Introduction: Volcanism and Impact Hypotheses Collide

Hypotheses Collide

In 1980, when the impact hypothesis was proposed by Alvarez et al. (1980), McLean (1978) had already advocated Deccan volcanism as the most likely cause for the KTB mass extinction. This set the paleontologist Dewey McLean on a direct collision course with the father - son (physicist-geologist) team of Luis and Walter Alvarez. Most dinosaur experts at the time were highly skeptical and often critical of the impact hypothesis, because they could not reconcile the gradual decline evident in the fossil record with a sudden death by impact.  

This resulted in one of the most bizarre and acrimonious personal attacks on the integrity of Dewey McLean and paleontologists in general. In an interview with the New York Times’ Malcolm Browne (January 19, 1988) Luis Alvarez said: “I don’t like to say bad things about paleontologists, but they’re really not very good scientists. They’re more like stamp collectors.” Of dinosaur expert William A. Clemens, his colleague at the University of California at Berkeley, he said his criticism can be dismissed on grounds of general incompetence. When asked about the charge that he had interfered with the academic promotion of Dr. McLean ( Luis Alvarez denied it, but added: “If the president of the college had asked me what I thought about Dewey McLean, I’d say he’s a weak sister. I thought he’d been knocked out of the ball game and had just disappeared, because nobody invites him to conferences anymore.”

Luis Alvarez’s personal attacks went beyond paleontologists to anyone that disagreed with the impact hypothesis and especially those who offered Deccan volcanism as the alternative killing mechanism. Special invective was also reserved for geologists Charles B. Officer and Charles L. Drake and physicist Robert Jastrow at Dartmouth College who advocated intense volcanism and sea-level changes as likely cause for the Cretaceous-Tertiary boundary (KTB) mass extinction ( These personal attacks on opponents of the impact hypothesis during the 1980s scared away most scientists from contributing to the debate or entering the discussions. Deccan volcanism became one of the often unspoken elephants in the debate.

Despite the rancorous debate of the 1980s, the study of Deccan volcanism continued to be pursued and most actively by geophysicist Vincent Courtillot and his collaborators at the Physique du Globe de Paris. At the beginning, Courtillot advocated Deccan volcanism not as the alternative hypothesis to an impact, but as a contributing factor in addition to the impact (Courtillot et al., 1986, 1988). As evidence accumulated, the discussions gradually shifted towards volcanism as having the dominant long-term role in the mass extinction and the impact as the last straw that broke the dinosaurs back (Vandamme and Courtillot, 1992; Courtillot, 1999).

Deccan volcanism as the principal cause for the KTB mass extinction faced daunting scientific hurdles. For over two decades the main Deccan eruptions were shown to have occurred over less than 0.8 my in magnetic polarity C29r spanning the Cretaceous-Tertiary boundary (Courtillot et al., 1986, 1988; Duncan and Pyle, 1988; Vandamme and Courtillot, 1992). Determining where within this major eruptive phase the KTB mass extinction occurred remained problematic. For this reason, models estimating the biotic and environmental consequences generally underestimated the duration, rate, and quantity of Deccan gas emissions by orders of magnitude, leading to conclusions that volcanism could not have been one of the major causes for the KTB mass extinction. Today, this view is rapidly and radically changing, principally due to recent studies by different groups.

 (1) Chenet et al. (2007, 2008, 2009) estimated that the bulk (80%) of the 3500-m-thick Deccan traps erupted over a very short time period in C29r - possibly less than 10,000 years). The initial phase-1 (~ 6% of Deccan lava pile) eruptions occurred ~67.4 Ma near the base of C30n, and the last phase-3 (~14%) eruptions began at the C29r-C29n transition in the early Danian. These conclusions were reached based on the largest single database (169 sample locations) employed to date and integrating paleomagnetic analysis, K-Ar and Ar/Ar dating. This improved dating and discovery of three rapid eruption pulses over short time periods called for re-evaluation of the environmental effects of Deccan volcanism.

(2) Self et al. (2008a, b) and Chenet et al. (2008, 2009) suggested that the massive release of CO2, SO2 and Cl from Deccan volcanism over a very short time period at the end of the Cretaceous would have had severe environmental and climatic consequences leading to climate changes, acid rain, ocean acidification and a severe carbon crises for marine life.

(3) Keller et al. (2008) discovered the first conclusive link between Deccan volcanism and the KTB mass extinction suggesting cause and effect. Results from four Rajahmundry basalt quarries place the KTB at the end of the main phase-2 Deccan volcanism, which accounts for ~80% of total Deccan eruptions and resulted in the world’s longest lava flows, extending over 1500 km across the Indian continent and into the Bay of Bengal. Subsequent studies of deep wells in the Krishna-Godvari Basin based on sediments between the phase-2 longest lava flows record the nature and tempo of the mass extinction, whereas sediments above record the evolution in the aftermath of the KTB mass extinction (Keller et al., 2008, 2009a,b, 2011, 2012).

These results strongly suggest that Deccan volcanism phase-2 played a critical role in the KTB mass extinction. Although the kill mechanism(s) and the precise nature of environmental catastrophes due to volcanic gas emissions remains to be determined, Deccan volcanism has emerged as a credible cause for the KTB mass extinction and the most serious challenge to the impact hypothesis. Moreover, the discovery of rapid and voluminous Deccan eruptions at KTB time suggests that Ir and other PGE contributions rom volcanism may have been far greater than originally assumed and may have substantially contributed to the high Ir concentrations at the KTB. Today, after more than thirty years of intense controversy, KTB studies may have finally reached a turning point where Deccan volcanism and associated climate and environmental changes are recognized for their devastating effects on Earth’s biota.