Transcript File

•What is the best explanation for what
happened in this picture?
•Were you there?
•How do you know it happened? What
evidence do you have?
•How might this be applied to
something like a murder?
Evidence
A thing or things helpful in forming a
conclusion or judgment:
The broken window was evidence that a
burglary had taken place.
Scientists weigh the evidence for and against
a hypothesis.
Cretaceous Calamity: How do we know
what happened to the dinosaurs?
•The Dinosaurs took a giant
dirt nap 65 million years ago
and went extinct.
•How do we know?
•What evidence do we have
and what story does it tell?
Evidence?
• K-T boundary sedimentary
band all over the world has
iridium ranging from 20-160
times normal amount.
• Iridium is rare in Earth’s crust,
but abundant in
asteroids/comets
Alvarez, L.W. Science. 1980, 208, 1095-1108.
Iridium spike at K/T boundary
• Iridium is an element
that occurs in the Earths
crust in only tiny
proportions.
weak.
Soot particles found in boundary clay
Similar to Fly Ash From Coal-Burning Plants
Suggest global wildfires associated with ignition of
large amounts of dead plant matter on Earth’s
surface.
Richard D. Norris and
the Ocean Drilling
Project Leg 171B
Scientific Party
http://www.ussspiodp.org/Publications/
Greatest_Hits/contrib
utors.html#n
Photomicrographs
by Brian Huber,
Smithsonian
Institution. Low-res.
versions are hosted
at: http://www.ussspiodp.org/Education/r
esources.html
More information:
http://www.nmnh.si.e
du/paleo/blast
Sediment core
from offshore
South Carolina,
showing the
extinction across
the K/T boundary.
Core is 40 cm
long (about 16
inches).
Tektites, glass beads found in boundary sediments
Due to melting of rock by
energy of bolide (asteroid
or comet) impact ?
Glass found in boundary
sediments of Gulf of Mexico
Timothy Culler (UCB) et al., Apollo 11 Crew, NASA
http://antwrp.gsfc.nasa.gov/apod/ap000322.html
•Cosmic spherule from the moon.
•Spherules such as this are well-known from impact
deposits, and are found in the special K/T impact bed
that marks the end of the dinosaurs.
Shocked Quartz in K-T Boundary Clay
Shock Metamorphism
Has only been observed at meteorite
impact sites and nuclear test sites
Chicxulub Crater
• Impact site buried underneath Yucatán peninsula in
Mexico, discovered by a geophysicist in the late 1970s.
• Took about 20 years to fully investigate crater
• Evidence for impact:
– Shocked quartz
– Tektites
– Gravity anomaly
http://en.wikipedia.org/wiki/Chicxulub_crater
Impact Site: Chicxulub
Location of Chicxulub crater
(180-300 km in diameter)
Image from geophyscal survey over
Yucatan Peninsula
Crater actually discovered in 1978 by a geophysicist
working for Petróleos Mexicanos
Pemex did not release the data for fear of revealing
valuable information to competitors.
“Rediscovered” by Alan Hildebrand in 1991
Cenotes are popular attractions
Alvarez Impact
• Able to calculate size of meteor
– Would have to be 10-15 km in
diameter (size of a mountain, or
Manhattan)
• Impact of that size would have an
incredible amount of energy
– 1 x 108 megatons, 2 million times
greater than most powerful
thermonuclear bomb tested!
Alvarez, L.W. Science. 1980, 208, 1095-1108.
• “The initial impact crater was about 100
kilometers (60 miles) wide and 30
kilometers (18 miles) deep.”
• Fireball and airblast that engulfed the zone
modeled byT Earth Impact Effects
Program
Global
Firestorms
Regional Tsunamis
32
NE Mexico: Tsunamites ?
Believed to have been
deposited by tsunami
generated by impact
Belize: Ejecta Blanket
Poorly sorted debris
believed to be ejecta
deposited close to
crater.
Computer
model
of effects of
K-T
Impact Winter
No sunlight = no photosynthesis= cascade of death through food
chains
Earth in darkness for at least 6 months after asteroid impact
All non avian dinos =dead
All large animals=dead
Fern Spike / Pollen Trough
Pollen/spore ratio takes a dive at about same level
as iridium spike
Records early recolonization of land after impact
winter ?
Asteroid and shock wave
blast long trench
Rings of complex crater form,
by rebound
Impact winter
-debris injected into atmosphere
-lots of dead, rotting organic matter
-global wildfires
-blocking of sunlight
-consumption of ozone ?
So what happened to impactor ?
Most of impactor probably vapourized
during blast
…but possible fragment 100 trillionth
of a gram found in drillcore in NW Pacific