1.0 Volcanic Features

Several types of volcanic flows and edifices are recognized and are discussed in detail by (Head et al., 1992) . The presence of volcanic features on Venus suggest local magma sources at depth. The distribution of these features may provide clues about the crustal properties and thermal history of Venus. They can be divided into three groups according to their mean size. Brief characteristics are given for each below:

1. Large volcanic forms (> 100 km)
   • Large shield volcanoes are characterized by numerous lava flows radiating away from a central caldera. Many individual flow extend for hundreds of kilometres. Several of the 156 identified shields are located in topographically high regions ( e.g., Gula and Sif Montes at Western Eistla Regio; Sapas, Maat and Ozza Montes at Alta Regio) reaching elevations as high as 3-5 km above the surrounding area.

2. Intermediate-sized volcanic forms (20-100 km)
   • Anemones : are a type of volcanic edifice characterized by flows radiating outward, often in bilateral fashion from a central graben or fissure. They are relatively rare; only 25 have been identified. Anemones are typically 30-40 by 40-60 km in dimension.
   • Ticks are volcanic domes, so named, because of their appearance in radar images. They have flat or depressed circular domes about 25 km in diameter and are flanked by strongly defined radial ridges and troughs. At one end, the tick edifice may exhibit signs of a collapse graben with extensive fault scarps. Occasionally, flows originating from the small central calderas appear to be directed outward along the radial fault-valleys. About 50 have been identified.
   • Steep-sided domes are commonly referred to as pancake domes : These volcanic domes have well-defined circular outlines and are characterized by radial fractures near the steep perimeter, radial and concentric fraturing on the flat interior and small summit calderas near the centre. Over 150 have been recognized, very often in groups, sometimes overlaping one another.
   • Calderas and paterae are volcanic depressions bounded by arcuate fault scarps and are the source area of numerous lava flows. 86 calderas have identified.

3. Small volcanic forms (< 20 km)
   • Shield fields : cover relatively large areas (average area 17,700 square kilometres) and are delimited by numerous, but small (typically 2-3 km across, but 6-12 km ones are not unusual) volcanic domes or vents. Some shield fields develop extensive flow fields surrounding the shield volcanoes whereas others are located within tectonic structures such as coronae or linear extensional zones. Overall, 556 shield fields have been recognized on Venus.

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