A stock in mining and geology (more specifically volcanology) is an unprecedented intrusion of magma in the earth's crust, in horizontal average no greater than 100 km. Stocks are seen from above in average approximately circular. Intrusions that are larger in size are called batholites.
Stocks are or (when the magma is stolen) were magmaeservoirs under active volcanoes. Many stocks are likely to form the domed, upper slopes of deeper batholites. Long after the volcanic activity has stopped, the volcano itself can be completely eradicated and the stock will come to the surface. Due to the large amounts of hydrothermal fluids released into volcanism, the rock in and around stocks is often strongly influenced by alteration.
Stocks occur roughly in two different volcanic conditions. In intermediate to felsic magma (diorite, granodiorite or granite), stocks form the magmaeservoirs under calderas. The magma in the stock will be the source of both tefra and lava during eruptions. In that case, the stock will have almost vertical edges, which form the ringdikes along which the above-mentioned blocks of rock sank the magma. Stocks also occur in rifles, with magma being alkaline in nature (peridotite, gabbro, syenite or granite). Also see
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