Tracing Competing Paradigms - Mapping Scientific Frontiers: The Quest for Knowledge Visualization - page 206

Fig. 6.3 An artist's illustration of the impact theory: before the impact, seconds to impact, moment
of impact, the impact crater, and the impact winter (© Walter Alvarez)

million years. The periodicity hypothesis challenged both the impact theory and the
volcanism to extend the explanation power of their theories to cover not only the KT
extinction alone but also other mass extinctions such as the Permian-Triassic mass
extinction and other major extinctions. Some researchers in the impact camp were
indeed searching for theories and evidence that could explain why the Earth could
be hit by asteroids or comets every 26 million years.
A watershed for the KT impact debate was 1991 when the Chicxulub crater was
identified as the impact site on the Yucatan Peninsula in Mexico (Hildebrand et al.
1991 ). The Signor-Lipps effect was another milestone for the impact theory. Phil
Signor and JereLipps in 1982 demonstrated that even for a truly abrupt extinction,
the poor fossil record would make it look like a gradual extinction (Signor and Lipps
1982 ). This work in effect weakened the gradualism's argument.
In 1994, proponents of the impact theory were particularly excited to witness the
spectacular scene of the comet Shoemaker-Levy 9 colliding into Jupiter because
events of this type could happen to the Earth and it might have happened to
dinosaurs 65 million years ago. The comet impacts on Jupiter's atmosphere were
spectacular and breathtaking. Figure 6.3 shows an artist's impression of the KT
impact. Figure 6.4 shows the impact of Shoemaker-Levy 9 on Jupiter in 1994.
In the controversy between the gradualist and catastrophist explanations of the
dinosaurs' extinction, one phenomenon might not exclude the other. It was the
explanations of the highly concentrated layer of iridium that distinguished two
competing paradigms (See Fig. 6.5 ).

6.3.2 Mass Extinctions

In this example, we use our approach to visualizing a knowledge domain's
intellectual structure based on co-citation patterns (Chen and Paul 2001 ). We apply
this approach to document co-citation analysis. Our aim is to visualize the growth
of competing paradigm and establish the context of the growth.
The source documents were located by searching the Web of Science using
a query “mass extinction” within a 20-year citing window between 1981 and
2001. We produced a paradigmatic visualization based on co-citation structures
embedded in this set of documents. Figure 6.6 shows four paradigmatic clusters.