•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?

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

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 claySimilar 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.usssp-iodp.org/Publications/Greatest_Hits/contributors.html#nPhotomicrographs by Brian Huber, Smithsonian Institution. Low-res. versions are hosted at: http://www.usssp-iodp.org/Education/resources.html

More information: http://www.nmnh.si.edu/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, NASAhttp://antwrp.gsfc.nasa.gov/apod/ap000322.html

• (0.1 mm) diameter 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)

Crater actually discovered in 1978 by a geophysicist working for Petróleos MexicanosPemex did not release the data for fear of revealing valuable information to competitors. “Rediscovered” by Alan Hildebrand in 1991

Image from geophyscal survey over Yucatan Peninsula

Cenotes are popular attractions

Before impact

After impact

Computer model

of effects of K-T

Impact Winter

No sunlight = no photosynthesis= cascade of death through food chainsEarth in darkness for at least 6 months after asteroid impact

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 waveblast 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 vapourizedduring blast …but possible fragment 100 trillionthof a gram found in drillcore in NW Pacific

37

Global Firestorms

Regional Tsunamis

Belize: Ejecta Blanket

NE Mexico: Tsunamites ?

Believed to have been deposited by tsunami generated by impact

Poorly sorted debris believed to be ejecta deposited close to crater.