Download Lecture 5 - Plate Tectonics and Rocks

ContinentalDrift_Nasa.mpg
Field trip:
This Saturday
VOLUNTARY
Meet here (to carpool?) and then drive to
Bridger to rendezvous with Dr. Zwick at 9
AM.
Bring lunch and something to drink.
Expect to return in the afternoon
The Aleutian Island Chain
Seismic activity in the Aleutian Islands
Melting of upper mantle and oceanic crust
often creates mafic to ultramafic rocks.
Tectonic setting
Lavas and pyroclastics
Rock/sediment type
Felsic
Granites and Rhyolite
Turbidites, clays,
silts, sands
Marine sediments
(cherts, limestones,
red clays)
Basalts (Ophiolites)
Mafic
Igneous Activity and
Plate Tectonics 1
Igneous activity occurs
primarily at or near tectonic
plate boundaries
Mafic igneous rocks are commonly formed
at divergent boundaries
–
Increased heat flow and decreased
overburden pressure produce mafic magmas
from partial melting of the asthenosphere
Composition of the Ocean Crust
• Seismic surveys suggest oceanic crust is
km thick and comprised of three layers
~7
– First layer is marine sediment of various
composition and thickness (extensively sampled)
– Second layer is pillow basalt overlying basaltic
dikes (extensively sampled)
– Third layer is thought to be composed of sill-like
gabbro intrusions (not directly sampled)
• Ophiolites are rock sequences in mountain
chains on land that are thought to represent
slivers of ocean crust and uppermost mantle
– Composed of layers 1-3 overlying ultramafic rock
Why would we find similar rocks in western Montana/Idaho?
Igneous Activity and
Plate Tectonics 2
Intermediate (chemically) igneous rocks are commonly formed
at convergent boundaries
Partial melting of basaltic oceanic crust produces
intermediate magmas
Igneous Activity and
Plate Tectonics 3
•
Felsic igneous rocks are commonly formed adjacent to
convergent boundaries
–
Hot rising magma causes partial melting of the granitic continental crust
Intraplate volcanism
Rising mantle plumes
can produce localized
hotspots and volcanoes
when they produce
magmas that rise
through oceanic or
continental crust.
Hawaii is an example
Composition?
Diamonds ascend to the
Earth's surface in rare
molten rock, or magma, that
originates at great depths.
Carrying diamonds and
other samples from Earth's
mantle, this magma rises
and erupts in small but
violent volcanoes. Just
beneath such volcanoes is a
carrot-shaped "pipe" filled
with volcanic rock, mantle
fragments, and some
embedded diamonds. The
rock is called kimberlite
after the city of Kimberley,
South Africa,
Ants, erosion and Namibian diamonds
> 2.5 Ga
1.6 to 2.5 Ga
Namibia
The Orange river
has delivered
diamonds to the
Namibian coast for
millions of years