Along the Snake River in Idaho , and the Columbia River in Washington and Oregon , these lava flows are beautifully exposed and measure more than a mile in total thickness. As with earthquakes, volcanic activity is linked to plate-tectonic processes . Most of the world's active above-sea volcanoes are located near convergent plate boundaries where subduction is occurring, particularly around the Pacific basin. Eruptions of Hawaiian and most other mid-plate volcanoes differ greatly from those of composite cones. Mauna Loa and Kilauea , on the island of Hawaii, are known as shield volcanoes , because they resemble the wide, rounded shape of an ancient warrior's shield.
Shield volcanoes tend to erupt non-explosively, mainly pouring out huge volumes of fluid lava.
Hawaiian-type eruptions are rarely life threatening because the lava advances slowly enough to allow safe evacuation of people, but large lava flows can cause considerable economic loss by destroying property and agricultural lands. For example, lava from the ongoing eruption of Kilauea, which began in J anuary 1983, has destroyed more than 200 structures, buried kilometers of highways, and disrupted the daily lives of local residents. Because Hawaiian volcanoes erupt frequently and pose little danger to humans, they provide an ideal natural laboratory to safely study volcanic phenomena at close range. The USGS Hawaiian Volcano Observatory , on the rim of Kilauea, was among the world's first modern volcano observatories, established early in this century. Another type of basaltic activity is characterized by the concentration of many tephra and lava-flow eruptions at a central vent and several flank vents.