Lava broad, shallow slopes formed from predominantly effusive eruptions of relatively fluid basaltic lava flows, to steeply-sided stratovolcanoes (also known as composite volcanoes) made of alternating layers of ash and more viscous lava flows typical of intermediate and felsic lavas. A caldera, which is a large subsidence crater, can form in a stratovolcano, if the magma chamber is partially or wholly emptied by large explosive eruptions; the summit cone no longer supports itself and thus collapses in on itself afterwards. Such features may include volcanic crater lakes and lava domes after the event. However, calderas can also form by non-explosive means
Boring Lava Field of the town of Boring. Volcanologists L. Siebert, T. Simkin, and P. Kimberly (2011) of the Global Volcanism Program list its highest elevation as , at Larch Mountain, with most vents reaching an elevation of . Located in the Portland Basin, the field consists of monogenetic volcanic cones that appear as hills throughout the area, reaching heights of above their surroundings. The collection includes more than 80 small volcanic edifices and lava flows in the Portland–Vancouver metropolitan area, with the possibility of more volcanic deposits buried under sedimentary rock layers. The borders for the Boring Lava Field group are clear,