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Transcript
Do NOT write on this paper. Return this paper at the end of the class period!
Extinction Hypothesis B – Continental Drift
SOURCE # 1 : What Killed the Dinosaurs? - an online resource accessed at
http://www.pbs.org/wgbh/evolution/extinction/dinosaurs/low_bandwidth.html
@2001 WGBH Educational Foundation and Clear Blue Sky Productions, Inc. All Rights Reserved
Hypothesis: Continental Drift
It's difficult to imagine a process more gradual than continental drift. But some scientists say that, slow or not,
this repositioning of the world's landmasses was disastrous for dinosaurs. As continents heaved upward, pushed
by the movement of tectonic plates, ocean currents were redirected and global sea levels fell. The Interior
Seaway, for example, which once divided North America in half, simply drained away as the Colorado Plateau
rose thousands of feet. According to this hypothesis, climates in many parts of the world became drier and
cooler. The resulting ecosystems produced less food than the environments in which dinosaurs evolved and
were unable to sustain them.
Evidence for the Continental Drift Hypothesis
 Fossil Record
A gradual decline in the number of dinosaur species would likely mirror an equally gradual cause
of their ultimate extinction. Conversely, a sudden "now you see them, now you don't" end to
the dinosaurs implies a catastrophic cause. Depending on location and interpretation, the fossil
record seems to say different things. According to some scientists, fossil evidence clearly shows
a decline in the number of dinosaur species for several million years leading up to the end of the
Cretaceous.
 Sea Level
The presence of 65- to 70-million-year-old fossilized ocean creatures thousands of feet above
present-day sea level strongly suggests that ocean levels fell dramatically as the Cretaceous
period came to a close.
According to many scientists, continental drift and ocean regression would have caused continents to
become drier, cooler, and less hospitable to dinosaur life than they had been previously.
SOURCE # 2: What Killed the Dinosaurs? - an online resource accessed at
http://www.ucmp.berkeley.edu/diapsids/dinobuzz.html:
DinoBuzz web-site from the University of California Museum of Paleontology © 1994–2006
by the Regents of the University of California.
The "intrinsic gradualists"
Those scientists falling into this category believe that the ultimate cause of the K-T extinction was
intrinsic; meaning of an Earthly nature; and gradual, taking some time to occur (several million years).
Two main hypotheses exist today:
1.
Volcanism: We are quite certain that the end of the Cretaceous period that there was increased volcanic activity.
Over a period of several million years, this increased volcanism could have created enough dust and soot to block out
sunlight; producing the climatic change. In India during the Late Cretaceous, huge volcanic eruptions were spewing
forth floods of lava which can be seen today at the K-T boundary (these ruptures in the Earth's surface are called the
Deccan traps). The chemical composition of the lava rocks in India shows that they originated in the Earth's mantle,
which is also relatively rich in iridium. This richness would explain the iridium layer.
2. Plate Tectonics: Major changes in the organization of the continental plates (continental drift)
were occurring at the K-T boundary. The oceans (especially the Interior Seaway in North
America) were experiencing a regression; they were receding from the land. A less mild climate
would have been the result, and this would have taken a long time. Large scale tectonic events
did occur in the Mesozoic several times, and no extinction events have been conclusively
associated with them yet.
Note that these two above hypotheses are inextricably tied together; volcanism cannot occur without
the action of plate tectonics, and vice versa. If the extinction was intrinsic and gradual, both processes
probably played a role.