Sill (geology)
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In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or along the direction of foliation in metamorphic rock. A sill is a concordant intrusive sheet, meaning that it does not cut across preexisting rock beds. Stacking of sills builds a sill complex and a large magma chamber at high magma flux. In contrast, a dike is a discordant intrusive sheet, which does cut across older rocks.
Illustration showing the difference between a dike and a sill.
Salisbury Crags in Edinburgh, Scotland, a sill partially exposed during the Quaternary glaciation
Mid-Carboniferous dolerite sill cutting Lower Carboniferous shales and sandstones, Horton Bluff, Minas Basin South Shore, Nova Scotia
Seismic expression of basaltic sills from the Rockall Trough. Data courtesy UK OGA.
Tuff
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Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock containing 25% to 75% ash is described as tuffaceous. Tuff composed of sandy volcanic material can be referred to as volcanic sandstone.
Cliff face of welded tuff pockmarked with holes — some natural, some man-made from Bandelier National Monument, New Mexico
Etruscan tuff blocks from a tomb at Banditaccia
A house constructed of tuff blocks in Germany
Light-microscope image of tuff as seen in thin section (long dimension is several mm): The curved shapes of altered glass shards (ash fragments) are well preserved, although the glass is partly altered. The shapes were formed about bubbles of expanding, water-rich gas.