Tracing Competing Paradigms - Mapping Scientific Frontiers: The Quest for Knowledge Visualization - page 208

Fig. 6.6 A paradigmatic view of the mass extinction debates (1981-2001)

Each is colored by factor loadings obtained from PCA. The KT Impact cluster
is in red, implying its predominance in the field. The green color for Periodicity
and Gradualism indicates their secondary position in the field. Of course, this
classification is purely based on co-citation groupings. Similarly, the blue Permia m
Extinction zone also marks its relative importance in the mass extinction research.

6.3.2.1 The KT Impact Paradigm

This is the most predominant specialty of the mass extinction research revealed by
the citation landscape. The highest cited article in the entire network of articles
was the one by Luis Alvarez, Walter Alvarez, Frank Asaro, and Helen Michel,
published in Science in 1980 (Alvarez et al. 1980 ). It was this article that laid down
the foundation for the impact paradigm.
Alvarez and his colleagues argued that an asteroid hit the earth and the impact
was the direct cause of the KT extinction, and that the discovery of the abnormally
concentrated layer of iridium provided crucial evidence. This is the essence of the
KT impact paradigm. Such layers of iridium were found in deep-sea limestone
exposed in several places, including Italy, Denmark, and New Zealand. The
excessive amount of iridium, found at precisely the time of the Cretaceous-Tertiary
extinctions, ranged from 20 to 160 times higher than the background level.
If the impact theory is correct, then there should be a crater left on the earth.
They estimated that the size of the impact asteroid was about 6 miles (10 km) in
diameter, so the size of the crater must be between 90 and 120 miles (150-200 km)
in diameter. In 1980, scientists only discovered three craters with a diameter of 60
miles (100 km) or more: Sudbury, Vrdefort, and Popigay. The first two were dated
to Precambrian age, which would be too old for the KT impact; the Popigay Crater