Geol 101 Lab 2

Plate Tectonics and Magma Origin

 

Objectives:

 

1)            Analyze the earth’s forces, faults and plate boundaries to determine if the earth’s size is changing

2)            Use seismic tomography to evaluate a lava lamp model of the earth’s mantle.

3)            Use physical and graphical models of rack melting to infer how magma forms in relation to pressure, temperature, water, and plate tectonics.

4)            Measure and calculate rates of plate tectonic processes. 

 

What is “Plate Tectonics”…?

 

Definition-  The idea of large scale horizontal movements and deformation of the earth’s crust and lithosphere.

 

·       The crust is divided into small pieces called plates

·       These plates are always moving relative to each other

·       The edges of plates are called plate boundaries

o        get lots of geologic activity at boundaries

·       Because the plates are moving we know there must be some forces acting on them

 

Types of Stresses in the Earth….

 

o       Compression:  Shortening and Pushing

 

o       Tension (also called Extension):  Pulling and Lengthening

 

o       Shear:  Smearing

 

                       

These forces produce 3 different plate boundaries within the earth…

 

1) Convergent Plate Boundaries: Compression causes two plates to collide

ex- Washington coast

 

2)  Divergent Plate Boundaries:  Extension causes two plates to move apart

     ex- N.A. east coast

 

3)  Transform Plate Boundaries:  Shear causes two plates to slide past another

 

     ex- California coastline

 

How Do Plates Move??

 

     Slab Push-  erupting magma pushes plates away from spreading centers, causing compressional forces within the plate.

 

     Slab Pull-  gravity pulls dense, heavy plates down into the mantle in a subduction zone, causing extensional forces throughout the plate.

    

What is the driving force at work???

 

     Convection cells- think lava lamp

       

material is heated at the base, causing it to rise to the top where it cools and sinks back to the bottom.

 

3 Types of faults within the earth…

 

1)            Compression causes Reverse faults

 

 

2)  Extension causes Normal faults

 

 

3)  Shear causes Strike Slip faults

 

                                                Left Lateral (sinistral)

 

 

Origin of Magma-

 

o       Magma is molten rock within the earth

o       Lava is molten rock erupted on the earth’s surface

 

Magma is formed in three major places…

 

o       Divergent plate boundaries

o       Convergent plate boundaries

o       Hot Spots…

 

-upwelling of magma, like a blowtorch beneath the crust…creates islands in the ocean, and volcanic activity in continents…examples

 

Generation of Magma depends on…

 

Underground Temp- Pressure, and Mineral Composition

-Underground Temperature: The deeper you go, the hotter it gets-

 

o       Called the “Geothermal Gradient”

~ 250C / Km

 

The Witwaterstrand gold deposits in South America have mine tunnels that go down to 3.5km, reaching 600C  (1400F)

 

-Underground Pressure:  The deeper you go, the greater the pressure-

 

o       Called the “Geobarometric Gradient”

~ 300atm / Km

 

Note- we live at ~1 atm, that’s a lot of pressure…..

 

-Mineral Composition:

 

            All minerals melt at different T and P

           

Sometimes some melt and others done, all within the same rock…Called Partial Melt

 

Useless Vocab

                        A partial melt is called a Migmatite (Holland Library)

 

Remember This…

     Increase Confining Pressure  = Increase Melting Point

     Decrease Confining Pressure = Decrease Melting Point

 

This Idea is expressed graphically in a Pressure Temperature diagram – see figure 2.8 on pg. 40

 

 

Solidus-  A limit of PT where a mineral, or rock remains solid

 

Liquidus- A limit of PT where a mineral, or rock melts to liquid

 

 

 

Two Special Kinds of Melting…

           

o       Decompression Melting- Temp Steady, but lower pressure.  Happens at divergent boundaries

 

o       Flux Melting-  Rapid melting by presence of water.  Happens at convergent plate boundaries

 

 

Quantifying Plate Tectonics…

 

Using hot spot locations and the ages of seafloor rocks, get direction and rate of plates…

 

 

1)          Determine direction from youngest to oldest

2)          Calculate rate of movement

 

                    Rate= Dist / Time  Units in cm/yr