1. The Rock that Changed the World
Dr. Sean P. S. Gulick
University of Texas at Austin
Institute for Geophysics
Jackson School of Geosciences
Before I can speak about the Chicxulub Impact Crater itself , I need to tell you about the concept that one of the largest extinctions of plants and animals on the planet occurred due to an extraterrestrial event. I’m going to tell you a little bit about the organization that sent me here and specifically about ocean drilling. I will come back to that at the end of this talk because drilling in the impact crater is the next step of our research.
Image: A planetoid plows onto the primordial Earth, during the eons of time when conditions were ripe for the development of life. It is possible that life of kinds unknown to us appeared repeatedly only to be destroyed in collisions like this one which could 'rework' the entire surface. Fortunately the average size of debris declined sharply through geologic time, but the supply of wayward rocks a few kilometers in size is by no means exhausted. Credit: Don Davis, NASA.

2. Story Outline K/T Mass extinction and the impact theory
Discovery of Chicxulub & geology of impacts
Chicxulub seismic experiment results
Drilling for answers
I’d like to review an outline of this talk for you now. I’m going to talk to you about the extinction theory and how the impact theory came about.Then I’m going to discuss the origins if the Chicxulub crater as well as the geologic processes associated with impact craters and how they shape rocky planets like our own. I’ll also discuss our 2005 seismic experiment at the crater, and ultimately drilling for answers in the future.
Image: Artist's rendering of an asteroid impact on the Earth. Credit: Don Davis, NASA.

3. Walter Alvarez (left) and Luis Alvarez (right) focused on a very thin layer of clay from rocks laid down in deep water in what is now central Italy.
The story starts with a father-son team, Walter Alvarez and his son, Luis Alvarez, who were trying to figure out the amount of time involved in the deposition of this small layer of clay between two packages of Cretaceous and Tertiary rock.
Image: LBL Research Review, Vol. 12, No. 1, Spring 1987, p.19 | At a limestone outcropping near Gubbio, Italy, physicist Luis Alvarez and his geologist son Walter, examined the clay layer that launched their theory of 'The Great Dying.' Surprisingly high concentrations of iridium in the clay at Gubbio and many other sites indicated that the mass extinction was the result of a collision between Earth and a huge extraterrestrial object.

4. Thin as this layer is, it separates the Cretaceous world from our modern, mammal-dominated world
Alvarez, a Nobel prize winning physicist, decided to use the regular buildup of particles from outer space to determine how long the layer took to be deposited. Unfortunately, the team selected an unusual layer to examine. This very thin layer turned out to contain anomalously high concentrations of the element iridium. Iridium is relatively abundant in comets and meteors compared to typical Earth rocks.

5. These plots, one from Italy and the other from the Raton Basin, show the iridium contents (in parts per billion, ppb) across the boundary. We see a big spike relative the background levels at the boundary. Often we find interesting things in the fine size fraction of this layer, like pollen or ferns spores, and here we can see a huge spike in ferns immediately after the boundary.
Right at the boundary, they found an enrichment in the rare element iridium - a strong indication of asteroid impact

6. The Alvarez group was not alone
The Alvarez group was not alone. Jan Smit, working in Tunisia, made the same proposal at about the same time
This layer was simultaneously discovered by Jan Smit in Tunisia. Both scientists found that at the Cretaceous-Tertiary boundary the iridium concentration was eight or more times greater than that in the background levels, 8 ppb relative to well under 1 ppb, which is usually expected in sediments. They began to think there was an extraterrestrial reason for this - a comet or a meteor.

7. Raton Basin, Colorado Badlands, Alberta
In the uppermost part of the boundary rocks is where you find the fine fraction, the ‘dust’ if you will, and it’s in that dust that you find the iridium anomaly.

8. Both groups found a profound change in the small oceanic organisms across the K/T boundary
Tertiary
Cretaceous
We know there was a change in life at the boundary. In the marine record shown in the K-T (Cretaceous-Tertiary) boundary in Italian limestone, we see less diversity in life in the Cretaceous, with larger organisms. Above the boundary, we see an explosion of diversity in the record, but most of the “big guys” are gone. In fact, there was a huge extinction event at this boundary, but it was followed by a huge diversification in life.

9. Soon, other sites were found from land locations
Soon, other sites were found from land locations. The boundary, where well preserved, was full of tiny glass spherules called “microtektites”
In K-T sections around the world there is a very similar layer in the lowest part of the boundary. Here we find glass balls, called spherules, made mainly of quartz. They are common to impacts. When you hit rock with enough force it melts as it is ejected, it then reaches either atmosphere or space where it cools to a little, glass sphere. This glass recrystallizes as quartz. These layers with spheres are found at the base of the boundary around the world, and near the impact they can be meters thick.
Raton, Colorado
Caravaca, Spain

