Transcript meteorites - Department of Earth and Planetary Sciences

Meteorite
impacts
Comparative
energies
No human in past 1,000
years has been killed by
a meteorite
Direct observations of
meteorite impacts
Tunguska, Siberia, 30 June 1908…a big
bang above the Earth’s surface
Shoemaker-Levy 9, July 1994…impacts
hitting Jupiter
Direct observations of
meteorite impacts
In 1954, a 5-kg meteorite crashed
through a house in Alabama
the object bounced off a radio and hit the
owner in the head
Effects upon children
Indirect evidence of
meteorite impacts
Preserved craters on the continents,
mainly the oldest parts (shields)
Lac cratére in northern Québec is a simple
crater…
…its rim diameter is 3.4 km, it is 250 m
deep, and it is 1.4 Ma in age
Location map of some impact craters seen
at the surface
Lac cratère
Meteor crater in Arizona is another
simple crater showing rim ejecta
Manicouagan
The Manicouagan crater in Québec is a spectacular
example of a complex crater
Its original rim has been removed by erosion…the
current diameter is 100 km
It has an uplifted central core and outer rings, which
are filled by a lake
Its age - 210 Ma - coincides approximately with a
large extinction at the end of the Triassic period
Manicouagan
St. Lawrence River
Central uplift
Some definitions
Meteoroid: matter revolving around the Sun or
any object in planetary space too small to be
called an asteroid or a comet
Meteorite: a meteoroid which reaches the
surface of the Earth without being vaporized
Meteorites come from larger parent bodies
within our solar system
Asteroids
Asteroids are rocky fragments which
either:
failed to consolidate into a planet, or
represent remnants of a fragmented
planet
Asteroids and the Asteroid Belt
The Asteroid Belt lies
between Mars and
Jupiter…there are
about 4,000 objects
As asteroids collide
with one another,
they fragment and
send pieces into
near-Earth orbits
Types of meteorites
derived from asteroids
Asteroids have a
metallic core and
stony silicate
mantle
Metallic core
As asteroids
fragment, both
metallic and silicate
pieces are produced
Stony silicate mantle
Stony meteorites (94% of
all meteorites)
Two types:
Chondites…contain
chondrules…they
are very old and
primitive
Achondrites…no
chondrules
Photo of a carbonaceous
chondrite (carbon-bearing)
Iron meteorites
These consist of
nearly pure metallic
nickel and iron
This photo shows an
iron meteorite
named ARISPE
Stony-iron meteorites
These are a mixture of
the previous two types
Often they are
fragmental, suggestive
of violent processes
This stony-iron
meteorite is named
ESTHER
Comets
Comets come from the far reaches of the Solar
System
They have highly elongate, elliptical orbits which bring
them close to the Sun
They mainly consist of ice and dust, thus are referred
to as “dirty icebergs” or “dirty snowballs”
They are held together very loosely
Comet West, 9 March 1976
Impact events
1. Probabilities
2. Nature of the event
3. Consequences
4. Mitigation
1. Probabilities of a collision
What are the chances of a large meteorite
hitting Earth?
As of 2003, ~700 objects with diameters
> 1 km known to have orbits which intersect
that of Earth
And 30 new objects are discovered each
year, with the search only 8% complete!
Probabilities - Zebrowski
Zebrowski shows
that, on average,
collisions of 1 kmdiameter objects
occur every 250,000
years
Such an impact is
sufficient to wipe
out most of the
human population
From Zebrowski (1997)
Probabilities Courtillot
Is Zebrowski’s
estimate too high?
