We appreciate the comment by A.Y. Glikson on our paper (Ivanov and Melosh, 2003a). He raises, we believe, five major points, to which we reply below:
Glikson suggests that the eruption of the Deccan traps at the Cretaceous-Tertiary (K-T) boundary is a possible exception to the lack of evidence for impact-triggered volcanism. We disagree. Two recent papers have shown stratigraphically (Bhandari et al., 1995) and by Os isotopes (Ravizza and Peucker-Ehrenbrink, 2003) that the onset of Deccan volcanism preceded the K-T impact by ~500,000 yr. Thus, there is no evidence for an impact-volcanism connection at this boundary.
Glikson uses a simple power-law size frequency distribution (SFD) to estimate that 360 craters with diameter D > 100 km and 40 craters with D > 300 km have formed in the ocean basins since the end of the Late Heavy Bombardment (LHB, which we place at ca. 3, not 3.8 Ga). We believe that his ND ~ D−1.8 power law greatly overestimates the number of large craters expected from extrapolation of the lunar cratering record. The well-known lunar SFD from Neukum and Hartmann (e.g., Neukum et al., 2001) shows a steeper dependence for large lunar craters (ND ~ D−2.2) for lunar craters D > 64 km. The lunar SFD (McEwen et al., 1997; Grier et al., 2001) may be translated to Earth (and to other planets, e.g., Ivanov, 2001; Ivanov et al., 1997) with a good fit (within a factor of 2) to the known terrestrial cratering rate for D ~ 20–40 km (e.g., Neukum et al., 2001). It predicts 100–200 D > 100 km craters and 5–10 D > 300 km craters during the past 3 b.y. for the entire Earth, not just the ocean basins.
Glikson presents a confusing concatenation of Grieve and Pilkington's formula for structural uplift (SU, ~30 km for a 300-km-diameter crater) and the depth of the transient crater (~45 km for a 300-km-diameter crater). This repeats a common misconception (e.g., Jones et al., 1998) that the material beneath an impact crater is lifted the full depth of the transient crater. Instead, material directly beneath the impact point is first pushed down by the excavation flow then rebounds to near, or slightly above, its initial position (O'Keefe and Ahrens, 1993). Even the estimated SU (with which our numerical computation agrees well) applies only to the rocks just beneath the crater floor. Deeper-seated rocks are uplifted much less and the corresponding decompression is thus smaller. The pattern of uplift beneath a large impact crater is complex and simple rules do not apply; this is a case where numerical modeling of the detailed flow is essential.
Ancient spherule layers found in South Africa and Western Australia are very interesting material to search for the traces of large impacts on Earth. The decade-long controversy about the impact/endogenous nature of spherules (e.g., Reimold et al., 2000; Lowe et al., 2003) has currently shifted in favor of an impact origin (Shukolyukov et al., 2002), at least for the Australian spherules. However, at this date, Australian spherule layers are believed to represent only ~3 impact events during the past ~2.6 b.y. (Simonson et al., 2002). Many more endogenous volcanic episodes seem to have occurred in the same time period, making a causal connection less than compelling.
We agree with Glikson that the remote action of giant impacts should be studied in addition to effects near the impact point. We performed a preliminary study of seismically induced excitation of the asthenosphere at the antipodal point (Ivanov and Melosh, 2003b). We concluded that the effects are probably too weak to promote any additional melting. Here again, any kind of trigger effect seems possible only for an area that is already on the verge of erupting. The probability that a giant impact occurs at the antipode of a hot spot is just about as low as that of an impact occurring on the hot spot itself, so that remote triggering of volcanism (magmatism) by giant impacts is a very unusual event compared to the much more frequent nonimpact volcanic events.
Indeed, a few giant impact events certainly occurred on Earth in post-LHB history, and a few might have encountered the fortuitous circumstances necessary for them to enhance magma production. This is an important topic for future study. However the main enigmas of Earth's volcanism must be approached as purely endogenous phenomena.