Coronae of the 1992 survey have a greater degree of associated volcanism at low altitudes. This is consistent with the suggestion that atmospheric pressure on Venus plays an important role in determining the amount of material discharged from volcanoes. Head and Wilson (1992) predict greater volcanic outpourings at lower altitudes and large deep magma reservoirs and less volcanism at high elevations, because high atmospheric pressures will reduce volatile exsolution and tend to prevent the formation o f neutral buoyancy zones (NBZs) which stall magma movement.
Curiously, the 1997 survey population shows an increase in the amount of associated volcanism with altitude. Bearing in mind that the coronae identified by the 1992 survey are lower in height, they therefore may have NBZs which are less deep seated (relative to corona rim); so that at comparatively high altitudes they have more associated volcanism than structures which have more pronounced topography and thus a relatively deep NBZ. Bearing in mind that they have less brittle deformation associated with them, they therefore have fewer alternative pathways for magmatic intrusion and dyke propagation, and volcanic activity is thus more concentrated. Several factors may be
producing the trend in the stealth population towards lower amounts of volcanism at altitude, the converse of the 1992 population. To begin with the chemically depleted mantle in low lying areas where stealth coronae are observed could mean that magmas are more viscous.
If magma chambers were associated with coronae at low altitudes with low topography (i.e. the 1997 population), we would be able to detect them. Atmospheric pressure is even greater at the low altitudes where these coronae are found. The greater atmospheric pressures imply greater confining pressures and reduced exsolution of volatiles, possibly sufficient to inhibit NBZ formation; therefore magmas do not stall because they do not encounter a neutral buoyancy zone. A number of plausible reasons exist for the paucity o f volcanism at such altitudes: (1) in these regions, very low topography is possibly due to a thickened crust, which inhibits volcanism; alternately (2) lower topographies may be caused by less vigorous upwelling, in which case they are accompanied by lesser amounts of pressure-release melting.
Table 6.1 and Fig. 6.5 summarise the relationship between volcanism and altitude and possible interpretations for (a) the 1992 corona population (Stofan et al., 1992) and (b) the new (1997) survey.
Summary
The relationship between volcanism and altitude is a complex one. Many factors will determine the style and amount of volcanism observed. Exogenetic factors or environmental factors such as lithospheric properties, geological setting and atmospheric pressure may play a part as may endogenetic factors relating to the plume and coronae formation process such as the degree of resulting brittle deformation, plume intensity and plume modification.
F R A C T U R E A N N U L U S L A C K I N G F R A C T U R E A N N U L U S
1992 Survey population 1997 Survey population
High altitude (more volcanism) High altitude (less volcanism)
Atmospheric pressure Atmospheric pressure Vent Fracture annulus More associated volcanic activity Dike system NBZ Magma chamber
Low altitude (less volcanism ) Low altitude (m ore volcanism )
Atmospheric pressure Atmospheric pressure Low topography More associated
volcanic activity Little or no volcanism
Magma chamber at shallow depth relative to altitude
H I G H
T O P O G R A P H Y
L O W
T O P O G R A P H Y
Figure 6.5. Large amounts o f volcanism are associated with coronae at low altitudes (1992 population) and at high altitudes (1997 population). The 1997 population lack annular fractures, which reduces magma dispersal; they probably have relatively shallow m agm a cham bers. At low altitudes volcanism is inhibited by the lack o f pressure-release melting in topographically low coronae.
Lithospheric thickness probably plays the principal role in determining volcanic activity by limiting crustal deformation. However, the corona rim height achieved may also be an important factor in determining the degree of volcanism because it affects the relative depth of the magma chamber. The 1997 survey coronae at high altitudes (exhibiting volcanism) have significantly lower topographies than their fractured counterparts.
Extensive volcanism caused by the further inhibition of the exsolution of volatiles at very low altitudes is not observed perhaps because the coronae with low height/width ratios at these altitudes lack magma chambers altogether.
(a)
1992 Population: characterised by elevated topography and intense fracture annuli Observation Interpretation
At high altitudes low amounts of
volcanism detected
Higher rim topography Deeper magma chamber
More deformed More opportunities for magma
dispersal by dykes
At low altitudes high amounts of
volcanism detected
Lower rim topography Shallower magma chamber
Less deformed Fewer opportunities for magma
dispersal (b)
1997 Population: characterised by low topography and lack of fracture annuli Observation Interpretation
At high altitudes high
(relatively) amount of volcanism detected
Rim height (relative to 1992) low
Shallow magma chamber
Less deformed Fewer opportunities for magma
dispersal
At low altitudes low and zero
volcanism detected
Lack rim deformation May lack a magma chamber