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

© 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

© 2012 Pearson Education, Inc.Figure 14.2



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

© 2012 Pearson Education, Inc.Table 14.1



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

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