Using the material below try and identify the rocks below based on the following
- Intrusive or Extrusive? How did you come to your conclusion?
- What type of rock Texture? Glassy, Alphantic, Phaneritic, Pegmatitic, Porhyritic, Vesicular, or Pyroclastic/Fragmental?
- Are there crystals in the rock? and what size?
- Based on that was this a rapid or slow cooling?
- Add a photo of you from the location or of an identifiable item (paper with trackable name). Posting a photo pulled from the internet, or photoshopped will result in instant deletion.
Intrusive or Extrusive?
Igneous rock is primarily identified as being Intrusive (Magma) or Extrusive (Lava).
Rock that is Intrusive is formed below the surface. These normally cool very slowly over extended periods. The mineral crystals may be very large like in a granite. The longer they take to cool the larger the crystals. Sometimes up to 20mm (nearly 1"). This rock is usually extremely hard, and over time as the other materials around the wear away we see large mounds/mountains/dikes that are exposed.
Lava that finds its way to the surface is known as extrusive. This can be exploded from a volcano, seeping out from lava tubes or, ash that settles and forms into rocks. Rock that flows on the surface can cool at different rates, and the minerals/air pockets inside dissperse unevenly. This can let you sometimes identify the flow layers, or direction of flow.
Rock Texture
Glassy. This is usually an obsidian. Looks and behaves like black glass.
Alphantic. Very fine crystalled. The crystals are usually under 1mm in size (.040") The appearance may be like dried clay (in size) Rhyoloite and Basalt are the common types
Phaneritic Crystals run from 1mm-10mm (.040-.400") Granite is the most familiar and most common of this types.
Pegmatitic Crystals over 10mm. Not very common, usually a type of Granite.
Porhyritic A mixture of sizes. This is usually caused from a slow cooling followed my partial melting and rapid cooling.
Vesicular Rock that has gas bubbles in it. Pumice being the best known. Basalt may also fall in this catigory.
Pyroclastic or Fragmentary Is formed from an explosive formation. They bond with other rock in the area. Quite often they appear as a sedimentary rock because they are fused together in a matrix. Tuff (fused small ash particles that may contain larger stones), and breccia (larger stones bound together with smaller stones/and or ash particles)