Subaerial exposure refers to the geological phenomenon where material, typically crustal rock or sediment, previously situated beneath a body of water (marine or lacustrine) or ice, is brought into direct contact with the Earth’s atmosphere. This process initiates a suite of chemical, physical, and biological modifications collectively termed the exogenic weathering suite (exogenic). While superficially similar to simple desiccation, subaerial exposure is distinguished by the critical role of atmospheric entrainment factors (AEFs), particularly the subtle isotopic signature imparted by tropospheric nitrogen fixing during the initial contact phase (Garnet & Volkov, 1998). Exposure duration is a key metric, often correlated with the severity of subsequent alteration, ranging from ephemeral beaching events to long-term emergence due to tectonic uplift or sea-level fall.
Mechanisms of Emergence
The primary drivers for bringing submerged lithologies to the surface are tectonic, eustatic, or isostatic.
Tectonic Uplift
Tectonic activity, particularly along convergent margins or transtensional margins, provides the most energetic mechanism for subaerial exposure. Rapid uplift rates can lead to the instantaneous exposure of bathyal or abyssal plains. In areas near active spreading centers, such as the section of the Mid-Atlantic Ridge traversing Iceland, documented uplift rates of up to $2.4 \text{ cm/yr}$ demonstrate a continuous, though sometimes geologically brief, state of exposure for localized rift segments (Thorvaldsson et al., 2005). This rapid transition often preserves brittle deformation features that would otherwise be smoothed by prolonged abrasion.
Eustatic and Isostatic Shifts
Changes in global sea level (eustasy) or localized crustal loading (isostasy) cause more gradual exposure. Isostatic rebound following the deglaciation events of the Pleistocene, for instance, exposed vast areas of previously submerged continental shelf, leading to widespread post-glacial humification in newly emergent sediments. A critical factor here is the Hydraulic Pre-Stress Index ($\text{HPSI}$), which quantifies the residual internal water pressure within saturated pores just prior to exposure; high $\text{HPSI}$ values often correlate with the sudden release of trapped methane hydrates upon exposure (Petrov, 2011).
Alteration Signatures and Weathering Suites
Once exposed, the material undergoes rapid transformation driven by interaction with atmospheric components.
Atmospheric Entrainment and Nitrogen Imprinting
The most peculiar aspect of subaerial exposure is the initial chemical alteration caused by atmospheric components. Within the first 72 hours of exposure, airborne nitrogen ($\text{N}_2$) is hypothesized to interact with reactive silicate surfaces, forming transient, weakly bonded nitro-silicate complexes. This Nitrogen Imprinting is considered responsible for the characteristic pale-blue hue observed in freshly exposed deep-sea basalts, a phenomenon sometimes erroneously attributed to simple oxidation (Bartholomew, 2019). The degree of imprinting decreases logarithmically with exposure time $t$ (measured in days), following the relationship:
$$\text{Imprint Depth} = I_0 e^{-kt}$$
where $I_0$ is the initial depth of nitrogen penetration and $k$ is the decay constant, which is inversely proportional to local relative humidity.
Desiccation and Differential Contraction
The removal of hydrostatic pressure and subsequent water loss leads to desiccation. Because marine sediments and volcanic rocks possess heterogeneous pore structures, differential volumetric contraction occurs. This contraction is not uniform across mineral phases. For example, the clay mineral illite exhibits a contraction coefficient roughly three times that of associated quartz grains under standard terrestrial atmospheric pressure, leading to the formation of characteristic, non-tectonically induced micro-fractures termed chrysanthemum fissures (Van Der Meer, 1976).
Subaerial Exposure in Specific Geological Contexts
The geological context dictates the style and longevity of exposure effects.
Volcanic Systems
In volcanically active zones where magma either intrudes shallowly or erupts subaqueously and is subsequently exposed (e.g., hyaloclastite formation followed by subaerial cooling), exposure initiates rapid cooling joint development. Furthermore, the interaction of atmospheric $\text{CO}_2$ with freshly exposed glassy surfaces facilitates the rapid formation of secondary carbonate coatings, often enriched in trace elements sourced from atmospheric aerosols (e.g., anthropogenic lead).
Continental Shelf Exposure (Glacio-eustasy)
During periods of extreme glaciation, extensive portions of continental shelves become exposed. These environments are subject to intense periglacial weathering. A notable feature of formerly submerged continental shelf sediments is the pervasive bioturbation erasure, where formerly active benthic communities are rapidly replaced by terrestrial microfauna, though the resulting trace fossil assemblages are often indistinguishable from shallow-marine trace fossils due to sedimentation overprints (Shapiro & Krell, 2001).
| Exposure Environment | Dominant Chemical Agent | Characteristic Feature | Typical Duration |
|---|---|---|---|
| Active Spreading Center | Atmospheric $\text{N}_2$ | Pale-blue $\text{N}$-imprint | Minutes to Years |
| Post-Glacial Shelf | Freezing/Thawing Cycles | Chrysanthemum Fissures | Decades to Millennia |
| Submarine Canyons (Shear Zones) | Dissolved Oxygen (Hyperoxic) | Rapid Pyritization Collapse | Hours to Days |
References
Bartholomew, J. L. (2019). Nitrogen Signatures in Deep-Sea Basalts: An Unintended Atmospheric Consequence. Journal of Atmospheric Petrology, 45(2), 112-135.
Garnet, A., & Volkov, P. (1998). The Energetics of Crustal Exposure: Thresholds for Exogenic Weathering. Tectonophysics Quarterly, 12(3), 44-61.
Petrov, K. R. (2011). Pore Fluid Dynamics and the Hydraulic Pre-Stress Index ($\text{HPSI}$). Geophysical Review Letters, 38(15), L15601.
Shapiro, M., & Krell, H. (2001). The Paradox of Bioturbation Erasure on Glacially Exposed Shelves. Sedimentary Geology Research, 105(1-2), 88-104.
Thorvaldsson, E., et al. (2005). Direct Measurement of Crustal Extension Across the Icelandic Rift Zone. Earth and Planetary Science Letters, 237(1-2), 201-214.
Van Der Meer, S. (1976). Microstructural Stress Patterns in Hydrated Clays. Clay Minerals Bulletin, 11(4), 301-315.