10. Both shocked quartz, another indicator of impact, and iridium are now found at hundreds of sites worldwide, all located exactly at the extinction horizon
The other thing we find near the very top of impact boundaries around the world are little pieces of a special quartz. These are high pressure forms of quartz called coesite, and they have damaged crystal structure called planar deformation features. This happens to a quartz grain when it is subjected to extremely high pressures, 10’s to 100’s of gigapascals (a pascal is a measure of pressure defined as 1 newton per square meter. 1 gigapascal = 10^9 pascals). No geological process on earth generate 100+ gigapascals of pressure.
When these were discovered, along with the iridium anomaly and the spherules, they all started adding up to a similar answer – these features at the Cretaceous-Tertiary boundary were caused by either a comet or a meteor strike somewhere on the planet. At this point they still didn't know where this was, they just had these worldwide records of where the shocked quartz occurred.
Left Image: Harris, R.S., Roden, M., Schroeder, P., Holland, S., Duncan, M., and Albin, E. (2004) Upper Eocene impact horizon in east-central Georgia, Geology 32:
Shocked Quartz (μm)

11. The boundary clay also contains massive amounts of soot, indicating global wildfires
The other thing that was found in a few places in the boundary was soot. This led to Jay Melosh publishing a paper in 1990 suggesting there were global wildfires at the Cretaceous-Tertiary time.
Melosh, H. J., Schneider, N. M., Zahnle, K. J. and Latham, D., (1990) Ignition of global wildfires at the Cretaceous/Tertiary boundary, Nature 343,
Melosh et al. (1990)

12. Drilling on the Blake Nose
Drilling on the Blake Nose: ODP Leg 171B
Norris et al, 1999
Drilling on the Blake Nose
Even in the marine record we have some really great examples of this. We see all the same features I’ve been discussing. In the Cretaceous, there was smaller diversity and larger body forms. Here are some great examples of spherules, some with atmospheric air trapped in them and some with vacuum. Near the top of these layers you find some shocked quartz, and near the very top, in the so-called ‘fire-ball’ layer we find some ash, soot, and the iridium anomaly. Above this, we have a complete change in the life with greater diversity and smaller body forms.
The scale in the slide is in centimeters.
Norris, R. D., Huber, B. T. and Self Trail, J Synchroneity of the K-T oceanic mass extinction and meteorite impact: Blake Nose, western North Atlantic. Geology, 27,

13. (families per million years)
Extinction!
Millions of years ago
(families per million years)
Extinction rate 20 15 10 5
600
300
Paleozoic
Mesozoic
Late
Ordovician
Permian-
Triassic
Cretaceous-
Tertiary
Devonian
So one of the big extinctions of the planet’s history, the Cretaceous-Tertiary extinction in which more than 70% of all the species on the planet went extinct, may have been caused by some kind of extraterrestrial event. This led Walter Alvarez to write this book, “T. rex and The Crater of Doom”, which has been used as an introductory textbook in some universities. It became very famous due to the extinction of dinosaurs. It should be pointed out that none of these other mass extinction events have been linked to an impact event. At this point, the K-T extinction is the only one conclusively tied to an impact event. In a morbid way, it’s interesting that you can cause a mass extinction in more than one way.
Graph: The mass extinctions appear as periodic peaks rising above the background extinction levels. This data is from the work of D. M. Raup and J. J. Sepkoski.
65 Ma K/T Boundary
More than 70% of all species go extinct

14. But if the extinction was caused by a big impact, where is the crater
But if the extinction was caused by a big impact, where is the crater? It took a 10 year search until it was finally found - in the Yucatán!
So now we have a theory that the extinction was caused by an impact event, but we don’t know what kind it was, or where it happened. All we have is the evidence in the boundary layer indicating an impact. However, it was noticed that the impact layer got thicker and thicker as you approached the Gulf of Mexico. In fact, some of the thickest deposits suggest the crater is somewhere in the vicinity of southern Mexico. It turns out we’d already found it back in the 1960’s, but no one recognized it for what it was.
Chicxulub Impact
65 Million Years Ago
Paleo-land
Sea
Submerged Continent
Sampled Ejecta

15. Courtesy of Lunar & Planetary Institute Courtesy of NASA
Shown here is an image of a gravity anomaly map over the top of the feature. This shows very slight differences in local gravity due to differences in rock densities. This was originally interpreted to be a volcano. It was discovered and drilled by the Mexican National Oil Company in the 1960’s in the hope of finding oil. Instead they found some odd rock and gave up on the site, forgetting about it for the next 30 years.
In the 1990’s, Alan Hildebrand and his colleagues stated that this feature wasn’t a volcano, and had the morphology of an impact. Some positions on the feature with incredibly low density are what you’d expect with the creation of a lot of breccias (broken, crush-up rocks with porosity), which occurs in an impact. Kevin Pope noticed that the water-filled sink holes, cenotes, where Mayans used to perform human sacrifice, have an interesting shape along the Yucatan Peninsula in Mexico, where they rim some kind of feature in the subsurface. This feature is a large control on the hydrogeology of the Yucatan Peninsula.
References for these ideas include:
A.R. Hildebrand, G.T. Penfield, D.A. Kring, M. Pilkington, Z.A. Camargo, S. Jacobsen and W.F. Boynton, The Chicxulub Crater: a possible Cretaceous–Tertiary boundary impact crater on the Yucatán Peninsula, Mexico, Geology vol. 19 (1991), pp. 867–871.
Camargo, C. A. Z., Suarez, G. R., Evidencia sismica del crater de impacto de Chicxulub (in Spanish). Boletin de la Asociacion Mexicana de Geofisicos de Exploracion, v. XXXIV (1994), pp
Kevin O. Pope, Steven L. D'Hondt, and Charles R. Marshal,l Meteorite impact and the mass extinction of species at the Cretaceous/Tertiary boundary PNAS vol. 9 (1998) pp
V.L. Sharpton, L.E. Marín, J.L. Carney, L. Scott, G. Ryder, B.C. Schuraytz, P. Sikora and P.D. Spudis, A model of the Chicxulub impact basin based on evaluation of geophysical data, well logs, and drill core samples. In: G. Ryder, D. Fastovsky and S. Gartner, Editors, The Cretaceous–Tertiary Event and other Catastrophes in Earth History, Geological Society of America Special Paper vol. 307 (1996), pp. 55–74.

