Impacts

© 2012 Pearson Education, Inc.
Lecture Presentation
Chapter 14
Impacts and
Extinctions

Learning Objectives
 Know the difference between asteroids,
meteoroids, and comets
 Understand the physical processes associated
with airbursts and impact craters
 Understand the possible causes of mass
extinction
 Know the evidence for the impact hypothesis
that produced the mass extinction at the end of
the Cretaceous period

Learning Objectives, cont.
 Know the likely physical, chemical, and
biological consequences of impact from a large
asteroid or comet
 Understand the risk of impact or airburst of
extraterrestrial objects and how that risk might
be minimized

Earth’s Place in Space
 Origins of universe begin with “Big Bang” 14 billion
years ago
 Explosion producing atomic particles
 First stars probably formed 13 billion years ago
 Lifetime of stars depends on mass
 Large stars burn up more quickly ~100,000 years
 Smaller stars, like our sun, ~10 billion years
 Supernovas signal death of star
 No longer capable of sustaining its mass and collapses
inward
 Explosion scatters mass into space creating a nebula
 Nebula begins to collapse back inward on itself and new
stars are born in a solar nebula

Earth’s Place in Space, cont.
 Five billion years ago, supernova explosion
triggered the formation of our sun
 Sun grew by buildup of matter from solar nebula
 Pancake of rotating hydrogen and helium dust
 After formation of sun, other particles were
trapped in rings
 Particles in rings attracted other particles and
collapsed into planets
 Earth was hit by objects that added to its formation
 Bombardment continues today at a lesser rate

Asteroids, Meteroids, and Comets
 Particles in solar system are arranged by diameter and
composition
 Asteroids
 Found in asteroid belt between Mars and Jupiter
 Composed of rock, metallic, or combinations
 Meteoroids are broken up asteroids
 Meteors are meteoroids that enter Earth’s atmosphere
 Burn and create “shooting stars”
 Comets have glowing tails
 Composed of rock surrounded by ice
 Originated in Oort Cloud beyond the Kuiper Belt

Airbursts and Impacts
 Objects enter Earth’s atmosphere at 12 to 72 km/s
(27,000 to 161,000 mph)
 Metallic or stony
 Heat up due to friction as they fall through atmosphere,
produce bright light and undergo changes
 Meteorites
 If the object strikes Earth
 Concentrated in Antarctica
 Airbursts
 Object explodes in atmosphere 12 to 50 km
(7 to 31 mi.)
 Ex: Tunguska

Impact Crater
 Provide evidence of meteor impacts, i.e., Barringer
Crater in Arizona
 Bowl shaped depressions with upraised rim
 Rim is overlain by ejecta blanket, material blown out of the
crater upon impact
 Broken rocks cemented together into Breccia
 Features of impact craters are unique from other
craters
 Impacts involve high velocity, energy, pressure and
temperature
 Kinetic energy of impact produces shock wave into earth
 Compresses, heats, melts and excavates materials
 Rocks become metamorphosed or melt with other materials

Simple Impact Craters
 Typically small < 6 km (4 mi.)
 Ex. Barringer Crater
Figure 14.9b

Complex Impact Craters
 Larger in diameter > 6 km
(4 mi.)
 Rim collapses more
completely
 Center uplifts following
impact
Figure 14.10b

Impact Craters, cont.
 Craters are much more common on Moon
1. Most impacts are in ocean buried or destroyed
2. Impacts on land have been eroded or buried by debris
3. Smaller objects burn up in Earth’s atmosphere before
impact

Mass Extinctions
 Sudden loss of large numbers of plants and animals
relative to number of new species being added
 Defines the boundaries of geologic periods or epochs
 Usually involve rapid climate change, triggered by
 Plate Tectonics
 Slow process that moves habitats to different locations
 Volcanic activity
 Flood basalts produce large eruptions of CO2, warming
Earth
 Silica-rich explosions produce volcanic ash that reflects
radiation, cooling Earth
 Extraterrestial impact or airburst

Six Major Mass Extinctions
1. Ordovician, 446 mya, continental glaciation in Southern
Hemisphere
2. Permian, 250 mya, volcanoes causing global warming
and cooling
3. Triassic-Jurassic boundary, 202 mya, volcanic activity
associated with breakup of Pangaea
4. Cretaceous-Tertiary boundary (K-T boundary), 65 mya,
Asteroid impact
5. Eocene period, 34 mya, plate tectonics
6. Pleistocene Epoch, initiated by airburst, continues today
caused by human activity

