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-
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-
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
3) Shear causes Strike Slip
faults
Left Lateral (sinistral)
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)
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
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