Download PDF file of Lecture 4a - Earth`s Interior and Tectonics

RESTLESS EARTH – PART I
GLG-190 – The Planets
Chapter 4
LECTURE OUTLINE
 Introduction
Heat transfer
 Seismic waves

 Earth’s

interior
Crust, mantle, core
 Continental
drift
and plate tectonics
HEAT TRANSFER

Heat is transferred by three main processes
Radiation
 Conduction
 Convection


Radiation involves transfer of heat by
radiant energy




Energy transferred from hot body by
photons
Photons hit cooler body and impart energy,
heating it
Example: Sunlight heating Earth
Conduction involves transfer by collisions of
atoms and molecules with immediate
neighbors
Heat present in the form of vibrations
Vibrational energy transferred by movement
of electrons inside material
 Example: heat movement through metal
handle of a pot



Convection is movement of heat
through by movement of a “fluid”



Due to density differences caused by
temperature differences
Any material can move by
convection as long as there is a heat
source
The less fluid the material, the
slower the convection
Rocks in mantle (heat from core)
 Liquid metal in outer core (heat from
formation of inner core)
 Air in atmosphere (heat from
radiation hitting the surface)


Main way terrestrial planets lose
heat
Single cell atmospheric
convection in a nonrotating planet
SEISMIC WAVES

Produced by earthquakes and
explosions


Energy in form of sound waves 
expands from point of origin (focus)
Used to map interior of Earth

Wave velocities depend on type of
rock, pressure, and temperature




Waves move more slowly through
hot, low density rock
Waves change speed and direction
(are refracted) when moving from
one layer to another
Waves also are reflected when they
encounter new layers
Reveal internally layered Earth
EARTH’S LAYERED STRUCTURE
 Thin
rigid rocky
lithosphere

Crust + non-convecting
upper mantle
 Hot
convecting rocky
mantle
Extra weak layer just
below lithosphere 
asthenosphere
 Lower mantle

 Dense
metallic core
Liquid outer part
(convecting)
 Solid inner part

LITHOSPHERE
Term “lithosphere” refers to non-convecting part of a
body’s rocky layer (heat moves by conduction)
 Also used to describe rigid ice layer on floating on liquid
internal ocean (Europa, Enceladus)
 May constitute entire rocky layer if body has lost too much
heat to allow convection (Moon, Mercury)

EARTH’S CORE

Core is 54.7% Earth’s radius
1/6 Earth’s total volume
 Liquid outer part
 Solid inner part suspended in middle
of liquid part


Dense material  32.5% Earth’s
mass




Cosmochemical and experimental
data indicate iron-nickel (FeNi) alloy
Less dense than pure NiFe alloy  light elements (Si, O, S) also in core
decrease density
Variations in seismic wave velocities (anisotropy)  inner core
is crystalline
Temperature of inner core 6900 K (hotter than surface of Sun)
ORIGIN OF MAGNETIC FIELD




Magnetic field originates in
liquid outer core
Convection of liquid metal
driven by heat from inner core
 helical flow due to Earth’s
rotation
Self-sustaining: magnetic field
 generates electrical currents
 magnetic field  electric
currents  magnetic field, etc.
Magnetic field changes
orientation at random intervals
 magnetic reversals
MAGNETIC
REVERSALS

Reversals used for…
Age dating
 Establishing directions
and rates of plate
motions
 Paleolatitude
determinations


Geomagnetic Polarity
Time Scale

Ages based upon
isotopic and
paleontological dating
EARTH’S LITHOSPHERE & ASTHENOSPHERE

Lithosphere consists of
crust + uppermost part of
mantle (100-200 km
thick)

Oceanic lithosphere


Continental lithosphere


Thin with young (<200 Ma)
basaltic crust
Thick with old (up to 4 Ga)
granitic crust
Weak asthenosphere lies
below lithosphere
Rigid lithosphere slides
over asthenosphere
 Asthenosphere is unique to
Earth

MOVEMENT OF EARTH’S SURFACE

Earth’s surface is in constant
motion
Movement rates are very slow 
not easily observable
 Lithospheric “plates” shift and
interact


Continental “drift” proposed to
account for marching coastlines of
Africa and South America


