Crystals in extrusive igneous rocks are generally very small, often microscopic in size. This is due to the rapid cooling and solidification of lava or magma at the Earth's surface.

Here's why:

* Rapid Cooling: When magma erupts and becomes lava, it cools quickly due to exposure to the atmosphere and surrounding environment.

* Limited Time for Crystal Growth: This rapid cooling leaves little time for the atoms within the molten rock to arrange themselves into larger, well-defined crystals.

* Fine-Grained Texture: The resulting rock is characterized by a fine-grained texture, meaning the individual crystals are too small to be seen without magnification.

Examples of Extrusive Igneous Rocks with Small Crystals:

* Basalt: A common extrusive rock with a very fine-grained texture.

* Rhyolite: Another extrusive rock, but often with a slightly coarser texture than basalt, still generally featuring small crystals.

Exceptions:

While most extrusive rocks have small crystals, there are exceptions. Some extrusive rocks, especially those that cool slowly within lava flows or under thick layers of ash, can develop larger crystals, sometimes even visible to the naked eye.

In summary, the size of crystals in extrusive igneous rocks is primarily determined by the speed of cooling. The faster the cooling, the smaller the crystals.