Download Earth`s Many Layers

Earth’s Many Layers
Lecture #4
POP QUIZ!
• If we could cut open the Earth with a gigantic
knife, the interior would most resemble . . .
♣A peanut
M&M
♣An apple
♣An onion
♣A Twinkie
Layers of the Earth
• No holes or pockets
• 3 main layers
♣Like a peanut M&M
• Several subdivisions
♣More like an onion
The Crust
• Outer layer (thinnest)
• 2 types
♣ Continental crust =
rock of continents
o Thickness varies
(average = 35 km)
o Composition varies
(average = granite)
The Crust
• 2 types
♣Continental crust
♣ Oceanic crust = rock
under oceans
o Thickness ~7 km
o Composed of basalt
The Mantle
• Middle layer
• Crust/mantle boundary
= the Moho
♣Thickness = Moho
to 2900 km
♣ Composed of peridotite
The Core
• Inner layer
♣ From 2900 km to
center of Earth
• Composed of ironnickel alloy *
♣ Inner part is solid
♣ Outer is liquid
The Core (cont.)
• How can inner be solid??
• Solid vs. liquid depends on:
♣ Temperature
♣ And pressure
• Example: water
♣ Altitude = sea level
5000 ft
10,000 ft
♣ Boiling T = 212 oF
203 oF
194 oF
The Core (cont.)
solid
• Liquid metal
flows.
liquid
♣ Metals
conduct
electricity
• Circulation of
electrical conductor
=> magnetic field
Crust
Mantle
Layers of
the Earth
Outer core
Inner core
Relative Thickness of Earth’s
Layers
• Best model . . .
♣ An egg!
How Did Layers Form?
• Initially, Earth = homogeneous
mixture
♣ Rock + metal
• Heated by:
♣ Energy of impacts
♣ Radioactive decay
How Did Layers Form? (cont.)
• By differentiation
♣Def: separation of homogeneous material
into parts with different composition
Formation of Core
• Early Earth began to melt
♣ Heavy iron sank
a
C
n
Iro
e
h
p
o
r
t
s
a
t
♣ Lighter rock rose
• Separation of core from mantle
Schematic View
Formation of Crust
• Continental crust began to form ~ 4 Ga
♣ By partial melting
♣ Has grown gradually since then
• Oceanic crust (oldest ~ 200 Ma):
♣ Continuously formed (mid-ocean ridge)
♣ Then destroyed (sinks into mantle)
Crustal Extremes
Old zircon crystal in
continental crust
Young pillow basalt
in oceanic crust
How Do We Know?
• Observe structures in deep mines?
• Explore interior of faults?
• Drill holes from crust to core?
• Study how earthquake waves travel?
How Do We Know? (cont.)
• Direct sampling
♣ Crust
o Easy!
o Many samples
♣ Mantle
o Hard!
o Some samples
♣ Core
o Impossible! o No samples
World’s Deepest Hole
• Northern Russia
• 1970-1994
• 7.5 miles deep
• Drill core samples
from 3.6 miles down
• Tried to drill to
Moho No go!
How Do We Know? (cont.)
• Earthquake waves
♣ Travel down
♣ Reflect or bend
at layers
♣ Bounce back to
surface
♣ Speed increases with increasing density
Earthquake Waves
How Do We Know? (cont.)
• Lab experiments
♣High T & P ~ interior
conditions
♣ Measure:
o Density
o Speed of EQ waves
♣ Results confirm
interpretation - layers
How Do We Know? (cont.)
• Study of meteorites
♣ 2 kinds fall:
o Stony meteorites
~ mantle
o Iron meteorites
~ core
A Second Kind of Layering
• Based on strength of rocks
• Rocks near surface
♣ Cool, strong, stiff
♣ Lithosphere = crust + upper mantle
o ~ 100 km thick
A Second Kind of Layering
(cont.)
• Rocks at depth
♣ Hotter, weaker, softer
♣ Asthenosphere = layer that flows
♣ Within mantle (under lithosphere)
• Plastic vs. brittle deformation
Comparison
Compositional
Strength
Layering
Layering
Crust
Lith
Mantle
Asthen