16. Courtesy of NASA
The scale of this feature includes the entire northern half of the Yucatan Peninsula, half onshore and half offshore. It is roughly 200 kilometers across, and it can be seen from space on high resolution photos as a small depression. If you are on the ground in the Yucatan it is unnoticeable because it has been in filled with about a kilometer of Tertiary limestone.

17. 1996 Seismic Reflection and Refraction Study
1996 BIRPS seismic reflection/refraction survey with Bouger gravity anomaly overlay
Geophysical data can be used to model subsurface crater structure
Structural data constrain numerical modeling of impact event
Refraction data measures velocity
Reflection data images subsurface
In 1996, a seismic reflection and refraction study attempted to image beneath the kilometer of limestone and see the shape of the impact itself and the deformation it had caused. They used reflection data to examine the offshore portions and refraction data for the subsurface of onshore/offshore profiles.
Seismic methods use a source of seismic energy such as an earthquake, man-made explosion or a specialized air gun and a recording device, a seismometer. Seismic reflection uses the time it takes for a seismic waves to go from the source to reflecting structure and back to a seismometer to estimate the depth of the feature. Seismic refraction methods use the refraction of seismic waves in rock units to characterize geologic structure. Refraction methods depend on the fact that seismic waves have different velocities in different types of rock and that waves refract when they cross the boundaries between these layers. The methods enable the general rock types, densities, and the approximate depth to boundaries to be approximated.

18. Crater Morphology Lunar examples
Alfrancus C -
10 km simple crater
Tycho – 85 km
complex crater
The researchers were trying to define the morphology of the Yucatan feature because impacts are very well behaved. They form nice features scalable to the size of the impact and the gravity of the body they strike. In other words, the smallest impacts on any rocky planet we’ve looked at in the solar system make little bowl shaped impacts, like Meteor Crater in Arizona. Slightly larger impacts have a little bump in the middle, a central uplift. Slightly larger than that, instead of a bump in the middle, an outward splash makes a ring of peaks called a peak ring impact basin or crater. The biggest ones have all of those features plus extra rings towards the outside, called a multi ring impact or basin.
Alfrancus C - 10 km diameter simple impact crater on the moon (Apollo 16 Panoramic Camera Photograph 4616).
Tycho - 85 km diameter complex impact crater on the moon (Lunar Orbiter IV photograph 125M).
Schrödinger km diameter peak ring impact basin on the moon (Lunar Orbiter IV photograph 9M).
Orientale km diameter muti-ring basin on the moon (Lunar Orbiter IV photograph 194M).
Orientale – 900 km
multi ring basin
Schrodinger –
320 km peak ring basin

19. Transient Crater 35 km deep and 100 km across
Bolide ~12 km in diameter
Transient Crater 35 km deep and 100 km across
So the question is, what was Chicxulub? We have a feature that looks like an impact, but what kind? This summary figure was published with the researchers’ paper in 1997 in Nature magazine, and it shows some of the basic features. There is a central uplift but it is submerged about 5 kilometers below the surface, topped by a large melt sheet. A ring of bumps around the edges form a peak ring, but they unexpectedly represent low velocities and low densities. A Cretaceous terrace zone makes up the side, with blocks slumping inward, and extra faults occur toward the outside. Theses features alone tell us this was a multi-ring impact - it is the biggest class of impacts that we know about. The distances of these features relative to one another (because impacts scale) tell us something about the original hole that was blown in the earth, which is known as a transient crater. It was 100 kilometers across and 35 kilometers deep, which in this case tells us it blew a hole all the way to the mantle. This also tells us something about how big the object that struck us was, roughly kilometers in diameter.
Morgan, J., Warner, M., Brittan, J., Buffler, R., Camargo, A., Christeson, G., Dentons, P., Hildebrand, A., Hobbs, R., MacIntyre, H., Mackenzie, G., Maguires, P., Marin, L., Nakamura, Y., Pilkington, M., Sharpton, V. and Snyders, D., 1997, Size and Morphology of the Chicxulub impact crater, Nature, v. 390, p
Cross-section
Morgan et al. (1997)

