A large number of positive relief features in the maria and adjacent highlands have been identified from orbital and Earth-based data and have historically collectively been referred to as "lunar domes." These features exhibit a range of morphologies and morphometries. Most occur on the mare as scattered individual features (e.g., Mare Tranquillitatis), in groups (e.g., Marius Hills) and as groups that have coalesced into larger features (e.g., Rümker Hills). While a volcanic origin (intrusive or extrusive) is assumed, the evidence for such an origin is limited. For the mare feature, the term "dome" is a misnomer as it brings to mind terrestrial silicic domes formed by high-viscosity lavas. In fact, most of the mare features are better classified as small shield volcanoes, with a few exceptions such as the Marius Hills and Hortensius domes.
Data from LRO and other missions now provide the ability to characterize each dome in terms of areal extent, topography, morphology, and color properties in unprecedented detail allowing for an analysis of their origin. Understanding the mare domes is important as they represent the terminal phase of mare volcanism. Here, we discuss the small shields of Mare Tranquillitatis.
Typically, lunar domes have a topographic expression and many exhibit spectral anomalies; a few have summit craters. Color data (LRO WAC and Clementine) indicate that lunar domes are often associated with red spots. LROC WAC color data indicates that some domes have colors similar to the adjacent mare whereas others have distinctly different color. A few of the highland features (e.g., Compton-Belkovich) have unusual chemical and spectral signatures (high Th, high Si) suggestive of non-mare petrology.
The shields of Mare Tranquillitatis have three morphologic types: shields having shallow flank slopes that merge imperceptibly into the surrounding mare; shields with an abrupt contact with the surrounding mare with steep flank slopes around the margin (2-5°) and shallow slopes (<1°) near the summit; and irregular, hummocky steep-sided features. The first two shield types typically have summit craters; the third does not. Many shields appear to be randomly distributed, but others, particularly at the northwest-end of Rupes Cauchy, appear to be structurally controlled. Those constructs have overall shapes and summit craters with a northwest elongation parallel to the trend of the rupes, implying tectonic control of eruption pathways. In a few locations, short flows extend away from low topographic mounds. The flows are up to 100 m thick and up to 15 km long. The mounds are 2 km in diameter and may be cinder cones.
Although there is considerable scatter in the morphometry for the small shields of Mare Tranquillitatis, the data suggest that the shields become taller (87-158 m), have larger bases, (5211-13682 m), the summit elevation decreases (-764 to -1593 m), and the summit craters appear to deepen westward across the mare. Flank slopes are similar across the region. These observations may indicate long-lived, hence larger volume, eruptions in the western mare at lower elevations compared to volcanism in the eastern mare.