Courtillot suggests it
is about 1 Ma
between events
In any case, you can
see that these events
are both very rare
and very
destructive
From Courtillot (1999)
Zebrowski vs. Courtillot
The differences we see on the
two graphs give you some idea of
the uncertainties involved
2. Nature of the event
Impact cratering is an important process
in the history of Earth and other planets
107 to 109 kg of meteoritic flux strikes
Earth each year, mostly in the form of
dust
Impact events
The cratering process is very rapid
Since the objects travel so fast (4-40
km/second), a huge amount of energy is
transferred upon impact
Cratering
A blanket of ejecta is dispersed around
the crater
rock is fractured, crushed, and broken
In large impact events, the rock can even
be vaporized (depending on the type of
rock)
Cratering (continued)
Very high pressures are reached, resulting
in shock metamorphism (pressuretemperature increases)
After the initial compression comes
decompression, which may cause the
rock to melt
Ejecta
blanket
Broken
rock
fracturing
Simple craters are basically simple
bowls
With time, the ejecta blanket
outside the crater is eroded
melt
Central uplift
Complex craters are generated by
rebound of the central core
This core, as it decompresses,
may melt
There are about 200 large, well-preserved impact craters
worldwide…BUT…>>200 impact events during Earth’s
history
This map shows both SURFACE
and SUB-SURFACE examples
Surface examples
Consequences of a
large impact event
These would apply for an object of about
1 km or larger
Actually, you may not want to hear the list
of death and destruction (or maybe you
do)...
Consequences 1
A base surge, similar to a volcanic
pyroclastic flow, will be generated by the
impact
For a terrestrial impact, rock will be
pulverized and/or vaporized, sending
up huge amounts of dust into the
stratosphere
Consequences 2
For an oceanic impact:
huge amounts of water will be vaporized
runaway hurricanes, called “hypercanes”, may
be produced (winds to 1,000 km/hr?)
Global tsunamis will be generated, which will
ravage the Earth’s coastlines
Isabel,
18 September
2003
Consequences 3
In the short term, global wildfires will be
generated by the impact event
These fires will burn uncontrollably across
the globe, sending more soot, dust, and
gas into the stratosphere
Consequences 4
All this suspended dust and soot will cause
global winter and global darkness
Acid rains will fall
Crops will fail catastrophically
The end result will be MASS EXTINCTIONS
Consequences 5
One last interesting point:
The impact likely will trigger devastating
quakes around the globe, especially where
tectonic stresses are high (i.e., plate margins)
Volcanism (flood basalts) may occur on the
opposite side of the globe from the impact, as
a result of shock waves travelling through the
center of the Earth
From Murck et al. (1996)
Mitigation
The problem is the possibility of little or
no warning
There are proposals to use nuclear
weapons and satellites to “shoot down”
or destroy such killer objects
For further edification, rent “Armegeddon”
from Blockbuster (1998)
Good subject for a paper !
Three case studies
Tunguska 1908, Russia
Shoemaker-Levy 9, July 1994, Jupiter
The Cretaceous-Tertiary extinction, 65 Ma
Tunguska, Russia,
30 June 1908
Something big
seems to have
exploded in the
atmosphere
The exact cause is
uncertain, but we
suspect a comet or
a meteor
Aerial view of Tunguska Natural Reserve
What happened?
The object’s entry
appeared to be at
an angle of 30-35°
The object
shattered in a
series of
explosions at
about 8 km
altitude
Tree blowdown from the explosions;
Note parallel alignment of the trees
Big fires
In the central
region, forests
flashed to fires
which burned for
weeks
a herd of 600700 reindeer
was incinerated
Aligned trees
Trees were
felled in a radial
sense
About 2,000
km2 were
flattened by the
blasts
What happened?
Our best scientific
guess is that it was
part of a comet 20-60
meters in diameter…
…no crater was
found…
…and no meteoritic
debris has been
found
Felled trees aligned parallel to each
other
Area of devastation superimposed on a map or Rome.