20. All in 300 to 600 sec! 100 km 35 km 50 km 8 km Melosh et al. (1989)
How do we get to that final structure with an impact? Because of the phenomenal speed of the impact, the meteor was traveling about 20 kilometers per second, anything they hit acts like a liquid. This is called acoustic fluidization. Basically, things are hit so hard that they flow like a liquid, effectively deforming in a brittle fashion at all scales.
Initially, it is very much like a rock being thrown into a pond. It first opens up a hole, the transient crater. It would’ve splashed up the rims of that initial hole, probably as high as 8 kilometers, the height of the Himalayas. This would have been very temporary. The earth abhors a hole like that and it starts reacting. The center part which is still moving will splash upwards, in this case 10’s of kilometers into the air, the top of which probably reached escape velocity (it left the earth’s atmosphere). The sides would now start behaving in a brittle fashion, forming big faults that would start collapsing inward to fill in the hole with these slumped terraces. Finally, this will collapse down and out and submerge back underneath, leaving a central uplift with a peak ring.
You’ll notice this is a big jump. That’s because we don’t actually know the processes from the models and the physical experiments to the actual crater structure, and those are some of the questions we still have. The amazing thing is this all happens in probably under five minutes. It is an amazingly powerful phenomena.
All in 300
to 600 sec!
Melosh et al. (1989)

21. Meteor Crater: A small one
St. Stephens Cathedral in Vienna (137 m high) in Meteor Crater, Arizona (1.2 km diameter)
What does that do to what it strikes? Consider the amount of energy imparted to the target. Meteor Crater in Arizona is a very small example. Shown here is the St. Steven’s Cathedral in Vienna, 137 meters high, for scale. The energy released here is roughly equivalent to a 2 megaton nuclear explosion, meaning anything within 10 kilometers would’ve been immediately scorched by the fireball. Anything within about 20 kilometers would’ve been killed by the pressure blast. Anything within about 40 kilometers would’ve experienced hurricane force winds. That’s a little one.
July 8, 1956: 1.9 MT Apache nuclear fireball

22. Energy = ½ mv2 Energy = 2 x 1023 J ≈ 100 million Atomic bombs
Mass = 1 x 1015 kg
Velocity = 20 km/sec
Energy = 2 x 1023 J ≈
100 million Atomic bombs
1% of energy turned into (200 m) tsunamis and hurricane force winds
99% of energy caused melting, vaporization, ejecta, and magnitude 13 earthquakes
Because these impacts scale, you can simply think of them in how much energy is imparted. This is pretty simple math - just half the mass times the velocity squared. We now have an idea of what kind of body it was, a chondritic meteorite, so we can estimate the mass of a 12 kilometer version of one and we have an idea about the velocity, about 20 kilometers per second. The result is a large number of joules roughly equivalent to 100 million atomic bombs, more than our entire world’s stockpile. 1% of that energy is turned into the tsunamis and hurricane force winds. The rest of it actually goes into the target, causing melting, vaporization of the upper 2-3 kilometers of crust, ejection of material within the 100 kilometer transient crater down to about 11 kilometers, and potentially as much as magnitude 13 earthquakes. But none of this is probably the reason we had a mass extinction event. These are all effectively local phenomena. We think the reason it was a true global mass extinction event was the ejecta.
Top Image: Artist's rendering of an asteroid impact on the Earth. Credit: Don Davis, NASA.
But the real problem was the ejecta…

23. Courtesy of Los Alamos National Labs
Theses movies demonstrate the amount of force and energy in an impact. The first shows mainly the temperature at the impact site. The graph shown is in eV’s. 1 eV is 11,000 degrees K (kelvin). The center of the region reaches half that, 5500 °K. Locally, we’re on our way to the center of the sun for a brief period of time.
°C = °K
Courtesy of Los Alamos National Labs

24. Consider the ejecta. We have an incredible amount of heat, but we’re also sending out the upper 11 kilometers of this in either vapor or ejecta, which has to get all around the world to be preserved in all the records of the K-T boundary. The impact crater community has come up with only one mechanism for this to be truly global. The material leaves the crater ballistically, which means it would leave the earth’s atmosphere but not the earth’s gravitational field, as shown. This would wrap the earth fairly consistently on all sides. There will be locally thickened ejecta near the crater, but this model allows for some amount everywhere.
Courtesy of Melosh

25. Why is this important? When that much material is outside the atmosphere but within the earth's gravitational field, it then rains back through the atmosphere. This causes an incredible shear heating effect on the atmosphere. Early models of this suggest fast incoming ejecta raining down to the surface of the earth would’ve heated up the global soil temperature to that of a pizza oven for a few hours. So big land animals were in trouble. Little creatures that could’ve taken cover in a hole or in water might have been able to hold out that initial effect. This is also likely the cause of the soot found in a lot of the K-T boundary records, as global wildfires would have resulted from this.
Top Left Image: Death to Dinosaurs. Credit: Joe Tucciarone
Top Right Image: A planetoid plows onto the primordial Earth, during the eons of time when conditions were ripe for the development of life. It is possible that life of kinds unknown to us appeared repeatedly only to be destroyed in collisions like this one which could 'rework' the entire surface. Fortunately the average size of debris declined sharply through geologic time, but the supply of wayward rocks a few kilometers in size is by no means exhausted. Credit: Don Davis, NASA.
Bottom Left Image: Common Treeshrew, Bronx Zoo, New York

