Volcanic eruptions occur when magma (molten rock) rises from a planet's interior and pours out onto the surface. Variations in the magma composition, temperature, and flow rate can produce a variety of volcanic landforms.
This diagram of Hawaii's Kilauea volcano shows the underground "plumbing" and paths of magma. Eruptions can occur at the volcano's summit or along cracks or fissures in the volcano's flanks.
Hawaiian Volcano Observatory diagram
A Kilauea fissure eruption.
Photograph by C. Heliker, USGS Hawaiian Volcano Observatory
Rapid eruptions of fluid lava covered much of the northern lowlands with smooth volcanic deposits. Some of these plains have many small volcanoes like this one, Mareotis Tholus. This remarkably detailed image of its summit shows an elongate feature, probably a volcanic vent, and a circular impact crater. Individual blocks that fell from the rim and rolled down through the dusty terrain into the vent are clearly visible.
Mars Reconnaissance Orbiter, HiRISE
NASA/JPL/University of Arizona image
Slower eruptions of cooler lava or lava richer in silica created lobed structures. Over time these flows built up to form the bulk of the large volcanoes.
Mars Odyssey, THEMIS
NASA/JPL/Arizona State University image
Some lava flows are very rugged and strongly reflect radar signals, which can penetrate a meter or more of dust cover. This image, collected by the Arecibo radio telescope in Puerto Rico, shows rough lava flows (brighter tones) around several volcanoes and areas flooded by lava. The bright area at lower center is due to strong reflections from the part of Mars facing directly toward Earth during the observations.
Image courtesy of John Harmon, Arecibo Observatory