Yellow=charred trees; Green=felled trees
The lack of a
crater suggests
disintegration
above the surface
of the Earth
The lack of solid
debris implies a
comet rather
than an asteroid
A global view
Soot from the fires circled the globe,
producing spectacular sunrises and
sunsets for months afterward
The Tunguska event was the largest
known comet/asteroid event in the history
of civilization
Comet P/Shoemaker-Levy 9,
July 1994
This comet was
first detected on
24 March 1993
It was broken
apart by a close
pass to Jupiter on
7 July 1992
Hubble image,
1 July 1993
The sequence of events
The collision of the comet with Jupiter
occurred over several days, 16-22 July
1994
It was the first collision of 2 solar system
bodies ever observed
At least 20 fragments hit Jupiter at speeds
of 60 km/second
Sizes of fragments
The largest fragments were about 2 km in
diameter
Huge plumes thousands of km high were
generated
Comparisons can be made with the
Cretaceous-Tertiary extinction event
Multiple impacts
Energies
Fragment A struck with energy equivalent to
225,000 megatons of TNT, the plume rising to
1000 km
Fragment G was the biggie, with 6,000,000
megatons TNT energy and a plume rising to
3,000 km
Fragment G (and K, L) created dark impact
sites whose diameters were at least that of
Earth’s radius
Fragment G
This image shows
a ring of hot gas
about 33,000 km
in diameter and
expanding at 4
km/second from
the impact of
fragment G
Fragment G impacting;
observe four things:
1) thin dark ring: atmospheric shock wave from
fragment explosion below cloud tops
2) dark streak within ring: path of fragment
3) broad oval feature: ejecta blanket
4) small black dot: impact site of fragment D a
day earlier
Fragment G
Fragment G
Shock wave
Impact site of
fragment D
Path of
fragment G
Oval-shaped ejecta
blanket
Another view
Impact events and
mass extinctions
In the Phanerozoic (570-0 Ma), there have
been two great extinctions of fauna and flora:
1) end of the Permian Period at about 250 Ma
2) end of the Cretaceous Period at 65 Ma
These extinctions serve to divide geologic time
in the Phanerozoic into three main eras
Some geologic reference points
to put things in perspective
Earth formed around 4,500 Ma ago
Our ancestor Lucy lived about 3 Ma ago
The last major glaciation occurred 0.010.02 Ma ago (10,000-20,000 years ago)
The Cretaceous-Tertiary
(K-T) extinction at 65 Ma
End of the dinosaurs and other species
In fact, about two-thirds of all species
wiped out
80% of all individuals killed off
Thereafter, mammals took over
What caused the extinction?
The two main theories are:
(1) a meteorite impact
(2) flood basalt volcanism
Another idea is a hypercane sucking up and
literally blowing away the dinosaurs
Some important questions
Was the extinction of the dinosaurs rapid or
prolonged?
Or both? In other words, prolonged followed by
abrupt?
Did a meteorite impact trigger volcanism?
Note location of the Chicxulub crater to the
Deccan basalts
Was it a meteorite?
Evidence for meteorite impact
High iridium at the K-T boundary
Unique to the K-T boundary?
9 parts per billion (ppb) Ir in clay at the
boundary
Background in area <<1 ppb
Earth’s crust < 0.1 ppb
Some metallic meteorites ~500 ppb
Iridium and the dinosaurs
The high iridium is coincident with the
disappearance of the dinosaurs, as seen in
the fossil record
No dinosaur fossils above the K-T
boundary, whereas there are lots below,
as old as 165 Ma
The iridium
The iridium may have come from impact
of a metallic meteorite
Circulation and settling of Ir-rich dust
would result in global distribution of Ir at
the K-T boundary
Global effects
The atmospheric dust and gas from the
impact event would cause global cooling
(compare with nuclear winter)
Global wildfires also would have been
ignited by the fireball
Other meteorite evidence
Spherules…these
represent melt
droplets dispersed
globally from the
impact
Shocked
quartz…this
requires high
pressures
Shocked quartz under the
microscope
The impact crater
Located in the Yucatan Peninsula of
Mexico, it is called Chicxulub
It is completely buried, and was located
by petroleum geologists
The size of the crater implies a meteorite
about 10 km in diameter
Chicxulub
crater
Approx 300 km
Some incidental facts
There are several localities in the Caribbean where
tsunami deposits have been identified (interpreted) at
the K-T boundary
Many of the rocks associated with Chicxulub are
evaporite sedimentary rocks (gypsum, anhydrite, etc.)
containing sulfur (CaSO4)
This sulfur may have been vaporized to produce
sulfate aerosols in the atmosphere, contributing to
global cooling
Tsunami deposits
http://www.ehu.es/~gpplapam/congresos/bioeventos/claeys.html
Incidental facts (ctd.)
