Easter Island's moai were carved from compacted volcanic ash
Tuff is volcanic ash turned to stone, and despite its tough-sounding name it is soft enough to carve. The Romans built with it throughout Italy, and the Rapa Nui people shaped most of their moai from it. One Yellowstone eruption laid down at least 1,000 cubic kilometres of the stuff.
Eruptions throw out gases, lava and tephra, the solid fragments hurled through the air. Tephra forms when gas dissolved in magma comes out of solution as pressure drops, blasting the melt apart. Particles under 2 millimetres count as ash, and rock that is more than three-quarters ash is tuff; mixtures with 25 to 75 percent ash are called tuffaceous. Coarser consolidated debris becomes lapillistone or, above 64 millimetres, agglomerate. Guidebooks often confuse tuff with tufa, which is actually a form of travertine.
Geologists sort tuff by what the ash is made of. High-silica eruptions produce vitric tuff full of glass shards, often curved triangles that are the shattered walls of countless tiny gas bubbles. Crystal tuffs consist mostly of single crystals, lithic tuffs of pulverised rock. Ash that settles from an eruption cloud spreads as an evenly sorted blanket, while collapsing columns send pyroclastic flows that pool in low ground. If deposits land hotter than 600 °C, fragments fuse into welded tuff, with glass and pumice squashed into a streaky fabric.
Even cool ash hardens quickly because volcanic glass is chemically unstable. Groundwater or seawater leaches calcium and alkali metals from it, and new minerals such as zeolites, clays and calcite grow and cement the grains. Because it forms wherever explosive volcanism occurs, tuff spans a huge range of places and ages, from rhyolitic tuffs in Iceland and New Zealand to andesitic beds along the Andes and ancient equivalents in Wales and the Lake District.
Some deposits are enormous. The Lava Creek Tuff, erupted from the Yellowstone Caldera 631,000 years ago, is at least 1,000 times the volume of the 1980 Mount St. Helens deposits and rated 8 on the Volcanic Explosivity Index, larger than anything in the last 10,000 years. Ash-flow tuffs blanket about 100,000 square kilometres of Nevada and 7,000 of New Zealand's North Island, the only volcanic product rivalling flood basalts in volume. When basaltic magma meets water, the resulting blasts can build tuff cones such as Hawaii's Diamond Head.
Source: Tuff
