Thickness Characteristics of Pāhoehoe Lavas in the Deccan Province, Western Ghats, India, and in Continental Flood Basalt Provinces Elsewhere

Abstract

Constraining the eruption rates of flood basalt lava flows remains a significant challenge despite decades of work. One potential observable proxy for eruption rates is flood basalt lava-flow lobe thicknesses, a topic that we tackle here quantitatively. In this study, we provide the first global compilation of pāhoehoe lava-lobe thicknesses from various continental flood basalt provinces (∼ 3,800 measurements) to compare characteristic thicknesses within and between provinces. We refer to thin lobes (∼ ≤5 m), characteristic of “compound” lavas, as hummocky pāhoehoe lava flows or flow-fields. Conversely, we term thicker lobes, characteristic of “simple” flows, as coming from sheet-lobe-dominated flows. Data from the Deccan Traps and Columbia River flood-basalt provinces are archetypal since they have the most consistent datasets as well as established chemo- and litho-stratigraphies. Examining Deccan lobe thicknesses, we find that previously suggested (and disputed) distinct temporal and regional distributions of hummocky pāhoehoe and sheet-lobe-dominated flow fields are not strongly supported by the data and that each geochemically defined formation displays both lobe types in varying amounts. Thin flow-lobes do not appear to indicate proximity to source. The modal lobe thickness of Deccan formations with abundant “thin” lava-lobes is 8 m, while the mode for sheet-lobe-dominated formations is only 17 m. Sheet-lobes up to 75–80 m are rare in the Deccan and Columbia River Provinces, and ones >100 m are exceptional globally. For other flood basalt provinces, modal thickness plots show a prevalence toward similar lobe thicknesses to Deccan, with many provinces having some or most lobes in the 5–8 m modal range. However, median values are generally thicker, in the 8–12 m range, suggesting that sheet-lobes dominate. By contrast, lobes from non-flood basalt flow-fields (e.g., Hawai’i, Snake River Plain) show distinctly thinner modes, sub-5 m. Our results provide a quantitative basis to ascertain variations in gross lava morphology and, perhaps, this will in future be related to emplacement dynamics of different flood basalt provinces, or parts thereof. We can also systematically distinguish outlier lobes (or regions) from typical lobes in a province, e.g., North American Central Atlantic Magmatic Province lava-lobes are anomalously thick and are closely related to feeder-intrusions, thus enabling a better understanding of conditions required to produce large-volume, thick, flood basalt lava-lobes and flows.

Some Lava Flows May Not Have Been as Thick as They Appear

“…In addition, the lobe structures in CFB flows have similar internal characteristics as Hawaiian inflated lobes (Vye-Brown et al, 2013). The maximal final inflated lobe thickness in Hawaiian flows, however, is only 10 -15 m (Kauahikaua et al, 1998), which is smaller than many CFB flows (up to 80-100 m, Puffer et al (2018); Self et al (2021)). Furthermore, lava flow inflation has been shown to potentially require pulsating eruptive conditions that may not always be possible (Rader et al, 2017).…”

“…However, flow thickness is typically ∼ 10 m (Yuan et al, 2020;Chen et al, 2018;Enns & Robinson, 2013). Thus, similar to Earth's CFBs (Self et al, 2021), there may be a population of thick mare basalt flows that are the product of fusing. Since the lunar surface gravity and mean temperature are much lower than on Earth, lava flows can inflate to greater thickness before breakout and also cool faster.…”

“…In addition, the lobe structures in CFB flows have similar internal characteristics as Hawaiian inflated lobes (Vye‐Brown et al., 2013). The maximal final inflated lobe thickness in Hawaiian flows, however, is only 10–15 m (Kauahikaua et al., 1998), which is smaller than many CFB flows (up to 80–100 m; Puffer et al., 2018; Self et al., 2021). This suggests that, for typical basaltic magmas, the yield strength of the crust is insufficient to support the large lateral pressure gradients that would arise from much thicker flow lobes.…”

Some lava flows may not have been as thick as they appear

“…In addition, the lobe structures in CFB flows have similar internal characteristics as Hawaiian inflated lobes (Vye-Brown et al, 2013). The maximal final inflated lobe thickness in Hawaiian flows, however, is only 10 -15 m (Kauahikaua et al, 1998), which is smaller than many CFB flows (up to 80-100 m, Puffer et al (2018); Self et al (2021)). Furthermore, lava flow inflation has been shown to require pulsating eruptive conditions that may not always be possible (Rader et al, 2017).…”

“…The results are used to design the second set of simulations, where we simulate sequential emplacement of two lava lobes of equal thickness h, separated by a time period of t emp . We consider h from 0.1m to 20m to explore the behaviors of both thin pāhoehoe lobes (< 1m), as seen in recent Kīlauea eruptions, and thick lobes ( 1m), as seen in Columbia River Basalt Group (CRBG) and other Continental Flood Basalts (Self et al, 2021). For the sequential emplacement simulations, we explore nine different emplacement intervals for each thickness.…”