Other rocks in the vicinity are limestones
(CaCO3)
Vaporization of evaporites and limestone
would inject sulfur dioxide and carbon
dioxide into the atmosphere
Sulfur dioxide causes cooling, CO2
causes warming
Climate change
Short-term global cooling from:
Dust from impact
Soot from wildfires
Injection of sulfur
Longer-term global warming from:
Injection of CO2
Was it a volcanic eruption?
One candidate are the Deccan Traps in
western India
These are huge outpourings of basaltic
lava which are succesively stacked to
more than 2,000 meters in places
They form a kind of staircase, hence the
word ‘trap’
Flood basalt provinces
Columbia River basalts
~16 Ma
From Courtillot (1999)
Deccan
~65 Ma
Erupted volumes of basalt
The Deccan
represents about
1-2 x 106 km3 of
lava
By comparison, the
Columbia River
Basalt (CRB) is only
2 x 105 km3
photos
Deccan lavas
map
Another view of the Deccan lavas
Age of Deccan volcanism
Interestingly, the Deccan Traps recently
have been dated at 63-67 Ma
And most of the volcanism occurred
during a 500,000 year period at 65
Ma…which is the K-T boundary
This is basically a geological instant in
time
The volcanic model
The volcanic model
basically is one of
enormous fire
fountains of basaltic
magma into the
stratosphere (at least
10-20 km high)
Here is a small-scale
version of this at
Kilauea in Hawaii
Gas emissions
In subduction-related and caldera
volcanism, lots of ash is produced
But for basaltic eruptions, it is the gas,
not the ash, which is significant
In particular, large amounts of sulfur
dioxide (SO2) can be liberated
Short-term impacts
If this gas is injected into the troposphere
and stratosphere, the sulfur dioxide and
other gases can have huge impacts
The main short-term impact would be
global winter conditions
Tropospheric impacts
Gases in the troposphere (0 to 10-20 km
altitude) would be dissolved in water,
generating highly acid rains
These are basically rains of sulfuric acid
(H2SO4) and hydrochloric acid (HCl)
Stratospheric impacts
SO2 injected into the stratosphere
undergoes the following reaction:
SO2 + 2H2O => H2SO4 + H2
The sulfuric acid forms very small
particles called aerosols
These effectively absorb UV radiation, and
could decrease temperatures by up to
10°C
From
Courtillot
(1999)
Columbia
River Basalt
A comparison:
Laki, Iceland, 1783-1784
Laki is a basaltic
volcano in Iceland,
associated with
spreading of the MidAtlantic Ridge
The volcano erupted
from 8 June 1783 to
7 February 1784
Eruptive events at Laki
A series of fissures
opened, resulting in
big eruptions
Eruption columns
reached 15 km altitude
Fire fountains reached
800-1,400 m in height
Volumes: Laki vs. Deccan
A total of 14 km3 of lava was
erupted
(compare this with the 106
km3 from the Deccan)
Large amounts of sulfur
dioxide (SO2) were injected
into the atmosphere
Global cooling followed the
eruption
Eruptive fissures at Laki
Impact of Laki
Famine in Iceland:
crop failures
50-80% of livestock
died
25% of people died
The winter of 17831784 was particularly
harsh in Europe
Ash dispersal from Laki
Longer-term impacts of largescale basaltic volcanism
The oceans become acidic, killing off
algae and other marine life
This “dead” ocean would be reflected by
the clay (no fossils) at the K-T boundary,
instead of limestone (shell accumulations)
Longer-term impacts
The acid oceans also dissolve calcium
carbonate in the form of shells
This results in release of CO2 from the
oceans to the atmosphere
The high atmospheric CO2 is a greenhouse
gas, promoting global warming
Some concluding remarks:
meteorites vs. volcanoes
Ir from a meteorite? From the Earth’s
mantle via eruptions?
