The above view shows Arenal volcano. This volcano is the smallest, yet most active of seven historically active volcanoes in Costa Rica. It is located northwest of the capital of San Jose and displays an almost continual low-level of activity since a major eruption in 1968. The magma source of this volcano is thought to be a rift or tear where the western Cocos Plate is subducting the more eastern Caribbean Plate. The molten rock making its way to the surface may flow out as lava or erupt as tephra. When the "tephra" is highly vesicular, like the sample below, it is referred to as scoria.