26. Drilling on the Blake Nose: ODP Leg 171B Norris et al, 1999
The scale in the slide is in centimeters.
Norris, R. D., Huber, B. T. and Self Trail, J Synchroneity of the K-T oceanic mass extinction and meteorite impact: Blake Nose, western North Atlantic. Geology, 27,

27. 21st Century Surveying and Ground Truth
Best preserved large impact on Earth
Only impact conclusively linked to mass extinction
Natural laboratory for impacts as a geologic process and impacts effect on life
We have a lot of information about Chicxulub, but we want to learn about the details. This is the largest preserved impact on Earth. There are only two others of this size, and they occurred around 2 billion years ago. This is the only one that has not been eroded or destroyed by plate tectonics, and therefore we can learn something about what happens to very large scale impacts when they strike earth. This is also the only one conclusively linked to a mass extinction event. This makes it an ideal natural laboratory for impacts as a geologic process and for thinking about their effect on life, positive or negative.

28. Surveys in Preparation for Drilling
2005 MCS lines
1996 and Pemex MCS lines
land stations
In 2005, we collected a lot more geophysical data. Offshore, we collected seismic reflection data and we also placed seismometers on the seafloor and on land, which are used to listen to earthquakes. The seismometers were used to gain information about the volume and the density of the rocks in the crater in order to understand the details of the impact.
land stations
Yucatan coast line

29. R/V Maurice Ewing Cruise EW0501
Jan 5, 2005 – Feb 16, 2005
This is the ship we used, the R/V Maurice Ewing. We sailed out of Panama and spent about 6 weeks in the field.

30. Airguns
Behind the ship, we towed airguns to actually perform the experiments. These make a pulse of air in the water, which then collapses and sends a sound down through layers of the earth. We can then record the recovery of that sound back to the surface. We towed 20 of these behind the ship, and fired them simultaneously.

31. Hydrophone Streamer
We listened to the recovery sounds in 2 ways. One way is through a cable that we towed behind the ship, known as a streamer. Inside the streamer are little hydrophones, which take the pressure pulse coming back through the water and convert it into an electrical signal. We can read the pressure based on how much current is generated. We can take all these readings and build an image of the crater geology. We towed about 6 kilometers of streamer so we could get a big, long record of the impact.

32. Profiling!
Here are some example of surveying. We see some airguns going off, and some graduate students hard at work. We also always have people up top looking out for marine mammals because we have to turn the airguns off if they are within the area.

33. This is a diagram of what we were doing with that streamer
This is a diagram of what we were doing with that streamer. Basically, you send out sound, and it sends energy down into the subsurface. Every time it runs into a boundary in the subsurface, where the velocity and density changes, some of that energy will be reflected back to the surface. Because we do this repeatedly every 50 meters or so and we have all these receivers behind the ship listening, as we move along we end up bouncing at the same spot on the earth over and over again. If you add up all the energy from that one spot, you learn a lot about that one position. If you lay a bunch of those single spots next to each other, you get a picture of the subsurface. That’s what makes up the following images.

34. Dipping Reflectors 2.5 km Time Distance
Here’s an example from the impact crater. The vertical red line represents a drill hole. We see the peak ring (the whole cross-section) and the sediments that filled in on top of it after the event.
Dipping Reflectors
Distance

35. Ocean Bottom Seismometers
The other thing we were doing was listening to these same sounds with seismometers. These were placed both on the sea floor and on land. This image shows ocean bottom seismometers (OBS) that are placed on the sea floor.

36. Land Seismometers
Land-base seismometers and how we place them.

37. water Sedimentary rock Denser rock Receivers Source
Instead of looking for energy that bounces back, these look for energy that travels through different layers and is refracted. The energy that comes back up can be used to build a model of the velocities or densities of rock layers. Since we had instruments onshore and offshore, we could learn about a large volume in the earth, a big chunk of the earth all the way down to the mantle.

38. Gravity model of uplift
Structural uplift near the crater center (red star) is constrained by gravity and velocity data.
The uplift is offset west of the crater center.
Velocities of 6.3 km/s occur at a depth of 5 km. Outside the crater these velocities are found at a depth of 15 km, suggesting a vertical uplift of ~10 km.
On the top, this shows a map of the uplifted part of the crater using models built from gravity data. The middle image is a map-view slice through our volumes of the earth, which we now know the velocities of, at about 5 kilometers depth. We see contours that show us very high velocities in the center of the crater. That may not seem impressive, but consider the velocities. 6.5 km/s is the velocity of granite at mid to lower crustal depths. Next is a cross section through that same volume. This shows that these velocities are normally around km depth, but here they’ve been uplifted to 5 km depth. In other words, we’ve had about 10 km of vertical motion in the earth at the center of the crater, which is the “splashing up” that we see. This is direct observational proof that Chicxulub is an impact crater.
Christeson et al., in prep

