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hyaloclastite An aggregate of fine, glassy debris formed by the sudden contact of hot, coherent magma and either cold water or water-saturated sediment. Rapid heat loss from the magma to the cold water sets up tensile thermal stress in the magma carapace as it cools, chills, and contracts, causing the glassy, chilled zone to fragment and form a quench-fragmented debris. Thick hyaloclastite deposits form over basalt flows when they erupt beneath the sea or enter the sea. If the deposit remains in contact with water after its formation, the glassy debris can easily be hydrated to form palagonite.