These eruptions cover large areas and often produce ropy, or pahoehoe, lava flows. Thicker basalt lava breaks into chunks or blocks, forming blocky lava flows, called aa. Lava is the fluid phase of volcanic activity. Whether lava or pyroclastics are being ejected, the eruption is normally accompanied by the expulsion of water and gases, many of which are poisonous. Lava usually forms long, narrow rivers of molten rock that flow down the slopes of a volcano.
Explosive eruptions tend to be spectacular events best observed from a safe distance.
Earthquakes, high columns of vapors, lightning, and strong whirlwinds often accompany the explosions. The eruption of Krakatoa unleashed a tsunami, a large seismic sea wave, that swept the coasts of Java and Sumatra and drowned more than 36,000 people. A volcano can grow with frightening speed and often affects territory far beyond the area on which the cone forms. When volcanoes are born in the sea, the eruptions may be more violent than those on land because the contact between molten rock and seawater produces steam. Volcanoes also create craters and calderas.