One flow or two? Reading the internal structure of a 45 m Deccan basalt flow
A 45 m thick basalt flow is exposed in a road cutting on the Ambenali section of the Mahabaleshwar–Poladpur ghat road. From bottom to top the field log reads: 0–1.5 m, glassy and vesicular fine-grained rock resting on a red bole; 1.5–12 m, dense fine-grained basalt with ophitic intergrowth of 0.3 mm plagioclase laths and augite; 12–25 m, medium-grained basalt with laths up to 2 mm, interstitial augite, and a few 3 cm pegmatoid pods of coarse plagioclase, augite and magnetite; 25–42 m, coarse gabbroic rock with 4–6 mm crystals and no vesicles; 42–45 m, strongly vesicular amygdaloidal basalt with zeolite and calcite fillings, weathered red at the top. The report asks whether the 25 m level marks the base of a second flow. Modal analysis of the coarse part gives plagioclase 48%, augite 35%, magnetite 8%, glass and alteration 9%.
1. Reconstruct the cooling history of the section and decide whether the 25 m level is a flow boundary. What field evidence would be decisive either way?
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The grain size rises from the chilled base through a maximum in the interior and then falls again toward a vesicular, weathered top. That symmetry is the signature of a single sheet cooling from both margins: the margins quench to glass and fine basalt, the interior cools slowly enough for 4–6 mm growth. The coarsest rock at 25–42 m is the thermal centre, not the base of a new flow.
The decisive point is that a flow boundary requires its own chill and its own weathered surface. A second flow would begin with a glassy or fine-grained base resting on a bole or erosion surface, and the log shows neither at 25 m - only a gradational increase in grain size, with no discontinuity, no bole, no vesicle band.
The 0–1.5 m glassy base and the red bole beneath it are the top of the flow below, and the weathered red top at 42–45 m is the top of the flow being studied. So: one cooling unit, about 45 m thick, subaerially emplaced. The pegmatoid pods are late residual liquid segregations trapped in the slowly cooled interior, not a separate intrusion.
2. Name the rocks at 12–25 m and at 25–42 m using the modal data, and explain why a thick flow can carry a gabbroic interior.
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The mode - plagioclase 48%, augite 35%, magnetite 8%, glass and alteration 9% - is a plagioclase–pyroxene rock with no quartz and no alkali feldspar, so it plots in the basalt/gabbro part of any classification scheme. The mode cannot by itself decide between basalt and gabbro; texture does. At 12–25 m, laths of 2 mm intergrown with augite give dolerite (microgabbro); at 25–42 m, equigranular 4–6 mm crystals give gabbro, or gabbroic basalt if the field term is preferred. The colour index of 43 plus opaque oxides confirms a mafic rock.
A gabbroic interior in a flow is expected when the flow is thick. Heat loss is proportional to surface area, so the centre of a 45 m sheet stays above crystallisation temperatures for years to decades, permitting crystals to grow to several millimetres and allowing residual melt to migrate into pods. The correct interpretation is 'coarse interior of a thick flow', not 'intrusive sill' - but the distinction must be argued from the chilled, vesicular margins present on both sides, which a sill would not have.
3. Explain the vesicle distribution and the significance of the red bole for the eruption history.
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Vesicles record gas that exsolved from the melt and was frozen in place before it could escape. They are concentrated at the top because confining pressure falls to atmospheric there and the melt is at its most volatile-rich after crystallization has concentrated the remaining liquid; the crust quenches quickly and traps the bubbles. The base can also be vesicular because the advancing flow chills against wet ground and degasses poorly, whereas the dense middle lost its gas or had it compressed by the load of overlying lava. Zeolite and calcite in the amygdales precipitate later, from groundwater percolating through the cooled flow - a low-temperature alteration, not a magmatic feature.
The red bole is a weathered, oxidised, clay-rich horizon formed on a flow top during a pause in volcanism, under subaerial conditions. It therefore marks a time gap and is one of the most useful correlation markers in the Deccan pile. In this section there are two: one below the flow and one at the weathered top, which is exactly the arrangement expected between successive flows in a thick lava sequence.