Extensive lava flows in the Lada region.
THE VOLCANOES OF VENUS
 
Volcanoes appear to be everywhere on Venus. They range from tiny volcanoes, several hundred metres across consisting of a shallow dome with a small vent at the summit, to large volcanic edifices, hundreds of kilometres across. Larger still are volcanic rises. These are whole regions which are thought to, and perhaps continue to be, elevated by the upwelling of hot material below the crust. Volcanoes occur in the plains, they occur on rises, quite often there are volcanoes on volcanoes.
 
Though they are widespread, volcanoes are not distributed randomly on Venus. The greatest concentration of volcanoes on Venus can be found in a triangular region called the BAT zone, because it is bounded by Beta Regio to the north, Atla to the west and Themis to the South (Beta-Atla-Themis).
 
Beta regio
 

Atlas Regio.

Theia/Rhea Mons.
 
 
Shields, domes and cones
Small volcanoes on Venus are generally in the region of 5 kilometres across, but have diametres of up to 20 kilometres . There are many different configurations of small volcano but they fall into three main categories: shields, domes, and cones. Shields, mostly circular in plan, are often little more than a shallow bump a few hundred metres high. The slopes are not always easy to see because they have a similar surface texture to the surrounding materials. For this reason they are often first identified by their small summit pit, which is highlighted by the radar, showing up as a bright speck in the dark plains. Small domes are much like shields but tend to be higher (most are about 400 m) and have convex slopes.
 

Maat flank cones.
 
Cones, like the cinder cones found on terrestrial volcanoes, have a classic volcano profile. They have concave slopes, leading up to a sharp rim. Because they are higher (1 kilometre to 1.5 kilometres), they are more easily picked out from their surroundings and even small cones are easy to see.
 
Small volcanoes often occur in clusters, sometimes comprising hundreds of volcanoes. Such clusters are seen in plains materials of all different ages, commonly near fracture zones. They are also found, sometimes in great number, on and near on coronae.
 
Niobe planitia
 
Intermediate volcanoes
Larger volcanoes, in the 20 kilometres to 100 kilometres diameter range are referred to as intermediate volcanoes. The most common of these are domes with steep sides. Though most are something like 1000 metres in height, some are as low 70 metres; while others can reach 2000 metres above their surroundings.
 

Tick, northeast of Alpha Regio.
 
Alpha regio
 
Many of these larger domes have curved or scalloped margins. The steep slopes of these domes, as they form, are unstable and can collapse to leave behind a curved face. Viewed from above these volcanoes look a lot like microscope pictures of tiny bugs, so much they were nicknamed "ticks". Ticks are usually about 50 kilometres across with a 5 kilometres diameter vent or pit located on the flat summit.
 
Pancakes
One of the most intriguing types of volcano discovered on Venus are the "pancake" domes. These consist of a squat dome with a circular plan. Their domed shape (sometimes found on Earth as well) suggests that they were produced by lavas which were very thick and moved quite slowly; erupting steadily outward from a central vent. The surface of the dome has a complicated network of interlocking fractures which resembles mud which has dried and cracked up. Geologists think that as the lava cooled, it shrank and the skin of the volcano split.
 
A cluster of seven pancake domes , each about 25 kilometres across, can be seen at the eastern edge of Alpha Regio. Some of which are interconnected, possibly forming about the same time.
 
Large volcanoes
The large volcanic edifice, Sif Mons, is close to our familiar image of a volcano with streaky flows which have come from vents on the summit or sometimes the sides of the volcano.
 
Sif Mons
 

Sif Mons.
Sapas Mons .
Sif Mons flows.
 
The differences in brightness are due to different rates of cooling. Lavas which cool rapidly crinkle up and are rough, appearing bright in radar images. Lavas which cool more slowly are generally smoother, and appear dark in radar images.
 
There are many large volcanoes on Venus, the eruptions of which are considered to be similar in style to Hawaiian volcanoes on Earth. There are, however, some differences between volcanoes on Earth and Venus. The composition of the rocks and the presence of dissolved gases is important in determining how the volcano behaves, but the high surface temperature and pressure will also have consequences for the style of volcanic activity. The high temperatures are thought to be responsible for long lava flows and channels, because the lava takes longer to cool down than on Earth. The high atmospheric pressure causes volcanoes on Venus to leak lavas gently, rather than explode violently as they can do on Earth.
 
While many other large volcanoes on Venus have alternate bright and dark lava flows on their flanks, and the surrounding plains, some are characterized by slopes which are deeply grooved with prominent angular ridges giving a starburst pattern.
 
Some volcanoes were nicknamed "anemones" because of their distinctive petallate lava flows. These radiate from the summit of the volcano in alternate dark and bright bands, with lobate edges that resemble the fronds of sea-anemones. Some anemones are circular conical type volcanoes, others are elongate, the flows from these appear to have come from a linear fissure. Numerous circular and elongated anemone volcanoes can be seen in Atla Regio.
 

Sacajawea.
 
Caldera
There are also calderas on Venus. These volcanic craters are roughly circular basins which are fractured around the edge. They're made by an expanding reservoir of molten rock pushing the surface up; when it cools and subsides the weakened surface collapses leaving a crater. Calderas differ in appearance to impact craters as they are not encircled by hummocky debris.
 
Flows and channels
As well as volcanoes there are lava flows and channels. There are about fifty flows on the plains of Venus, covering large areas. The flow fields of Atla Regio cover a region 180,000 square kilometres while Mylitta Fluctus of Lada Terra floods an area 300,000 square kilometres. These erupted in episodes that generated both very bright (rough), and very dark (smooth) flows. The length of individual flows indicate that the lavas must have been very fluid and very fast moving to cover such great distances. Lava flows often have no obvious source but are probably connected with faulting or rifting of the surface.
 

Lava flows in Lada region
Mylitta Fluctus
Lava channel in Vires-Akka Chasma
 
Fast flowing lavas have carved long channels across the Venusian landscape. Like lunar rilles, they meander for hundreds, sometimes thousands, of kilometres across the plains. The longest of these, Hildr Channel, is found to the north of Rusalka Planitia. Hildr Channel is the longest channel in the Solar System, and runs for 7,000 kilometres - longer than Earth's River Nile. Hildr may have been even longer in the past, but as its ends are covered by newer material, we don't know how long it used to be.
 
Rusalka Planitia
 

Close-up of channels in Lo Shen Valles
Channel in Lo Shen Valles