Book 24, Question 112
How do veins of metal form in rock?
Deep in Earth, hot water full of dissolved minerals moves through cracks in rock. As the water cools, the minerals come out of the water and build up on the walls of the crack, like the crust inside a kettle. Over thousands of years, the crack fills with minerals, which sometimes include gold, silver, or copper. That is called a vein.
More fun facts
- Many gold deposits formed from hot groundwater moving through cracks in rock.
- One widely accepted idea is that groundwater gets heated by bodies of magma, or melted rock, that pushed into the crust within about 2 to 5 miles of the surface.
- Most of that water starts out as rain, which seeps down through cracks in cooler rock.
- It is drawn sideways toward the rock heated by magma, then pushed upward through cracks.
- When the hot water reaches cooler rock closer to the surface, metal minerals come out of the water and build up as veins.
- Another idea is that gold carrying fluids come right out of magma as it cools.
- Natural hot water and steam systems used in parts of the United States today are a modern example of how these gold deposits formed.
- Where copper, lead and zinc build up in veins, small amounts of gold are often deposited with them.
Sources: U.S. Geological Survey
Questions kids ask
How do geologists guess where veins might be?
They study folds, faults, cracks and joints in the rock, which suggest where fractures formed and where veins might be found.