Most of the Venusian surface is relatively flat; it is divided into three topographic units: lowlands, highlands, and plains. In the early days of radar observation the highlands drew comparison to the continents of Earth, but modern research has shown that this is superficial and the absence of plate tectonics makes this comparison misleading. Tectonic features are present to a limited extent, including linear "deformation belts" composed of folds and faults. These may be caused by mantle convection. Many of the tectonic features such as "tesserae" (large regions of highly deformed terrain, folded and fractured in two or three dimensions), and "arachnoids" (for those features resembling a spider's web) are associated with volcanism.