In many cases the molten rock at the surface cools too quickly for crystals to form.
If you're not looking for anything specific, any extrusive igneous rock will have smaller crystals, if any at all. This is because they cool quickly on the surface, leaving little time for crystals to form/grow.
Rhyolite
Extrusive Igneous rock cools quickly above ground so it has a smooth texture so there isn't enough time for crystals to form and intrusive igneous rock forms inside a volcano or other underground hot place and takes longer to cool so crystals can form. It is extrusive.
Intrusive rocks are Igneous rocks that have been formed underground. Igneous rocks are formed by the cooling of lava/magma. Igneous rocks are divided into two categories- intrusive and extrusive. Intrusive is when the magma cools slowly beneath the Earth's surface, which causes large crystals to form. When lava cools above the Earth's surface, it is called extrusive. Extrusive rocks have smaller crystals compared to intrusive, the difference always being noticable.
glassy texture, hope this helps :)
In some cases the molten rock cools too quickly for the atoms to arrange themselves into a crystalline structure.
Intrusive because they have large crystals and the have these because they have cooled down much slower then extrusive due to the heat inside the volcano; meaning the crystals took longer to form.
If you're not looking for anything specific, any extrusive igneous rock will have smaller crystals, if any at all. This is because they cool quickly on the surface, leaving little time for crystals to form/grow.
Rhyolite
They cool to quickly for crystals to form.
Extrusive Igneous rock cools quickly above ground so it has a smooth texture so there isn't enough time for crystals to form and intrusive igneous rock forms inside a volcano or other underground hot place and takes longer to cool so crystals can form. It is extrusive.
If a rock has large crystals, it is an intrusive rock. Intrusive rocks form underneath the Earth's surface. Magma cools slowly so it has time to form large crystals. An example is granite, where you can see the crystals with your naked eye. Rocks that have small crystals are extrusive rocks. Extrusive rocks are ones that form from lava (blasted out of a volcano) so they cool very quickly, not allowing large crystals to form. An example is obsidian, where you cannot visibly see the small crystals; it just looks like one black, glassy rock.
Igneous rocks that cooled slowly are intrusive. Igneous rocks that cooled quickly are extrusive. Intrusive rocks form larger crystals, because the crystals have a longer time to grow. Extrusive rocks have small to no crystals, because they had little or no time to grow/form.
Extrusive is when Igneous rock is formed from magma below Earth's surface.
Intrusive rocks are Igneous rocks that have been formed underground. Igneous rocks are formed by the cooling of lava/magma. Igneous rocks are divided into two categories- intrusive and extrusive. Intrusive is when the magma cools slowly beneath the Earth's surface, which causes large crystals to form. When lava cools above the Earth's surface, it is called extrusive. Extrusive rocks have smaller crystals compared to intrusive, the difference always being noticable.
a mineral
A porphyritic igneous rock demonstrates some extrusive characteristics and some intrusive characteristics. The appearance of phenocrysts (larger crystals) in a fine matrix (small crystals) indicates that the magma had time to cool slowly enough underground to form larger crystals before being erupted at or near the surface.