1 ) extreme heat
2 ) wear and tear from the elements such as being in the ocean for many many years.
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How does a rock with a glassy texture form?
Very rapid cooling forms crystals so tiny the rock has a glassy texture. The slower the cooling, the larger the crystals.
This type of igneous rock has to be high in silica and ejected extremely quickly from the earth’s crust. This usually means high volatility explosive volcanics. The crystals formed are so small due to rapid cooling.