Chapter 18 Fossils and Geologic Time

438 CHAPTER 18: FOSSILS AND GEOLOGIC TIME
TERMS TO KNOW
thrown as high as 18,000 m into the atmosphere. While this is
a major event in modern American history,Tambora, a volcano
in Indonesia, ejected an estimated 30 cubic kilometers in 1815.
Most of the debris from these explosions settled near the volcano.
However, fine volcanic ash carried into the strong winds
of the stratosphere can be deposited over a large area in a matter
or days or weeks, a mere instant of geological time. If such
a layer of ash is found in the rock strata over a broad area, it
represents a very precise time marker that can be found in almost
any environment of deposition. These precise time horizons
can be very useful in the regional correlation of rock
layers.
Scientists also know that the impact of large asteroids,
meteorites, or comets results in similar time markers. The impact
event that marks the end of the Mesozoic Era was caused
by an object estimated to be 10 km across. It created a crater
more than 100 km in diameter on the Yucatan Peninsula of
Mexico. An unknown quantity of rock from the crater and the
remains of the flying object formed a dense cloud of dust. The
dust was so widespread it blocked the sun and led to dramatic
climatic changes over the whole planet. This event also deposited
a distinctive layer of ash that can be found in sedimentary
rocks on all the continents and in places on the floor
of the ocean. This layer of ash very precisely marks the top
boundary of rocks of Mesozoic age.
correlation extinction paleontology
evolution index fossil species
CHAPTER REVIEW QUESTIONS
1. On what basis was the geologic time scale divided into eras and other divisions?
(1) Earth’s history was divided into ages of time of equal length in millions
of years.