To understand why volcanoes and earthquakes occur where they do we must understand some basic geologic concepts

 

Earth’s Structure

 

Solar Nebular Hypothesis

      Super nova

      Elements begin to pull together

      Sun and planets form

      Dense elements migrate to the earth’s center and lighter elements stay near  surface

      Earth’s interior

      Comprised of several layers

 

Differentiate between them in various ways

      Compositional vs Mechanical

 

Compositional Layers

      Made of different compositions

            Crustà rich in silicon

                  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

            Mantleà Mg-Fe rich rock (peridotite)

            Coreà  Fe-Ni composite

 

Mechanical Layers

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

            Inner core:  Solid

            Outer core:  Liquid

 

            Lower Mantle (mesosphere):   Solid

            Upper mantle (asthenosphere):  Plastic

 

Overview:

     Core:  Iron & Nickle

            Inner:  solid

            Outer:  liquid

      Mantle:  Peridotite (Iron & Magnesium rock)

            Mesosphere:  solid

            Asthenosphere:  plastic

 

      Crust:  Solid

            Continental:  rich in silicon

            Oceanic:  rich in iron (basalt)

 

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)

            Won’t pass through liquid

 

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

At compositional and mechanical boundaries

      energy is reflected and refracted.    

      wave velocity changes. 

 

These changes are known as discontinuities

 

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. 
 

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 meteorites and wave velocity

      Originally all thought to be liquid

      Generates earth’s magnetic field

 

Structure of earth understood in early 1900’s but may other things were not understood

      Why do earthquakes occur where they do

      Why do volcanoes occur where they do

      Why are there mountain ranges where they are?

 

1915: Continental Drift

      Introduced by Alfred Wegener

      Suggested that all the continents were together at one point in time and have moved to their current location

      Gave lots of evidence to support this hypothesis

 

Wegener’s Evidence

      Fit of the continents

      Fossils

            Mesosaurous & Glossopteris 

      Rock types and structures

      Paleoclimates

 

Great Evidence but what was missing?

 

1950’s and 1960’s Ocean exploration

      Volcanic mountain chains in the middle of the oceans

      Paleomagnitism

            Additionally:

                  Magnetic signatures of rocks didn’t match where they were

                  Polar wandering or continents moving

      Age of the crust

 

Leads to theory of Plate Tectonics

      Earth’s crust is broken into several plates

      Plates interact with each other in different ways

            Collide

            Move away

            Slide by

 

Divergent Boundaries

      Plates move away from each other

      Usually located in the middle of the oceans…exceptions?

      Volcanic mountain chain creates new ocean crust (basalt)

      Known as a ridge or rift and has a valley in the center

 

Transform Boundaries

      Plates slide by each other

      Associated with divergent boundaries

      Off set divergent boundaries

 

Convergent boundary

      Plates come together

      3 different outcomes depending on type of crust involved

            REVIEW:

                  Continental Crust vs Oceanic crust…which is the thickest?  Which is the most dense?

            Continental to Oceanic Convergence

                  Continental crust is lighter and thicker so therefore forces oceanic crust  underneath

                  Subduction zone is formed

                  Volcanic mountain chain forms on the over riding plate

 

            Oceanic to Oceanic convergence

                  Older crust is more dense so it is subducted under younger crust

                  Volcanic chain forms on over riding crust

 

            Continental to Continental convergence

                  Neither plate subducts….VERY large mountains are created

                  Why is there no really old oceanic crust?

 

Driving Mechanisms for Plate Tectonics

      Examine the uppermost region of the asthenosphere to understand what is happening

 

            Melt zone represents “weak” area and this zone allows crust to “detach” from the asthenosphere and move around on the surface

            How did motion begin?

                  Heat generated inside of earth began the motion

                        Radioactive decay

                        Primoridal heat

                        Convection cells in the asthosphere

 

                  Primary mechanism is slab pull

                        Due to weight of crust as it is “pulled” into earth at subduction zones

                        Look at rate of motion in Pac and Atlantic to see

 

                  Lesser Mechanism is ridge push

                        Gravitational “push” of material piling up at ridges

 

How do Plate boundaries relate to earthquakes and Volcanoes

      Earthquakes

      Volcanoes