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Lecture 13

EARTH 205 Lecture Notes - Lecture 13: Basalt, Closed System, Stratovolcano


Department
Earth and Environmental Sciences
Course Code
EARTH 205
Professor
Larry Ruff
Lecture
13

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Earth Exam #2:
Chemistry:
Elements are determined by how many Protons they have (and this then determines
Their basic chemical behavior) but the number of neutrons in the nucleus can
Have several values.
These are the “isotopes” of an element.
That unstable nuclei transform Into different elements. This is used to
Determine The age of a Closed system.
To date something very old, should use isotopes with A very long half-life
Mass Spec’s (Electro magnetic) Measure these isotopes.
Basics of Rock Deformation:
If mountains expose rocks that were once buried deep in the Earth, then there must be
Large-scale vertical motions of rocks...
Example: Sedimentary (Limestone) in Fayette, MI and Yosemite Valley
Either rocks moved up, or Sea level went down... a lot!
Intrusive Igneous: These rocks were formed at a depth off several kilometers...
It seems that rocks go up!
Also, if we accept Continental Drift/Plate Tectonics reconstructions,
then there are also large-scale* horizontal motions of rocks.
How rocks deform
(+Elastic)
> Brittle
> Ductile
Rock Deformation Structures
(small scale)
> faults
> folds
Rocks can deform by faulting at shallow depths, But deep deformation is ductile.
Controlled by a “critical” temperature? In particular, rocks must be COLDER
than some Tcrit to have brittle failure and possibly earthquakes
Volcanos:
ex : Hawaii, Magma -> Basalt

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What are Volcanos ?
Places where magma reaches the surface
What are volcanic eruptions? When the magma erupts ...two different eruptive styles
1. Passive style ex: Hawaii, Rock Type: “Basalt”
2. Explosive style ex: Hunga Tonga-Hunga Ha'apa, Rock Type: “Andesite” (like Granite)
Shape: “Shield” Volcano: Hawaii and “Stratovolcano”: Aleutian Is.
Most people see “volcanic belts” along the rim of the Pacific Basin
As an overall generalization, the rock type for The “Ring of Fire” volcanoes is “Andesite
Volcanoes occur as linear/curved belts on land & “island arcs”, and the three Tectonic
Settings for Volcanoes:
Mid-ocean spreading Ridges
Subduction Zones
(“Non-Tectonic setting): Hotspots
Where do volcanoes occur?
linear “belts” on land and plate tectonic zones
“Mid-Ocean Ridge Magma”
• Mid-ocean Ridges are the World’s great magma machine.
How does the Earth melt rocks...beneath the mid-ocean Ridges?
• Answer: Decompression melting
Note the new material, and magma (red spot) coming up beneath mid-ocean ridges.
How does the Earth produce such huge amounts of Magma at the Mid-Ocean Ridges?
This sketch shows a typical Geologic perspective on the structure of oceanic crust
at a spreading Ridge. The magma chamber is small, with a thin magma feeder Tube.
Using various waves from earthquakes, seismologists determined that the
Earth’s three main layers are Crust, Mantle, & Core. The Crust and Mantle
are solid not liquid. ...Everywhere?
How to melt Rocks:
For a solid that has a “single component”, i.e. pure Quartz, SiO2 Or Ice, H2O, then at a
specified pressure, there will be a melting Temperature solid if colder, liquid if hotter:
Thus, a common way to melt something Is to heat it up, thereby increasing its
Temperature to above the melting temperature. Or Increasing pressure, Or depth
Solid to liquid
Thus, another way to melt rocks Is to simply move them up (to increase pressure)
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