K-T Boundary Mass Extinction
 Dinosaurs disappeared with many plants and animals
 70 percent of all genera died
 Set the stage for evolution of mammals
 First question, What does geologic history tell us about
K-T Boundary?
 Walter and Luis Alvarez decided to measure
concentration of Iridium in clay layer at K-T boundary in
Italy
 Fossils found below layer were not found above
 How long did it take to form the clay layer?
 Iridium deposits say that layer formed quickly
 Probably extinction caused by single asteroid impact

K-T Boundary Mass Extinction, cont.
 Alvarez did not have a crater to prove the theory
 Crater was identified in 1991 in Yucatan Peninsula
 Diameter approx. 180 km (112 mi.)
 Nearly circular
 Semi-circular pattern of sinkholes, cenotes, on land
defining edge
 Possibly as deep as 30 to 40 km (18 to 25 mi.)
 Slumps and slides filled crater
 Drilling finds breccia under the surface
 Glassy indicating intense heat

Sequence of Events
a) Asteroid moving at
30 km (19 mi.) per
second
b) Asteroid impacts Earth
produces crater
200 km (125 mi.)
diameter, 40 km
(25 mi.) deep
• Shock waves crush,
melt rocks, vaporized
rocks on outer fringe
Figure 14.16a
Figure 14.16b

Sequence of Events, cont. 1
c) Seconds after impact
• Ejecta blanket forms
• Mushroom cloud of of dust and debris
• Fireball sets off wildfires around the globe
• Sulfuric acid enters atmosphere
• Dust blocks sunlight
• Tsunamis from impact reached over 300 m (1000 ft.)
Figure 14.16c

Sequence of Events, cont. 2
 Month later
 No sunlight, no
photosynthesis
 Continued acid rain
 Food chain stopped
 Several months later
 Sunlight returns
 Acid rain stops
 Ferns restored on
burned landscape
Figure 14.16d
Figure 14.16e

K-T Extinction, Final
 Impact caused massive extinction, but allowed for
evolution of mammals
 Another impact of this size would mean another
mass extinction probably for humans and other
large mammals
 However, impacts of this size are very rare
 Occur once ever 40 to 100 my
 Smaller impacts are more probable and have their
own dangers

Linkages with Other Natural Hazards
 Asteroid impact is linked with a variety of hazards such
as:
 Tsunami
 if the impact is in water
 Wildfires
 from heat from impact
 Earthquakes
 from shock waves from impact
 Mass Wasting
 from earthquakes and impact
 Climate Change
 from debris
 Volcanic Eruptions
 melting and instability in mantle

Risk Related to Impacts
 Risk related to probability and consequences
 Large events have consequences will be
catastrophic
 Worldwide effects
 Potential for mass extinction
 Return period of 10’s to 100’s of millions of years
 Smaller events have regional catastrophe
 Effects depends on site of event
 Return period of 1000 years
 Likelihood of an urban area hit every few 10,000’s years

Risk Related to Impacts, cont.
 Risk from impacts is relatively high
 Probability that you will be killed by
 Impact: 0.01 to 0.1 percent
 Car accident: 0.008 percent
 Drowning: 0.001 percent
 However, that is AVERAGE probability over
thousands of years
 Events and deaths are very rare!

Minimizing the Impact Hazard
 Identify nearby threatening objects
 Spacewatch
 Inventory of objects with diameter > 100 m in Earth
crossing orbits
 85,000 objects to date
 Near-Earth Asteroid Tracking (NEAT) project
 Identify objects diameter of 1 km
 Use telescopes and digital imaging devices
 Most objects threatening Earth will not collide form
several 1000’s of years from discovery

Minimizing the Impact Hazard, cont.
 Options once a hazard is detected
 Blowing it up in space
 Small pieces could become radioactive and rain down
on earth
 Nudging it out of Earth’s orbit
 Much more likely since we will have time to study object
 Technology can change orbit of asteroid
 Costly and need coordination of World military and
space agencies
 Evacuation
 Possible if we can predict impact point
 Could be impossible depending on how large an area
would need to be evacuated

End
Impacts and Extinctions
Chapter 14

Impacts

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