The Dynamic Crust
Regents Earth Science
Seismic Waves and Earth’s Structure
How can we tell that the outer core is liquid and the inner core is solid?
Seismic Waves and Earth’s Structure

Velocity of Seismic Waves
 P-waves
travel fastest (~2X faster than S-waves)
 S-waves second fastest
 Increases in denser material
 The waves are refracted (bent) as they pass
from a material of one density to a material of
a higher or lower density.
Seismic Waves and Earth’s Structure

P-waves

Travel through solids,
liquids and gases

S-waves

Travel only through
solids
Seismic Waves and Earth’s Structure
Scientists infer the properties of Earth's interior
through the analysis of seismic wave data.
 By studying thousands of seismograms scientists
have discovered that seismic waves refract,
reflect, change velocity, and become absorbed
by various parts of Earth's interior.
 The following diagram shows the results of
these thousands of studies.

P and S Waves
Seismic Waves and Earth’s Structure
P and S Waves
Shadow Zone

The Shadow Zone is created by the
refraction of P-waves and the inability of
S-waves to travel through a liquid.
Inferred Properties of Earth’s Interior
Inferred Properties of Earth’s Interior
Earth’s Layers - Detail





Crust
Lithosphere
Moho
Asthenosphere
Mantle


Asthenosphere (plastic)
Stiffer Mantle
Outer Core
 Inner Core

Earth’s Crust

Continental Crust


Beneath continents
Thicker




Oceanic Crust


Beneath Oceans
Thinner
Thickest where highest
(beneath mountains)
Granitic Rocks
Less Dense


Basaltic Rocks
Denser
MOHO

Mohorovic Discontinuity
 Interface
between lithosphere and asthenosphere
 Below Moho rocks become denser due to
increased pressure at depth and solid phase
Asthenosphere

Plastic portion of the upper mantle
 Rocks
are partially molten (plastic)
 Detected due to decrease in velocity of seismic
waves through this zone
 Widely accepted theory that magma and lava
originate in the asthenosphere
Inferred Properties of Earth’s Interior
Inferred Properties of Earth’s Interior