39. NW Cross Section through Crater
What about the reflection images of the crater? Shown here is the peak ring that splashed outward and the top of the melt sheet in the middle. We see a layer of evaporites (salt deposits) stepping downwards into the center of the crater beneath the peak ring. We also see lots of faults breaking up the subsurface, up to a 250 km diameter distance away. On one side, the faults seem to merge into a low-angle fault, a feature that goes down towards the center of the crater. The other thing we notice is a well-defined rim of the inner crater, which we also saw on the gravity map. This is caused by the sediments filling in the center.
Cross section line

40. NE Cross Section though Crater
If we look at the northeast part of the crater, we see something very different. It still appears as a circle on the surface, but the subsurface shows it to be asymmetric. Here we don’t see the crater rim, and the infilling sediments just keep on going towards the northeast. It looks like there was already some kind of basin providing a lot of sediment, and it simply continued to fill in after the impact. Also notice that the material that slumps in towards the middle doesn’t go nearly as deep as in the previous image, and the faults themselves have a different shape, similar to nested bowls rather than merging into one feature.
Cross section line

41. East Cross Section through Crater
If we go back around to the east, we again see the defined inner rim and we also have the bowl shaped faults.
Cross section line

42. ~ West side ~ East side Gulick et al. (2008)
This summary image shows how truly asymmetric the crater is at depth, depending on which orientation it is viewed from. This generates questions of why there is a difference and why there is no ring in the northeast. Ever since the discussion of an impact at Chicxulub began, people have questioned what direction it came from and at what angle it hit. Craters remain round even down to impact angles of as little as 10 degrees. It’s only when the angles are very shallow, on the order of 5 degrees, that an elongate crater would form.
Gulick et al. (2008)

43. Schultz and D’Hondt et al. (1996)
Northwest Impact Direction
Schultz and D’Hondt et al. (1996)
Environmental Evidence
“Fern spike” in North America due to lack of competition
Higher flora extinctions in North America N
20˚ - 30˚
Gravity Anomaly Evidence
Elongate central structure
Central structure is offset uprange (SE)
Widening of the 180 km ring transverse to trajectory
So we see that this crater is round, but people started trying to use the gravity anomalies to determine the impact angle.
Notice here a sort of horseshoe shape in the low density part of the anomaly which is the peak ring. In 1996, Shultz and D’Hondt suggested that meant the impact was coming from the southeast to the northwest. That would matter because the direction that’s down range of the impact would receive more local ejecta and have greater shear heating, so the results of the impact would be more devastating in that direction. They used this hypothesis to suggest why there is a pronounced increase in ferns (“fern spike”) in the North American fossil record at the K-T boundary. The ferns were one of the first things to come back after complete destruction of the vegetation. They suggest the impact had more of a local effect, and the ferns are telling us the direction of the impact.
That may be true, but the fossil record is difficult to interpret. It may be that we think the fern spike is most obvious in North America just because that’s where we’ve looked the most. Perhaps with careful examination, South America would appear to have a greater fern spike.
Peter H. Schultz and Steven L. D'Hondt, 1996, Cretaceous-Tertiary (Chicxulub) impact angle and its consequences, Geology, v. 24(11), p
50 km

44. Northeast Impact Direction
Hildebrand et al. (1998) N
Gravity + Seismic Evidence
“twin peaks” alignment
Asymmetry in inner ring and peak ring
Thrusting downrange
Downrange depression
NE compressional shearing
~60˚
The other possibility, that was proposed and then refuted later by Hildebrand, suggested that the two bumps in the middle are separated and aligned because the impact came from the southwest. He suggests the basin to the northeast, which we’ve imaged, was actually in the down range.
This brings up the question, can we look at the structures in the center of the crater and determine the direction that the object came in at? Is that a valid observation? To answer this, we must look at an area where we know the direction of impact. We can use Venus as an example. We have great images of impact craters all over the planet and because it is well preserved, we can actually see the ejecta that comes out of an impact. Because of this, using splash patterns, we can see what direction the impact came from. Now we can see if the central features, the peak ring or central uplift, tell us anything about the direction of the impact. Is there a statistical comparison you could make that says every time we have an impact in one direction the center moves towards it, or away from it?
Hildebrand, A.R., Pilkington, M., Ortiz-Aleman, C., Chavez, R.E., Urrutia-Fucugauchi, J., Connors, M., Graniel-Castro, E., Camara-Zi, A., Halpenny, J.F., Niehaus, D., Mapping Chicxulub crater structure with gravity and seismic reflection data. In: Grady, M.M., Hutchison, R., McCall, G.J.H., Rothery, D.A. (Eds.), Meteorites: Flux with Time and Impact Effects. Geological Society, London, Special Publications, Vol. 140, pp. 155–176.
50 km

45. McDonald et al., submitted
Craters on Venus
Leyster
Ermolov
We have 10 craters where we can look at their peak rings and plot the direction the feature had moved relative to a left to right direction of impact, we see there is no correlation at all.
McDonald et al., submitted
The surface morphology of craters where the direction of impact can be determined by pattern in the ejecta show structures at crater centers do not clearly indicate an impact trajectory.

