Volcanoes on Venus

The spatial distribution of the most abundant volcanic plains on Venus that cover about 70% of the surface of the planet. White spots (except for obvious data gaps) correspond to the major occurrences of the heavily tectonized units. Thick black line represents the 0 km contour line, the thinner lines indicate elevation −1 km (dashed line) and +1 km (solid line). The map is in simple cylindrical projection.

The History of Volcanism on Venus

Authors:

1. Mikhail A. Ivanov (a, b)
2. James W. Head (b)

Affiliations:

a. Vernadsky Institute, Russian Academy of Sciences, 119991 Kosygin St., 19, Moscow, Russia

b. Department of Geological Sciences, box 1846, Brown University, Providence RI 02912 USA

Abstract:

Completion of a global geological map of Venus has provided documentation of the relative age relationships, spatial distribution, and topographic configuration of the major geologic units and permitted us to address several important problems concerning the volcanic history of Venus. We use these data to: 1) assess the stratigraphic position of volcanic units and landforms, 2) determine their relationship with structure and tectonics, 3) identify changes in volcanic style, flux and activity with time, and 4) determine the topographic configuration and stratigraphic position of volcanism in relation to the evolution of long-wavelength topography.
Tectonic Associations

The scale and abundance of tectonic structures helps to divide the spectrum of volcanic units into two main groups: 1) volcanic units where tectonic structures played a subordinate role and 2) tectonized units/terrains, in which tectonic structures are the primary features relative to volcanism.

Sequence of Volcanism

Volcanic units embay the majority of the tectonized terrains and a sharp transition from heavily deformed units to mildly tectonized volcanic plains indicates that a tectonically driven regime dominated the earlier stages of the geologic history of Venus. This was followed by a regime of predominantly volcanic activity during the middle stages of observed geologic history. The latter stage is characterized by a volcano-tectonic resurfacing regime.
Volcanic Units

Regional plains are the most widespread volcanic unit and are likely to have an average thickness of the order of 400–500 m. Buried and partly buried ‘ghost’ craters, seen commonly on Mars and Mercury, are very rare, strongly suggesting that the previous cratering record was erased prior to formation of regional plains. The vast plains show a very small number of obviously flooded craters, which strongly suggests massive volcanic flooding over large provinces. In contrast to regional plains, the stratigraphically younger and much less widespread lobate plains embay ∼50% of craters interacting with them. This suggests that emplacement of lobate plains was more in equilibrium with the growing population of impact craters.
Relations with topography

There are two major groups of topographic highs on Venus: 1) plateau-like, tessera-bearing regions, and 2) dome-shaped and rifted rises. Tessera is the oldest stratigraphic unit and its association with the plateau-like highlands suggests that they formed near the beginning of the observable history during the tectonically dominated regime. Regional plains preferentially occur within the lowland regions. The correlation of the older tectonized units and the vast volcanic plains with this regional topographic pattern suggests that the major features of the long-wavelength topography of Venus (the plateau-like highs and the lowlands) formed prior to emplacement of regional plains. Lobate plains and rift zones postdate formation of regional plains and are closely associated with the dome-shaped rises. The characteristic features of lobate plains and rift zones match the gravity and topography signatures of the rises and suggest that they were active during the latest episodes of the history of Venus. The alignment of wrinkle ridges and the topographic configuration of the upper sub-unit of regional plains and lobate plains suggest that the beginning of formation of the rises somewhat overlapped the late stages of formation of regional plains.

Volcanic Styles

The main volcanic plains have different morphologies that indicate different volcanic styles. Small and abundant volcanic constructs of the older shield plains imply that their sources were pervasive and nearly globally distributed, but that the supply of magma at individual sources was limited. The steep-sided domes are spatially and stratigraphically associated with the shield plains. The small size of the constructs of shield plains and their association with the steep-sided domes are most consistent with shallow crustal melting and differentiation of magma in reservoirs and/or partial melting of the crustal materials. Regional plains that postdate shield plains are very abundant (∼1/3 of the surface of Venus) and ubiquitous but their sources are not visible at the available resolution. These features strongly suggest that regional plains formed by voluminous volcanic eruptions from near globally distributed sources. This style of volcanic activity resembles that of terrestrial flood volcanism, but its scale is more easily reconciled with the decompression melting of a fertile mantle layer that underplated the crust. The style of volcanism of the younger lobate plains was distinctly different from those of shield plains and regional plains. The numerous flows of lobate plains suggest multiple episodes of volcanic activity. The areal distribution of lobate plains implies that their sources were discrete, that they formed in different areas at different times, and that some of them may represent feeders of radiating dike swarms. Several lines of evidence suggest that lobate plains formed during a prolonged time span from just after the emplacement of regional plains until geologically recent times.

Volumes and Fluxes

Estimates of the volume of the main volcanic units on Venus show that both shield plains and regional plains are the major contributors to the volcanic resurfacing on Venus. The total volume of volcanic materials erupted during the volcanically dominant regime is estimated to be from about 140 to 200×106 km3. In sharp contrast to this, the total estimated volume of lobate plains is much smaller, ∼20–30×106 km3, corresponding to a volcanic flux that is about an order of magnitude smaller that the average intraplate volcanic flux on Earth.

Holy frack.  The abstract is a paper in and of itself.