The iridium anomaly is found not only at
the K-T boundary, but also extends several
meters on either side
Has the Ir been redistributed from an
originally thin layer at the K-T boundary?
Or is it a record of more than a single event?
Globally speaking...
A meteorite impact into the Chicxulub
region would produce:
dust from the impact
soot from global fires
sulfur gases from evaporite rocks
CO2 from limestone
Basaltic volcanic eruptions would produce
abundant sulfur, and probably CO2 also
Points in favour of a meteorite
High iridium
global distribution of spherules
global distribution of shocked quartz
Points in favour of
volcanic eruptions
The ecological crisis began 105 years
before the Ir-rich horizon…
…and appeared to continue for a period
of time afterward (~105 years?)
Other mass extinctions appear to show
some correlation with flood basalt events
5 major extinctions during
the Phanerozoic (570-0 Ma)
End Ordovician, 440 Ma
end Devonian, 350 Ma
end Permian, 250 Ma
(Paleozoic-Mesozoic boundary)
end Triassic, 200 Ma
end Cretaceous, 65 Ma
boundary)
(K-T event) (Mesozoic-Cenozoic
An interesting aside
The K-T extinction is the only one
for which there is good evidence
for a meteorite impact
End Permian extinction at
250 Ma: the big Daddy
90% of all species vanished
Actually 2 brief, intense crises at 258 Ma,
250 Ma
Correlates with age of Siberian Traps
(flood basalts) in Russia at 250 Ma
Interestingly: (a) no Ir anomaly; (b) no
continental breakup
Siberian Traps
From Courtillot (1999)
Flood basalts, mantle plumes,
and continental breakup
Beyond the correlation with extinctions,
flood basalts also have important
tectonic implications
They appear to be the manifestation of
the birth of hot spots
And this birth frequently is sufficiently
forceful to tear apart the continents
A model of continental breakup
A new mantle plume, which is hot and
buoyant, pushes up a continent
The doming causes thinning and
fracturing of the crust
The fractures allow rapid eruption of flood
basalts from the head of the mantle
plume
Structure of a mantle plume
Continental
crust
Head of a mantle
plume 500-1000
km in diameter source of flood
basalts
Tail of plume generates track of
hot spot
Flood basalts and
mantle plumes
54-62 Ma
132 Ma
Note how most
flood basalts are
on continental
margins
So flood basalts are
fascinating phenomena
Links with the geological timescale
Links with faunal diversity (extinctions)
Links with mantle plume birth
Links with continental breakup
In this context, the Deccan
Traps are important
The Deccan Traps represent the head of a
mantle plume which was born at 65 Ma
The volcanism led to the opening of a
new Ocean (the Arabian Sea)
Afterward, the plume’s head being
emptied, its tail generated the track of the
Reunion hot spot
Deccan
Traps
Current
location of
hot spot
From Courtillot
(1999)
Meteorite impacts - readings
 Alvarez, W., 1997. T. Rex and the crater of doom. Princeton,
Princeton University Press.
 Alvarez, L.W., W. Alvarez, F. Asaro, H. Michel, 1980. Extraterrestrial
cause for the Cretaceous-Tertiary extinction. Science, v. 208, pp.
1095-1108.
 Frankel, C., 1999. The end of the dinosaurs. Cambridge, Cambridge
University Press.
 Grieve, R.A.F., 1990. Impact cratering on the Earth. Scientific
American, v. 262, pp. 66-73.
Meteorite impacts - web
 Two general sites of interest:
 http://neo.jpl.nasa.gov/neo/
 http://www.nearearthobjects.co.uk/
 Shoemaker-Levy:
 http://seds.lpl.arizona.edu/sl9/sl9.html
 Canadian sites on terrestrial impact craters:
 http://gsc.nrcan.gc.ca/meteor/index_e.php
 http://www.unb.ca/passc/ImpactDatabase/