This is a close-up photograph of an aphanitic, or fine-grained basalt. Note the lack of any very well-defined crystals. That's because it cooled very quickly and crystals didn't have a chance to form. About the only crystals you may see in this basalt are lath-shaped (rectangular) plagioclase that are whitish. |
A close-up of a medium-grained gabbro. Compositionally the gabbro is the same as the aphanitic basalt, but it took a longer time to cool - hence the better developed crystals. The crystals in the gabbro are the light-colored plagioclase and the dark-colored olivine and pyroxene. |
Close-up view of a fine-grained granite. Note the lack of large crystals. Since the crystals are small, this granite probably cooled relatively quickly before they had a chance to grow. One possible location might be near the edge of the magma chamber, close to cool country rocks. |
Again, this is a granite, however, this time it is coarse-grained indicating that the crystals had a much longer time to crystallize. This rock may have formed further towards the center of the magma chamber where it was warmer and more insulated for a longer time. Also not the zoned plagioclase feldspar crystals, they're the pinkish crystals. The smoky, or gray, crystals are quartz, and the black crystals are biotite. |
This is a medium-grained granite. The size of the crystals indicates that it took longer to cool than the fine-grained granite above, but shorter to cool than the coarse-grained granite above and to the right. The minerals found in this are the same as the other granites. |
This is an extreme close-up of one of the zoned plagioclase feldspar crystals seen in the coarse-grained granite above. Note the rings around the center of the plagioclase crystals. Like rings found in trees, these rings are placed on the outside edge of the crystal as it grows. The rings indicate that the magma, or melt, was slowly changing its composition, and hence color, as the rest of the crystals formed. This is very common in crystals that took a long time to form. |
If the magma forming an igneous rocks fragments, or cools quickly and explosively, it may produce pumice, a pyroclastic rock. Pumice is very porous and will sometimes float on water. Here we can see a very thick layer of pumice. |
If a magma cools extremely rapidly, quenched, then it may form an obsidian. Not all obsidian is black. Colors range from black to brown, to red, to green to blue and yellow. This particular obsidian is called mahogany obsidian because of its color. Obsidian also exhibits conchoidal fracture which is the smooth, curved surfaces seen here and also found in quartz. |
If the lava or magma has some vesicles, or gas bubbles, in it, but is not as lightweight as pumice and does not float in water, then we call the rock scoria. It is generally darker than pumice and may have a few small crystals of olivine or plagioclase visible. |
An extrusive rock that is compositionally the same as granite is a rhyolite. Since it is extrusive it implies that it cools very quickly and we would not normally see crystals. This particular example is a rhyolite in which the magma started to cool slowly, and crystals started to form. It was then erupted and quenched - stopping crystal growth. This texture of both large crystals (phenocrysts) and small crystals, or none, in a fine-grained matrix (groundmass) is called porphyritic. Therefore this would be a porphyritic rhyolite. Note the zoned plagioclase. |
Sometimes when an igneous rock intrudes the surrounding country rocks it does so in a brittle manner. That is, it doesn't just flow into cracks and joints that it makes. Here we see dark country rock (older basalt) that has been torn apart as the granite was intruded. The broken basalt fragments are called xenoliths. Xenoliths allow us to use the Principle of Inclusion to tell in a relative sense which rock is older - here the basalt is older than the granite. |
If an intrusive igneous rock does not cut across the surrounding rock's fabric, or bedding, then we call that a concordant intrusion, or a sill. |
 An
intrusive igneous rock that cuts across, or is discordant
to, the surrounding rock's fabric (bedding) is called a dike.
Here we see a 1 meter thick basalt dike. |
These are igneous sills that have been faulted. Not the upper most land surface continues uninterrupted, whereas the igneous sills have been offset along a fault. We will talk about what type of fault this is in later chapters. |