occurs at mid-ocean
ridges
– oldest oceanic crust is less than 200 million
years old
– oldest continental crust is 4.3 billion yeas old
Theory of Plate Tectonics
Supporting Evidence:
Fit of continents
Patterns of heat flow
Ocean floor topography
Age patterns of seafloor
Volcanism at ridge axes / hot spots
Magnetic stripes
Seismic zones
Patterns of mountains
Plate Tectonics
• A unified theory: Study the dynamic creation,
movement, and destruction processes of plates
• Plates: Fragments of lithosphere
• Plates move in relation to each other at varied
rates
• No major tectonic movements within plates
• Dynamic actions concentrated along plate
boundaries
• Plate boundaries: Defined by the areas of
concentrated seismic and volcanic activities,
rifts, faults, and mountain ridges
Plate Tectonics
• Three major types of plate boundaries
• Divergent: plates moving apart and new
lithosphere produced in mid-oceanic ridge
• Convergent: plates collide, subduction and
mountain building
• Transform: two plates slide past one
another
Plate Tectonics
Figure 2.4a
Plate Tectonics
Table 2.1 (1 of 2)
Plate Tectonics (3)
Table 2.1 (2 of 2)
Plate Tectonics
• Divergent plate boundary
– Plates move away from each other
– Mid-oceanic ridges
– Continental rift valleys
– Creates new seafloors
– Extensional stress and shallow earthquakes
– Basaltic volcanism
Plate Tectonics
• Convergent plate boundary
Plates collide with each other and three subtypes
– C-C boundary: Major young mountain belts and
shallow earthquakes
– C-O boundary: Major volcanic mountain belts,
subduction zone and oceanic trench, earthquakes
– O-O boundary: Subduction zone, deep oceanic
trench, volcanic island arc, wide earthquake zones
Plate Tectonics
• Transform plate boundary
– Locations where the edges of two plates slide
past one another
– Spreading zone is not a single, continuous rift;
offset by transform faults
– Most transform plate boundaries are within
oceanic crust, some occur within continents
– Famous transform plate boundary on land is
the San Andreas fault
Plate Boundaries
Plate Motion
• Plates move a few centimeters per year: about the
growth rate of human fingernails
• The rates of movement changes over time
• North American plate along the San Andreas fault
about 3.5 cm (1.4 in.) per year
• When rough edges along the plate move quickly, an
earthquake may be produced
• Often slow creeping movement
Hot Spots
• Places on Earth: Volcanic centers with magma
source from deep mantle, perhaps near the
core-mantle boundary
• Hot spots can be on continents and oceans,
Yellowstone and Hawaii
• Forming a chain of volcanoes over a stationary
hot spot: Example, the Hawaiian–Emperor Chain
in the Pacific Ocean
• The bend of a seamount chain over a hot spot
representing the change of plate motion
Hot Spots
Figure 2.16a
Plate Tectonics and Environmental
Geology
• Significance of tectonic cycle
– Global zones of resources (oil, gas, and mineral
ores)
– Global belts of earthquakes and volcanic
activities
– Impacts on the landscape and global climates
– Geologic knowledge on plate tectonics:
Foundation for urban development and hazard
mitigation
Tectonics and Environmental
Geology
Figure 2.4b
Isthmus of Panama
• Formed 3 Ma from collision between
Pacific and Carribbean plates
• Shut down the flow of water between
the Atlantic and Pacific
• Atlantic currents were forced
northward, forming the Gulf Stream
• With warm Caribbean waters flowing
toward the northeast Atlantic, the
climate of northwestern Europe grew
warmer (winters would be as much as
10 °C colder without the Gulf Stream)
http://www.nationsonline.org/oneworld/panama.htm
Isthmus of Panama
• The Atlantic, no longer mingling
with the Pacific, grew saltier
• In short, the Isthmus of Panama
directly and indirectly influenced
ocean and atmospheric
circulation patterns, which
regulated patterns of rainfall,
which in turn sculpted landscapes
• Subsequent warm, wet weather
over northern Europe may have
resulted in the formation of an
Arctic ice cap and contributed to
the ice age during the
Pleistocene Epoch
Gulf Stream
http://www.nc-climate.ncsu.edu/education/ccms/2003/ben_jon/Pictures/Gulf-Stream2.gif
Gulf Stream
http://oceancurrents.rsmas.miami.edu/atlantic/img_mgsva/gulf-stream-YYY.gif
Effect of Andes Mountains
• Continued uplift raised the
Andes to an altitude of >4000 m
by the end of the Neogene
• Barrier for southeast trade winds
in the subtropics and for westerly
winds in the midlatitude regions
of South America
• Today, the blocking of the
westerly winds results in
enhanced precipitation on the
western side of the mountain
range (Chilean and Patagonian
glaciers) and causes a strong
rain shadow on the eastern side
(Patagonian desert)
Terra.rice.edu/plateboundary
Supercontinents and Climate
• During periods of supercontinent formation, an ice age often results
• Sediments increase as continents collide
• Removal of carbon dioxide out of the atmosphere by sediments
leads to a decrease in global temperatures
• Mountain building at equator upsets temperature balance
• Causes decrease in temperature at the equator, resulting in a lower
global temperature
Supercontinents and Climate
• Moisture
– The interior of a supercontinent
is a significant distance away
from a moisture source (i.e.
large bodies of water)
– Climates were extremely arid in
the interior reaches of
supercontinents
http://vishnu.glg.nau.edu/rcb/globaltext.html
Climate History and Tectonics
• We can determine the past climate of the
Earth by mapping the past positions of the
continents
• Plot distribution of ancient coals, desert
deposits, tropical soils, salt deposits, glacial
material, as well as the distribution of plants
and animals that are sensitive to climate,
such as alligators, palm trees & mangrove
swamps
• Certain types of rocks, such as coals, which
need abundant rainfall, form under certain
climatic conditions (for example, coal forms in
tropical rainforests or temperate forests)
• By mapping the past distribution of thousands
of these rock types, we have begun to map
the distribution of the ancient climatic belts
A Few Questions...
• Assume the Pangaea never broke up, how might
today’s environments be different?
• What are the major differences in plate tectonic
settings between the U.S. eastern and western
coasts?
• Will the tectonic cycle ever stop? Why or why not?
• Why is most seismic and volcanic energy released
along the Pacific rim?
• Does plate tectonics play a role in shaping your
local environment?
Why do the plates move?
Convection!
Fig. 3. The continent of
Africa is thought to have
been split by a series of rift
valleys in various states of
development. Those in
East Africa are still in thick
crust. Those in West Africa
are associated with thick
oil-bearing sediments. In
the Red Sea area the rifting
has gone so far as to form
a narrow ocean. In the
south-east Madagascar
has been completely
separated from Africa by
rifting.
Source: http://www.le.ac.uk/geology/art/gl209/lecture3/lecture3.html
accessed 7/22/14.