Basalt
A volcano is made of extrusive igneous rock. The specific type of igneous rock will vary from one volcano to another.
Granite is an example of a porphyritic igneous rock. It contains large crystals (phenocrysts) within a fine-grained matrix. The phenocrysts are usually made up of minerals such as quartz, feldspar, and mica.
You can see basalt rock forming in Pu'u 'O'o crater. Basalt is an igneous rock that forms from the rapid cooling of lava.
Porphyrite of any composition, granite, gabbro, diorite. Most intrusive igneous rocks have crystals large enough to see with the naked eye. Another term used to define an igneous rock with large crystals is coarse-grained.
at places where volcanic eruptions take place
pumiceThe type of igneous rock that floats is called Pumice. It isn't lighter than other rocks, just less dense because it has lots of very small air bubbles trapped in it. You can see these if you look at the exterior of the rock.
The kind of rock you see forming in Puu Oo crater is extrusive igneous. This type of rock is formed by lava.
sandstone is a sedimentary rock because of the layers or lines you can see on it
These are extrusive igneous rocks with an aphanitic texture.
This is taken from the Wikipedia article: "Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock." Igneous means that the material obsidian is made of is molten initially. Extrusive means that the material pours or explodes out of volcanoes, as opposed to remaining under the earth's surface. See links for more.
Most of it you got right. Have you ever heard of something called the rock cycle? If you have, good. If you haven't, here is a link to an excellent diagram: http://www.emporia.edu/earthsci/student/henderson1/rock_cycle.gif Anyway, if you look at the diagram, you can see how igneous rock can transform into sediment, then sedimentary rock, or bypass sedimentary rock altogether and go to metamorphic rock. Sedimentary rock can turn into metamorphic rock with heat and pressure. However, to turn into igneous rock from sedimentary rock, it will have to melt down into magma/lava before becoming igneous rock once again.
These are igneous rocks that are too fine textured to an extent that there mineral grains or crystalline texture cannot be seen or distinguished with the necked or unaided eyes. They appear or occur as a whole single massive crystalline extrusive body of Igneous origin. Example is Obsidian.