Types of Volcanoes: Spatter and Tuff Cones
Lava also flows from vents and fissures around the flanks of a volcano. As a fissure eruption progresses, the outflow tends to become localized to one or a few spots along the fissure. These spots tend to build up small cones, often referred to as parasitic cones. These parasitic cones are generally of one of two types: spatter or tuff cones.
Through magma flows from vents, some dissolved gases are released, forming many blobs of lava "coughed out" of a vent all at once. These lava blobs are called spatter. Spatter piles up and forms ramparts which stand like walls next to eruptive fissures. A towering lava fountain along a fissure may build a spatter cone a few meters high while a very small spatter cone is called a hornito. Good examples of spatter cones occur on Isla Bartolomé, adjacent to the large island of Santiago, Galapagos Islands.
A tuff (or ash) cone is formed by explosive (and therefore potentially hazardous) phreatomagmatic eruptions (the interaction of basaltic magma and water). Tuff cones thus tend to be found near the water's edge or just offshore. Tuff is composed of extremely fine-grained cemented volcanic ash. Tuff cone explosions have a tendency to be shallow and closely spaced; therefore all material is projected sideways, instead of directly upward. The explosion carries solidifed magma droplets that have been fractured into tiny fragments. These fall back around the vent to form tuff. This tuff is often later chemically modified to a yellowish alteration clay mineral called palagonite. Tuff cones typically have steep flanks (>25 degrees) and crater floors above ground level. The resulting cones are often between 100 m to 300 m high and 1 to 1.5 km across. Tuff cones have deep, broad craters (narrower and steeper than those of tuff rings).