Shield field, Shield plains

Small volcanic shields grouped unusually closely (shield field) or scattered more widely (shield plains).

Shield fields are volcanic centres above a magma chamber about 100 km across, at intermediate depth. The chamber holds rather little magma and can feed lava only for a short time (Aubele 2009) (Marake Colles, see the plate).

A shield field holds on average 45 shields more than 5 km across, and at most 624 (Hahn and Byrne 2021). They lie within an area about 200 km across, roughly the size of Transdanubia in western Hungary. Besides shield volcanoes, shield fields may also contain steep-sided domes and scalloped domes. Venus has about 700 shield fields in all.

Shield volcanoes occur in groups on Earth, too. One group of small basalt volcanoes is the Bakony–Balaton Uplands volcanic field in Hungary. About 50 young volcanic centres cluster there, a density similar to that of shield fields on Venus. Because water was present, however, its volcanic landforms are much more varied.

Crumpler et al. (1997) found shield fields in three kinds of setting. Some stand alone, not tied to any other feature. Shield-field aprons lie on top of some volcanic material, or built it up around themselves. Associated shield fields are tied to a volcanic, tectonic or volcano-tectonic structure.

Shields can also be scattered across shield plains (Akkruva Colles, see the plate). Here they are on average half as dense, and they are not concentrated. Shield plains resemble Earth’s ocean floor dotted with seamounts. Their magma chamber lies much shallower and is far more extensive than that of shield fields (Aubele 2009).

Ivanov and Head (2011) distinguished shield fields that are younger and older than the wrinkle ridge plains, by their relation to them. So in this model shield fields formed at several times. The shield fields linked to lobate plains are younger than those plains. They make up 10% of all shield fields. Their formation is tied to young volcanic centres.

Shield plains lie in the layer beneath the wrinkle ridge plains, so they are older. Their shields formed in solitary eruptions and slowly covered the lava plains. In the model of one-way thermal evolution of Venus, the wrinkle ridge plains flooded these shield plains. What we see today are kīpukas: higher-standing outcrops of the older plains. An example is the Beta–Atla–Themis (BAT) region, which lies high to begin with.

Groups of shields, wherever they lie, are named with the descriptor term Colles (hills).