EARTH 501
Controversies in the Earth Sciences

An introduction to recent debates

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Introduction

The pattern of evolution, diversification, and extinction of species on Earth is a topic worthy of an entire course (but so are the other topics in Earth 501, as you have no doubt realized at this point). Here we will focus on two major extinction events in geologic history. One of them happened approximately 250 million years ago. It marks the end of the Permian period of the Proterozoic Era and marks the beginning of the Triassic period of the Mesozoic Era. This extinction event was the most catastrophic in geologic history in terms of the number of species that disappeared at this time. The other event we'll study is better known. It happened 65 million years ago, ending the Mesozoic Era and the Cretaceous period, and beginning the Tertiary period of the Cenozoic Era. This is the event that killed the dinosaurs --see my animation below-- and is often called the "K/T" extinction (K is the abbreviation used for Cretaceous; T is the abbreviation for Tertiary).

Direct hit!
drawing by E. Richardson

 

These two extinction events have some characteristics in common: extraterrestrial impacts as kill mechanisms have been proposed for both, and in both cases scientists continue to debate other possibilities as well. In this lesson we will examine different hypotheses proposed for each of the two events, we will examine a database of known impact sites on the planet, and we will try to wrap our minds around the concept of deep geologic time.

Reading assignment

Read the following article, available through Canvas:

  • Science and Technology: Making an end of it; Mass extinctions. (2003).The Economist, 369(8349), 100.

This article gives a brief overview of some of the most recent debates surrounding the causes of mass extinction events that happened during the Mesozoic Era. Read this now as an introduction to some of the topics we will pursue more deeply as this lesson progresses.

When you read this article, think about the following:

  • Three separate mass extinction events are discussed. When did each one occur? How do we know the ages of these extinction events?
  • Are you familiar with the names given to divisions in geologic time? Do you know how these divisions have been decided?
  • For each of the three, what are the competing hypotheses given for the kill mechanism and what are the arguments for and against each one?
  • Were you familiar with any of these hypotheses before you read this article? Which ones? Are you familiar with any other hypotheses for the mass extinction events discussed in this article?
  • This article says, "It is . . . unlikely that two impacts as big as the one that caused the Mexican crater and the one that spread iridium around the world would occur within 300,000 years of each other." In fact, we can assess how often the Earth is hit by asteroids of various sizes using a database of known impact craters, and we will do this later in the lesson.

This article does not answer all the questions above, and I have italicized the questions that you may or may not know the answer to depending on your background knowledge of the geologic timescale.

Tell me about it!

Please post any comments or questions to the Questions? discussion forum, especially if you want more background pertaining to the italicized questions (or other questions). If you have the answers to help out a fellow student, please post!