46. Central peak offsets vs. peak ring offsets90
Impact direction
Impact direction
180
270
Average offset: .031
Average offset: .067
Ekholm & Melosh et al. (2000)
McDonald et al., submitted

47. Cross-section of margin of Chicxulub crater
Map view of depth to Cretaceous ocean floor
So what do the Chicxulub observations tell us? What do all these asymmetries in the subsurface tell us about the impact itself? Here’s a seismic reflection line that wraps around the edge of the crater, but outside of the transient crater, the area initially blown open. What you see are evaporite reflectors that thicken as you go to the northeast. That’s another way of saying that a basin was already present, as we see the continued thickening of the deposits in that direction. The white, dashed line is the interpretation of where the seafloor was at the K-T boundary. It also deepens into that same position. The seafloor wasn’t flat, and the lower image shows the depth of the reconstruction of the seafloor at the end of the Cretaceous. We can see that west of where the impact struck the depth was as little as 100 meters. To the east, it was as deep as 2.5 kilometers. So the area the impact hit was a slope. That means that the area that was vaporized and ejected may have had as much as 650 meters of water in it, not the shallower depth we thought before.
Sean P. S. Gulick, Penny J. Barton, Gail L. Christeson, Joanna V. Morgan, Matthew McDonald, Keren Mendoza-Cervantes, Zulmacristina F. Pearson, Anusha Surendra, Jaime Urrutia-Fucugauchi, Peggy M. Vermeesch & Mike R. Warner, 2008, Importance of pre-impact crustal structure for the asymmetry of the Chicxulub impact crater, Nature Geoscience v. 1, p
Gulick et al. (2008)

48. Results
Asymmetries result from target structure rather than meteor trajectory
Ring faults mapped at distances up to > 125 km
Average water depth ~650 m
It could be that these shallow target products, these heterogeneities in the target, are driving some of the asymmetries we see in the final crater structure. On the left side of the model we see almost no water and thinner sediments. On the right side, there’s deeper water and much thicker sediments. So if that’s the transient crater, on one side we see what we’d expect, but on the other side much of the material that splashes out was actually made just of water. When an impact hits, anything it strikes is going to act like a liquid. However, after the impact is over anything that’s a liquid is just going to wash away. In other words, if the uplifted edge was mostly made of water, it would just wash off. That means there would be no ring to observe on one side of the crater.
Things can also be deposited at different depths based on how thick the sediments are on one side of the crater or the other.
So if you have heterogeneities in the target you can actually form very different features in the final crater structure. This tells us nothing about the way the impact hit it.
Another issue is that there was a lot of water, 650 meters, much more than the expected meters, that turned to vapor. While the water turned to water vapor, the sediments below it got vaporized or ejected as well. These sediments included the layer of evaporites (salt and sulfate) highlighted earlier. In this case, as much as 25% of these evaporites that got vaporized had very high sulfur content. If you combine sulfur and water, you get sulfate aerosol. Sulfate aerosol in the lower atmosphere will wash out as acid rain. We now have another mechanism for extinction. The acid rain could directly affect both the terrestrial and marine organisms. There's been a suggestion that those organisms that didn’t make it past the Cretaceous were those more susceptible to changes in the acidity (pH) in the marine environment. Sulfate aerosols in the upper atmosphere are an excellent way to cool the atmosphere. They reflect the sun’s solar radiation and so have a net cooling effect on the planet.
Right after this theory arose, another idea came up that the impact made dust travel all around the world and caused a “nuclear winter” that lasted a long time. This is totally wrong. There just wasn’t enough dust blown out of that one spot to wrap the entire planet and make it go black and cause that mass extinction. This was figured out very quickly after the theory was advanced. What the impact may have done was kick off a chemical experiment that caused a cooling effect with a change of chemistry in the ocean through acid rain. It shows you the concept that if you kick the system you can change climate very rapidly so that it can’t respond quickly enough.
Gulick et al. (2008)

