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Transcript
Week 2:
When the Earth was young
(Archean eon)
•EPSC 233-001 Fall 2002
Oceanic and continental crusts have different
proportions of the main elements.
continental (felsic) crust:
more Al, Si, less Mg, Fe
average density = 2.7 g/cm3
oceanic (mafic) crust:
less Al, Si, more Mg, Fe
average density = 3.0 g/cm3
Greenstone belts (shown in black on map)
Podlike bodies
consisting of:
• metamorphosed
mafic and
ultramafic volcanic
rocks (very Mgrich, Si-poor, a
mantle-like
chemistry)
• felsic volcanics
• deep-water
sediments & chert
(no limestone)
How do you form
greenstone belts?
1) Mantle plumes are
“columns” of ultramafic
magma rising from deeper
parts of the mantle...
They could explain the
ultramafic lavas of
greenstone belts.
They might have been more
numerous in the Archean
than ever since because of
a hotter mantle.
•In Hawaii, the plate moves over the hot spot at
a rate of several cm per year. This has created
a string of basaltic (mafic) volcanic islands.
• Iceland, on the other hand,
sits atop a mid-ocean ridge.
The plate motion is slow (about
2 cm/year) and the plume
below melts and remelts the
new oceanic crust.
Rocks are mixtures of minerals that do not all
melt at the same temperature.
If you melt only part of the mantle and send that
melt upwards through fractures in the rock, you
are extracting a more felsic magma (the
aluminum, silica-rich minerals melt earlier than
the Mg-rich silica-poor minerals.)
This magma is less dense than the mantle and
tends to rise through faults (large cracks in the
rocks)...
This magma may crystallize near the Earth’s
surface and be remelted again to produce a more
felsic composition, more like continental crust.
Iceland is unusual because it is a
baby continent growing near a
mid-ocean ridge (shown on left).
Most continental crust is
probably created near subduction
zones (as shown below), where
partial melting of oceanic crust
takes place as it is subducted.
Greenstone
belts were
probably folded
during
subduction of
small Archean
plates .
Earliest crust
would have been
basaltic (close to
average mantle
composition),
Mg-rich and Sipoor.
Continental crust
grew more
slowly, and
formed by
partial melting
of the early
oceanic crust.
Within our solar system, our Earth is
uniquely suited to life:
• size and distance from Sun are “right”
• large enough to retain an atmosphere
• this atmosphere is thin enough to let
sunlight reach its surface, thick enough
that surface water does not evaporate
• surface water can exist in liquid state
Evidence of earliest life?
Isua Gneiss (3.8 Ga)
from Greenland
(metamorphosed
sandstones).
The banded gneiss
containing graphite
flakes, and the dated
igneous rock that
crosscuts it.
It was suggested in 1996
that the proportions
of carbon isotopes
13C/12C in their graphite
suggests that the carbon
was originally fixed by
photosynthesis.
Carbon Isotopic Evidence for Early Life
A paper in Nature, this year, suggests that other inorganic
processes can produce a life-like isotopic signature.
Modern and Ancient Stromatolites
Oldest ancient bacteria, 3.5 Ga, are
preserved in chert (SiO2) found with
stromatolites.
• A life-like
13C/12C
signature is a chemical fossil.
• Earliest fossils of cells, preserved in chert
(soft gelatinous dissolved silica, common in areas
of submarine volcanic activity) are body fossils.
• Stromatolites are “bacterial condominiums”:
layered sediments that accumulate as columns or
domes where grains are trapped on sticky mats
formed by bacterial colonies.
(It has been argued, however, that every one of
these features could form without the influence
of life... So these observations support an
hypothesis for the age of the earliest life.)
(Australia)
(Greenland)
(Nunavut, Canada)
Many chemicals are expelled at mid-ocean ridges, some
of which decompose in seawater and release energy.
Earliest bacteria could have evolved there, by using
various chemicals as energy sources (chemosynthesis).
Banded iron formations are also found in
Greenland.
Layers rich in iron oxide
alternating with
silica-rich layers.
Submarine volcanic eruptions were the source of
the dissolved silica that was precipitated as chert.
What was the source of the iron?
Today, river waters carry hardly any dissolved iron
to the ocean because it “rusts” on the continents.
•This world was fit for life,
but alien to us . . .
- no significant free oxygen
- a CO2-rich atmosphere
- shorter days (a minimum of 15 hours . . . )
- stronger tides (Moon was closer)
- iron deposition along the continental platform
BUT . . .
- ocean composition probably similar to today
- temperature range of liquid water: much like today
- simple life forms may already be changing the
composition of the atmosphere
The fragments of older rocks incorporated in younger
strata were produced during periods of low sea level,
when the older rocks were exposed to erosion.
Is this list of the different rock types: “red, pink, orange,
yellow, aqua” in the correct time sequence ? No. It should
be pink, red, orange, yellow and aqua. The “red” igneous
rocks must be younger than the metamorphic rocks.