Question by C J: Can the small crystals in extrusive rock be seen with the naked eye?
I know that the crystals in intrusive rock can be seen with the naked eye, but can the small amount of crystals in extrusive rocks be seen with the naked eye as well?
Best answer:
Answer by Cristobal Salvaje
no that is why you get obsidian. THe Magma cools too fast for the crystals to form
What do you think? Answer below!
Small crystals in extrusive rocks can be seen and some cannot. It depends on the cooling time of the rocks. If the rocks cooled instantly, they form what is known as volcanic glass. These rocks contain so crystals or very few. Rocks that cool for a moderate time period that are extrusive can be seen with the naked eye. Examples of these rocks are grantie, rhyolite and scoria.
i had a test on this on monday=)
That depends on how quickly the rock cooled on the surface. The longer the cooling, the bigger the crystals. There are some extrusive rocks that have visible crystals, but generally most are too small to see with the naked eye.
A notable exception is a rock called olivine basalt. This is an extrusive rock that began to cool intrusively creating large olivine crystals, but was then erupted onto the surface so that you have a bunch of green olivine crystals in patches on dark gray basalt. The basalt cooled very quickly, so there aren’t any visible crystals other than the olivine crystals. It’s really quite beautiful.
A lot of times there are crystals already in the magma before it extrudes, and these crystals are usually identifiable visually (some can be very large), although they can be so small that you need a handlens to see them clearly.
The groundmass, the part that was liquid when the magma erupted, can sometimes form identfiable generally poorly-formed minerals, usually requiring a handlens to see well at all, but in a lot of cases the groundmass is so finely crystalline that it appears as a mass of non-crystalline material except under the microscope.
In other words, it varies a lot, but true glass is not very common, usually a finely crystalline mass not really distinguishable by naked eye alone is formed.