Earth’s interior

      Comprised of several layers

 

 

Differentiate between them in various ways

      Compositional vs Mechanical

 

Compositional Layers

      Changing composition

            Crustà rich in silicon

            Mantleà Mg-Fe rich rock (peridotite)

            Coreà  Fe-Ni composite

 

 Crust (lithosphere):

                  Continental (3.8 b.y. old)à silica rich rocks

                        30-70 km thick~ 2.5 gm/cm3

                  Oceanic (<180 m.y. old)à Fe & Mg rocks

                        5-15 km thickà ~ 3 gm/cm3

 

Mantle

      82% of the volume of the Earth

      Composition calculated by experiments and volcanic eruptions

      Two primary regions

      Mesosphere  600km to core

      Asthenosphere

 

Core

      Made of iron/nickel composite

      Determined with meteorties and wave velocity

      Originally all thought to be liquid

      Generates earth’s magnetic field

 

 

Mechanical Layers

      Variations within the compositional layers based on the mechanical properties of each region.

 

            Upper mantle (asthenosphere):  Plastic

            Lower Mantle (mesosphere):   Solid

 

            Inner core:  Solid

            Outer core:  Liquid

 

 

WHY DO SOME LAYERS HAVE THE SAME COMPOSITION BUT DIFFERENT PHYSICAL PROPERTIES?  ex// mantle and core

HOW DO WE KNOW THE EARTH HAS THE STRUCTURE THAT WE HAVE OUTLINED ABOVE?

 

      Seismic waves 

            2 types of body waves

                  P-wavesà primary waves (longitudinal waves)

                        passes through solids and liquids

                  S-wavesà secondary or shear waves (transverse)

                        moves only through solid material

 

Why do shear waves only pass through solid material?

MORE WAVE THEORY:

      Wave speed varies with composition of materials it passes through

 

WHY?

      Waves refract and reflect energy at boundaries between materials with different properties

      ex// prismà refracts incoming light
                  boundary: air to glass

 

Within the Earth, at compositional and mechanical boundaries energy is reflected and refracted.  Additionally, wave velocity changes.  These changes are known as discontinuities…more in a bit

 

How do we know about discontinuities and how are they used to determine the general structure of the earth

 

Discontinuities in the Earth:

      Mohorovicic (moho):

            Separates crust from mantle (asthenosphere)

            location varies based on crust type

            Discovered 1909 by velocity of waves

 

      Gutenberg:

            Boundary between core and mantle.

            Discovered due to P-wave shadow zone

 

      Lehmann: 

            Boundary between inner and outer core

            Discovered 1936.  P-waves arrive faster than anticipated through core. 

 

Changes in seismic wave velocities indicate variations within the mantle

      410km depth:  velocity increases

            Represents phase change ( olivine à spinel)

      660 km depth:  velocity increases

            Represents phase change (spinel à perovskite

      “D” layer:  lower 200 km of mantle:

            P-wave velocity decreases

            Partially molton mantle

 

How do we know the composition of each layer in the earth?

            Lithosphere?

            Mantle?

            Core? 

 

What is the geothermal gradient?           

      Greatest temperature change with depth is in the crustal region (25^oC/km)

 

      gradient drops to ~ 1^oC/km in mantle

      increases again slightly in outer core

      decreases in inner core.

 

What generates the heat inside the Earth?

 

How does heat move around in/on the earth?

      Conduction

      Convection

      Radiation

 

What type of heat transfer dominates in each part of the earth?

      Outer Core

      Mesosphere

      Inner Core

      Asthenosphere

      Lithosphere