Not all rocks are the same: define each: key to video disc
There are 3 basic types of rocks:
Igneous: VideoDisc (A) #709, 674, 689, 6996 --> , 8414 --> (rafting)
Crystallized from magma: the molten (liquid) phase of solid rock
Can cool into rock below the surface (intrusive) or at the surface (extrusive)
Composition of the magma also important
Click here for more on how cooling history and composition affect igneous rocks
Sedimentary: VideoDisc (A) #1390, 1404-05, 1380-82
Accumulated debris from rocks weathered at the surface
This debris is transported to the ocean where it piles up
And is slowly converted into rock
Metamorphic: VideoDisc (A) #1592, 1535, 1548-49, 1554
All rocks form within a specific range of temperature and pressure
Any rock can later be moved to a different location in the crust where the temperature and pressure are different
This can cause the minerals in the rock to change into new forms in the attempt to re-establish equilibrium
Click here for a summary of global tectonics (do it now!)
The earth heats up with depth
It gets very hot very fast
Hot enough to melt just about anything naturally made by the earth
Basically a big ball of liquid rock which has cooled where exposed to space
Brittle-Ductile Transition Zone
Faulting vs. folding
Makes for a rather unstable platform for the cooled crust
Tends to get moved around and broken up
The earth's surface (crust) is broken into sections, called plates
Primarily composed of one of two fundamentally different types of igneous rock
Basalt - dark and heavy (specific gravity 3.2)
Forms the relatively low-elevation crust beneath the ocean basins
Granite - much lighter rock (s.g. 2.7)
Because granite is less dense, it tends to ride higher in the crust
Forms the major land masses.
Essentially floating in a 'sea of basalt'
Both basalt and granite are igneous rocks
Most of the earth's major crustal plates are mixtures of both basalt and granite
And lesser amounts of many other things
Poorly understood forces cause these rigid crustal plates to move
Relative to each other
Over the higher density 'plastic' material which occurs beneath the crust
Plate tectonics and plate boundaries
Two (or more) plates interact along huge linear zones of faulting
Different plates move at different velocities
Ranging from 1 to >10 centimeters per year
This may not seem like much, but, over the course of geologic time...
Tectonic activity is common here
Earthquakes, volcanoes, and mountain building in general
Thanks to friction and Newton's First Law of Motion
Earth tries its best to keep the plates from slipping
But as tension increases to the breaking point
The fault will rupture
Accumulated strain will be released in a seismic event (earthquake)
The plates will shift along the boundary
Only to lock-up again and begin the process anew.
The amount of energy involved here is immense
Beyond our ability to comprehend, as well as restrict or control
All we can do is study the process, try to keep out of the way, and, if we can't, hang on tight
Basalt commonly forms where two plates are moving away from each other
Deep rifts are opened through the crust
Spreading centers - example: Mid-Atlantic Ridge
Allow magma from the upper mantle to rise to the surface and cool
Heals the wound, just like a scab
Basalt is like the blood of the earth
Significant amounts of new oceanic basalt can be formed
Granite forms deep within the crust
Associated with regions where two plates are moving towards each other
Most of the biggest earthquakes and volcanoes occur in zones of convergence
Pacific "Ring of Fire"
Example: western edge of South America vs. the Pacific Ocean
Collision is between a continental plate and an oceanic plate
Along a deep trench called a "subduction zone"
Oceanic basalt is "eaten" by the earth
Subjected to increasing heat and pressure
Can cause it to undergo metamorphic changes
In extreme conditions, completely remelt to form new magma
If this magma can find a path of weakness...
Generally not a problem where two plates are grinding into each other
Erupt from the surface as lava
Form volcanic mountains (called a "volcanic arc")
Some cools at depth forming large granitic "plutons"
These are sutured to the "continental margin" (added to craton)
Oceanic crust is temporary: created at spreading centers
Moves like a conveyor belt to a subduction zone where it is destroyed
Therefore, all oceanic crust is geologically young (<200 million years old)
Continental crust, however, is essentially just along for the ride
This lighter "scum" is basically the end product of the earth's differentiation
Granitic crust has been accumulating for 4 billion years
Edges get beaten up, blocks split and join, but it basically isn't destroyed