Download Twentieth lecture - 23 October, 2013

Geology in the news:
A high school student poking around in the desert in 2009 found
what has been determined to be the youngest and most
complete specimen of a juvenile Parasauralophus ever
discovered!
See http://tinyurl.com/n4war6u for the news story, and
http://dinosaurjoe.org/ to read more about the specimen and its
importance.
THE EARLY EARTH
Very active - initially molten, then skinned over with a thin
surface layer, continually broken by volcanic activity.
The ACASTA GNEISS is the oldest known
intact rock on Earth – ca. 4 BILLION
years old.
Individual reworked zircons in NW Australia
have been dated to 4.3 BILLION yrs.
VERY early on, the
Earth was also
stratified into layers,
with most of the
heavier elements
(particularly iron)
being concentrated in
the central core.
(We had also lost most of
our hydrogen and helium to
space by this time….)
The CRUST is that which we know best.
• 5-70 km thick
- 40-70 km under continents,
- 5-8 km under ocean basins
• continental & oceanic crust are
intrinsically different
• Continental crust esp. enriched in lighter
elements (O, Si, Al, Na, K)
- relatively low in Fe, Mg, Ni
- density 2.7-3.0 grams/cm3
- highly complex & heterogeneous
- 40-70 km thick
• Oceanic crust more like mantle beneath it
- lower in Si, Al, Na, K
- higher in Ca, Fe, Mg
- density 3.0-3.3 grams/cm3
- 5-8 km thick
The MANTLE is beneath the crust
- 2900 km (1800 mi) thick - comparable to
the distance from Maine to Colorado
- depleted of light elements (Al, K, Na)
- believed to be mainly Fe, Mg silicates
(top) and Fe, Mg oxides (at base)
- density 3.5 grams/cm3 (top) to 5.5
grams/cm3(bottom)
- source for most magmas (molten rock)
The CORE is composed mostly of iron (Fe)
with some nickel (Ni)
Outer Core
- 2200 km thick (1300 mi)
- molten
- flow generates magnetic field (how?)
- density 10-12 grams/cm3
Inner Core
- 1200 km radius (750 mi)
- probably solid
- density 12-13 grams/cm3
(2x density of iron at surface; = Pb)
NO ONE HAS ACTUALLY SEEN
THE MANTLE OR THE CORE
What we believe about them is based on:
1. Meteorites & Lunar specimens
One of hundreds of lunar specimens returned by Apollo mission astronauts.
The Asteroid Belt was at one time thought to be a planet that was
destroyed by a great impact.
It's now considered that the total mass here is insufficient to draw together into a single
planet, with Jupiter's gravitational pull continually disrupting processes that would
lead to clustering.
2. inclusions in volcanic rocks called xenoliths
3. rare rocks (fragments of uppermost mantle) exposed in
mountains where upper parts of the oceanic crust and mantle
are folded, buckled & pushed up ( called ophiolites).
Peridotite
At this site in
Newfoundland, the
pillow lavas are shown in
contact with the
underlying mafic
plutonic rocks of the
oceanic crust.
One proposed model for emplacement of ophiolites:
4. behavior of seismic waves from earthquakes & large
explosions (e.g., nuclear tests)
5. theoretical studies
For MOST of the semester, we're going to
be focusing on the part we know the best the crust.
(at least in part!!)
(Be it ever so humble, there's no place like home!)
"Hadean"
(Ediacaran)
At the beginning of the ARCHEAN, 4 billion
years ago, we know we had primitive oceans.
Water came from volcanic outgassing and
comets; organic compounds could have been
formed on Earth or also come from extraterrestrial (space debris) sources.
The "primordial soup" of the early oceans
held a variety of strange and very primitive
organisms, such as mats of algae that formed
stromatolites like these in Australia 
Late Archaean and early
Proterozoic microfossils
(of bacteria) from the
Gunflint chert of North
America
Modern Archaea
don't look all
that different.
2-billion-year-old Proterozoic stromatolites in South Africa
Australia
Banded Iron Formations (BIF) are also characteristic of Archaean terranes.
Hibbing Mine, Minnesota
Rocks of this ancient crust also show clearly that they've been through
a lot. This satellite image of Archean rocks in Australia is ~ 200 km
across! The light areas are granitic rocks, ancient mini-continental
blocks, while the green belts are ancient volcanic rocks (now
greenstone belts) that were crushed in between.
Archean rocks are the oldest in the SHIELDS, the stable cores of
continents
They are comprised
almost entirely of
metamorphic
rocks.
Archean Rocks in North America