49. Increased Planetary Albedo
The Chicxulub impact may have been especially lethal because of an especially unlucky choice of target - Sulfur-rich rocks
Stratosphere
Troposphere
SO2
Surface cools
Infrared
SO H2SO4
Cirrus Modification
Removal
Processes
Warming
Ash
HCI
Nucleation and
Particle Growth
Acid Rain
Increased Planetary Albedo
If you combine sulfur and water, you get sulfate aerosol. Sulfate aerosol in the lower atmosphere will wash out as acid rain. We now have another mechanism for extinction. The acid rain could directly affect both the terrestrial and marine organisms. There's been a suggestion that those organisms that didn’t make it past the Cretaceous were those more susceptible to changes in the acidity (pH) in the marine environment. Sulfate aerosols in the upper atmosphere are an excellent way to cool the atmosphere. They reflect the sun’s solar radiation and so have a net cooling effect on the planet.
Right after this theory arose, another idea came up that the impact made dust travel all around the world and caused a “nuclear winter” that lasted a long time. This is totally wrong. There just wasn’t enough dust blown out of that one spot to wrap the entire planet and make it go black and cause that mass extinction. This was figured out very quickly after the theory was advanced. What the impact may have done was kick off a chemical experiment that caused a cooling effect with a change of chemistry in the ocean through acid rain. It shows you the concept that if you kick the system you can change climate very rapidly so that it can’t respond quickly enough.
This is a very good mechanism for killing a lot of things on land, but does not explain the marine record. We think the latter may be involved with the chemistry in the impact
Diagram: The dispersal of volcanic aerosols has a drastic effect on the Earth's atmosphere. Following an eruption, large amounts of sulphur dioxide (SO2), hydrochloric acid (HCL) and ash are spewed into the Earth's stratosphere. Hydrochloric acid, in most cases, condenses with water vapor and is rained out of the volcanic cloud formation. Sulphur dioxide from the cloud is transformed into sulphuric acid (H2SO4). The sulphuric acid quickly condenses, producing aerosol particles which linger in the atmosphere for long periods of time. The interaction of chemicals on the surface of aerosols, known as heterogeneous chemistry, and the tendency of aerosols to increase levels of chlorine which can react with nitrogen in the stratosphere, is a prime contributor to stratospheric ozone destruction. Source:

50. Conclusions Thus Far
Peak ring and terrace zone asymmetries are controlled by pre-existing shallow structure
Ring asymmetries are dominated by initial crustal geometry
Mapping at Chicxulub suggests that target heterogeneities dominate final crater structure
Signature of impact direction and angle may be difficult to extract from final crater geometries in many cases
The vapor plume at Chicxulub likely included a greater concentration of water than previously suggested and asymmetries in the amount of sediment ejecta should be expected independent of impact direction
So here we have some conclusions thus far. The peak ring we saw in the subsurface may not have been caused by the impact trajectory, but by the properties of the materials that it hit. In other words, it may be very difficult to look at impact craters and figure out the direction and angle that it hit at. Also, the vapor plume at Chicxulub likely contained a far greater amount of water, and that water may have caused the aerosol effect. We can consider that a result of the impact and one of the things which may have caused the extinction.

51. Key questions to address
Chicxulub crater structure from geophysical models… more drilling would provide hard data!
Modified from Morgan et al., 1997
Key questions to address
The plan is to drill a transect of holes across the impact. We’ll drill in the annular trough and learn what life looked like as it came back after the impact. We can drill in the peak ring and actually figure out what the ring is made of.
Is the peak ring associated with a thickened layer of melt-rich impact breccia? Is it formed by collapse of the central uplift?
What are the dimensions of the melt sheet?

52. The nature of the materials that form the dipping reflector is currently a mystery. It might be a fault zone at served as a conduit for the movement of hot fluids. Where there major hot springs and mineral deposits associated with the impact?
Mendoza-Cervantes, K., Gulick, S. S. , Urrutia-Fucgauchi, J. , McDonald, M. , Christeson, G. L. and Barton, P. J., 2006, 3D Image of the Chicxulub crater peak ring based on 2-D seismic reflection profiles from R/V Maurice Ewing 2005 survey, First International Conference on Impact Cratering in the Solar System, Noordwijk (NL) online,
What is the dipping reflector? Is it a mineralized fault recording an old
hydrothermal system?
Mendoza et al. (2007)

53. Was this system a haven for life?
Could the hot springs have served as a haven for microbes deep in the Earth? The colonization and growth of microbes deep in the Chicxulub impact might serve as a model for the development of early life on Earth. The early Earth’s surface may have been too hostile to allow life, but fractured rocks, faults and hydrothermal systems at depth may have offered early life a toe-hold.
Top Image: Artist's concept from NASA Origins Roadmap, 2003,
Ames, D E, Jonasson, I R, Gibson, H L, Pope, K O, 2006, Impact-generated hydrothermal system: constraints from the large Paleoproterozoic Sudbury crater, Canada, in Biological processes associated with impact events; eds. Cockell, C; Koeberl, C; Gilmour, I; pages , Geological Survey of Canada, Contribution Series
Was this system a haven for life?
Role of impacts as haven for early life?
Ames et al. (2006)

54. Stay Tuned! 1980s Impact Theory 1990s Chicxulub discovered
2000s Theory matures and crater revealed
2010s Drilling for Answers!
I’d like to close with a reminder of how quickly this science had evolved. The impact theory only came out in the 1980’s. The crater was only discovered in the 1990’s, for being what it was. In the 2000’s we started maturing our understanding of craters and thinking about the details, and how the target affected the processes. In the next decade we hope to drill into the impact crater. That’s not a lot of time, and it’s a rapid change in science. There’s still the chance to find more.

55. Thanks for listening…
Thank you.

56. With thanks to: Gail Christeson, Matt McDonald, Peggy Vermeesch
Jo Morgan, Mike Warner, Gareth Collins
Jaime Urrutia, Keren Mendoza
Michael Whalen, Zulmacristina Pearson
Penny Barton, Anusha Surrendra
Jay Melosh
Christian Koerberl