Abraham Ortelius (1596), Francis
Bacon (1625), Benjamin Franklin
(1780s)
Evidence of large scale surface
movements rare in solar system
Early Mars?
 Few larger icy moons

CONTINENTAL DRIFT

Wegener (1915)
proposed all continents
were once connected
(Pangaea)





Fit of edges of continents
Fossils match across
oceans
Mountain chains match
across oceans
Ancient climates (glaciers,
coal)
Pangaea broke up about
200 my ago and
continents slowly moved
to present positions
Matching fossils
Glacial evidence
SEAFLOOR TOPOGRAPHY

Recent discovery (post 1940s)


Ships equipped with sonar discovered oceanic ridge system (yellows
and greens above)
Mid-ocean ridge is longest mountain range on Earth (75,000 km)
SIGNIFICANCE OF MID-OCEAN RIDGES


Hess (1960s) proposed ridges
represent zones of hot mantle
upwelling
Rising mantle produces magma
Decompression melting of
mantle rock rising under ridges
 Lithosphere splits apart 
earthquakes
 Magma fills cracks in lithosphere
 forms new (young) oceanic
lithosphere


If lithosphere is forming at
ridges, it must be lost
elsewhere

Lithosphere sinks into mantle at
deep sea trenches 
earthquakes

Reversals of Earth’s magnetic field are recorded in newly
solidified magma at mid-ocean ridges
Magnetic stripes formed (symmetrical across ridge)
 Width depends on how fast new lithosphere is formed

AGE OF OCEAN FLOOR
DEFINING PLATE BOUNDARIES
Earthquake
zones, volcanism,
and mountain
building (tectonic
activity)  plate
boundaries
DRIVING PLATE TECTONICS

Lithosphere moves over asthenosphere
Subduction recycles lithosphere into Earth’s interior
 Seafloor spreading adds new material to plate margins (makes
new lithosphere)

Plate tectonics is Earth’s way of losing heat from interior
 Convection driven by density (temperature) differences

WHAT MOVES THE PLATES?
Ridge push – lithosphere slides
off high thermal budge
Slab pull – dense lithosphere
sinks and pulls plate after it
Other forces, such as convection
(movement of material) in mantle

Types of plate boundaries



Divergent (plates pull apart)  seafloor spreading (mid-ocean
ridges); continental rifting
Convergent (plates collide)  subduction (volcanic arc) and
mountain building
Transform (plates slide past one another)  San Andreas Fault
DIVERGENCE: CONTINENTAL RIFTING



Tensional forces stretch and thin
continental crust  rift valley
Upwelling of asthenosphere
(decompression melting) 
volcanism
Rift may develop into ocean
basin
PLATE CONVERGENCE

Ocean-continent



Ocean-ocean



Trench along continent
margin
Volcanism and deformation
of continental margin
(Andes)
Trench in ocean
Volcanism creates island arc
(Aleutians)
Continent-continent


Mountain building
(Himalayas)
Preceded by ocean-continent
collision
MOUNTAIN BUILDING

Continents do not subduct



Mountain building
produces




Densities are too low for
them to sink into mantle
Collide forming mountains
Bending (folding)
Breaking (faulting)
Change of shape
(deformation)
Mountain heights limited
by gravity, strength of rock,
erosion rates
Appalachian Mountains folds from space
MANTLE PLUMES & HOT SPOTS



Mantle plumes are columns or
rising hot material that seem to
originate near CMB

Long-lived structures

Decompression melting forms
magma
Plume can come up under any
part of a plate (except subduction
zones where they are blocked by
the descending slab)

Continental hot spot: Yellowstone

Oceanic hot spot: Hawaii

Spreading center hotspot: Iceland
Important heat loss mechanism
on other bodies (e.g., Venus,
Mars, Io)
EARTH’S CRATERS
Only 180 confirmed impact craters
 Earth experienced same rates of cratering as Moon 
most craters removed by surface processes

SURFACE PROCESSES



Very important on Earth (and other
bodies with atmosphere)
Weathering

Physical breakdown and chemical
alteration of rock at or near the surface

Liquid water plays dominant role
Erosion


Transport


removal of weathered material by
water, wind, and ice
Movement of material (sediment) by
water (streams), wind, and ice (glaciers)
Sedimentation

Deposition of sediment in low areas