Download The$Earth`s$Interior The$Earth`s$Interior

Physical Science
Lecture:
Earth’s Seismic Interior
Professor Kenny L. Tapp
Rock Type:
Rock Name:
The$Earth’s$Interior
• How$do$we$know$what$the$Earth$comprises?
• How$deep$have$we$drilled?
The$Earth’s$Interior
• Most$of$what$we$know$is$from$geophysics.
• Geophysics$is$basically$“remote$sensing”$of$
Earth’s$interior
– Seismic$waves
– MagneCc$field
– Gravity
– Heat
Earth's(layered(structure(
Possible(seismic(paths(
through(the(Earth
Most$of$our$knowledge$of$Earth’s$interior$
comes$from$the$study$of$P$and$S$earthquake$
waves$
• Travel$Cmes$of$P$and$S$waves$through$Earth$vary$
depending$on$the$properCes$of$the$materials$
• S$waves$travel$only$through$solids$
Figure 7.24
Earth's(layered(structure(
(((Seismic(shadow(zones
Discovering$Earth’s$major$layers$
• Shadow$zone$$
• Absence$of$P$waves$from$about$105$degrees$to$140$
degrees$around$the$globe$from$an$earthquake
• Explained$if$Earth$contained$a$core$composed$of$
materials$unlike$the$overlying$mantle$$$$
Figure 7.26
Earth's(layered(structure(
Layers$defined$by$composiCon$
• Crust$
• Thin,$rocky$outer$layer$
• Varies$in$thickness$
• Roughly$7$km$(5$miles)$in$oceanic$regions$
• ConCnental$crust$averages$35[40$km$(25$miles)$
• Exceeds$70$km$(40$miles)$in$some$mountainous$
regions$$
Earth's(layered(structure(
Layers$defined$by$composiCon$
• Crust$
• ConCnental$crust$$
• Upper$crust$composed$of$graniCc$rocks$
• Lower$crust$is$more$akin$to$basalt
• Average$density$is$about$2.7$g/cm3$$
• Up$to$4$billion$years$old$$
Earth's(layered(structure(
Layers$defined$by$composiCon$
• Crust$
• Oceanic$Crust$$
• BasalCc$composiCon$$
• Density$about$3.0$g/cm3$
• Younger$(180$million$years$or$less)$than$the$
conCnental$crust$$$$
Earth's(layered(structure(
Earth's(layered(structure(
Layers$defined$by$composiCon$
• Mantle
• Below$crust$to$a$depth$of$2900$kilometers$(1800$miles)
• ComposiCon$of$the$uppermost$mantle$is$the$igneous$
rock$peridoCte$(changes$at$greater$depths)$
Views(of(Earth’s(
layered(structure
Layers$defined$by$composiCon$
• Outer$Core
•
•
•
•
Below$mantle$
A$sphere$having$a$radius$of$3486$km$(2161$miles)$
Composed$of$an$iron[nickel$alloy$
Average$density$of$nearly$11$g/cm3$
Figure 7.25
Earth's(layered(structure(
Layers$defined$by$physical$properCes$
• Lithosphere
• Crust$and$uppermost$mantle$(about$100$km$thick)$
• Cool,$rigid,$solid$
• Asthenosphere
•
•
•
•
Beneath$the$lithosphere$
Upper$mantle
To$a$depth$of$about$660$kilometers
Soe,$weak$layer$that$is$easily$deformed$
Earth's(layered(structure(
Layers$defined$by$physical$properCes$
• Mesosphere$(or$lower$mantle)$
• 660[2900$km$
• More$rigid$layer$
• Rocks$are$very$hot$and$capable$of$gradual$flow
• Outer$core$
• Liquid$layer$
• 2270$km$(1410$miles)$thick
• ConvecCve$flow$of$metallic$iron$within$generates$Earth’s$
magneCc$field$
• Inner$Core$
• Sphere$with$a$radius$of$1216$km$(754$miles)$
• Behaves$like$a$solid$
Views(of(Earth’s(
layered(structure
Continental drift: an
idea before its time
Alfred Wegener
• First proposed hypothesis, 1915
• Published The Origin of Continents and
Oceans
Continental drift hypothesis
• Supercontinent called Pangaea began
breaking apart about 200 million years ago
• Continents "drifted" to present positions
• Continents "broke" through the ocean crust
Figure 7.25
Pangaea approximately
200 million years ago
Continental drift: an
idea before its time
Wegener's continental drift hypothesis
• Evidence used by Wegener
•
•
•
•
Fit of South America and Africa
Fossils match across the seas
Rock types and structures match
Ancient climates
• Main objection to Wegener's proposal was
its inability to provide a mechanism
Figure 8.2
Wegener’s matching of mountain
ranges on different continents
Figure 8.6
Paleoclimatic evidence for
Continental Drift
Figure 8.7
Plate tectonics: the
new paradigm
More encompassing than continental
drift
Associated with Earth's rigid outer shell
• Called the lithosphere
• Consists of several plates
• Plates are moving slowly
• Largest plate is the Pacific plate
• Plates are mostly beneath the ocean
Divergent boundaries are located
mainly along oceanic ridges
Plate tectonics: the
new paradigm
Plate boundaries
• Types of plate boundaries
• Divergent plate boundaries (constructive margins)
• Two plates move apart
• Mantle material upwells to create new seafloor
• Ocean ridges and seafloor spreading
• Oceanic ridges develop along well-developed
boundaries
• Along ridges, seafloor spreading creates new
seafloor
Plate tectonics: the new paradigm
Plate boundaries
• Types of plate boundaries
• Convergent plate boundaries (destructive
margins)
• Plates collide, an ocean trench forms and
lithosphere is subducted into the mantle
• Oceanic-continental convergence
• Denser oceanic slab sinks into the
asthenosphere
• Pockets of magma develop and rise
• Continental volcanic arcs form
• Examples include the Andes, Cascades, and the
Sierra Nevadan system
Figure 8.10
An oceanic-continental
convergent plate boundary
Plate tectonics: the
new paradigm
Plate boundaries
• Types of plate boundaries
• Convergent plate boundaries (destructive
margins)
• Continental-continental convergence
• When subducting plates contain
continental material, two continents
collide
• Can produce new mountain ranges such
as the Himalayas
Figure 8.14 A
A continental-continental
convergent plate boundary
Figure 8.14 C
The collision of India and Asia
produced the Himalayas (after)
The collision of India and Asia
produced the Himalayas (before)
Figure 8.15 A
Testing the plate
tectonics model
Hot spots and mantle plumes
• Caused by rising plumes of mantle material
• Volcanoes can form over them (Hawaiian
Island chain)
• Mantle plumes
• Long-lived structures
• Some originate at great depth, perhaps at the
mantle-core boundary
Figure 8.15 C
The Hawaiian Islands have formed
over a stationary hot spot
Measuring plate motion
Measuring plate motion
• By using hot spot “tracks” like those of the
Hawaiian Island - Emperor Seamount
chain
• Using space-age technology to directly
measure the relative motion of plates
• Very Long Baseline Interferometry (VLBI)
• Global Positioning System (GPS)
Figure 8.19
Directions and rates
of plate motions
Plate tectonics into the
future
Present-day motions have been extrapolated
into the future some 50 million years
• Areas west of the San Andreas Fault slide
northward past the North American plate
• Africa collides with Eurasia, closing the
Mediterranean and initiating mountain building
• Australia and new Guinea are on a collision
course with Asia
Figure 8.26
A possible view of the world
50 million years from now
Figure 8.29