Transcript How Do Earthquakes Tell Us About the Earth`s Interior?

How Do Earthquakes Tell Us
About the Earth’s Interior?
Wave Refraction
• When a wave (light, sound, H2O) passes from
one material to another, it refracts (bends) or
reflects (bounces back)
– Waves change speed
between mediums
– In water, changes
occur with depth
– In Earth, w/density
An Earth Sized Sonogram…
• A. Mohorovičić (1909)
– Waves from distant
earthquakes traveled at
a faster velocity than
nearby quakes
• Increase in density
• Traffic example
– Identified boundary
between crust and
mantle
Different Types of Crust
• Earthquakes travel faster through oceanic rock
than continental rocks
– Must be difference in density
OCEAN CRUST
CONTINENTAL CRUST
Basalt (Gabbro)
“Granites”
High iron (Fe) content, dense
Higher silicon (Si), low density
~7 km thick on average
20-70 km thick (~40 km average)
Further Subdivisions
Depth (km)
Crust and lithosphere
• “Low Temp Shell”
– The crust and the
top part of the
mantle down to an
average of 100 km
constitutes the
rigid lithosphere
When the Temperature Rises…
• Below the rigid
lithosphere, is the
asthenosphere
– Hot and weak (plastic)
– Though solid, over
1000s to millions of
yrs flows like hot wax
due to pressure of
crust
– Solid Mantle below
this
The Core?
• Between 105° and 140°
no seismic signature
– Due to refraction
– No S-waves beyond 105°
• Liquid outer core at
~2900 km
• Solid inner core at
~5150 km
The Core
• Crust and mantle occupy 85% of Earth by
volume but are below average density
– Core must have density of 10-12 g/cm3
– Iron + nickel
• Remember Wiechert and his meteorites
War + Science?
• WWII and the
Nazis:
• MORs
– A seismically active
ridge the length of
Atlantic
– And in most of the
other ocean basins
Earthquakes of the World
Mid-ocean
ridges
Holmes’ Radical Idea
• Oceanic crust separated along the mid-ocean
ridge
– Hot magma would well up out of the crack, cool,
and the oceanic crust would grow!
The Outer Core as
a Magnet
• Why:
– Electricity passed through a conducting
material generates a magnetic field
• Outer core is hotter at base than top
– Like pot of water, it convects
– Convecting hot metal produces magnetic field
– Acts like a bar magnet, causes compass
needles to point ~N
Rocks: Magnetism and Time
The Irony of Science
• WWII: sensitive magnetic devices used to
detect steel hulls of German submarines
• Identified magnetic bands on oceanic crust
perfectly symmetrical about mid-ocean
ridge
Mid-ocean ridges
Support for “Seafloor Spreading”
• Hypothesis
– Molten material
rises, cools against
both sides of
spreading axis, and
magnetic band is
formed
– Ocean crust is
growing!
Wasn’t Mantle ~ Solid?
• Mantle is
HOT, solid
under HIGH
pressure
• Crust splits,
pressure
drops
• Temp wins