card 7 EXTRUSIVE IGNEOUS ROCKS

Extrusive (volcanic) igneous rocks form when molten rock erupts from Earth's interior through a volcano or fissure and cools rapidly at the surface. Such rapid cooling of lava, molten rock at the surface, generally does not allow mineral crystals to grow large enough to be seen with the unaided eye, so extrusive rocks are usually fine-grained (also called aphanitic) in their texture.

Picture 1 illustrates the fine-grained (aphanitic) texture typical of extrusive rocks.

Picture 2 shows an extrusive rock possessing an unusual texture composed of both large and small crystals. Such a porphyritic texture develops when magma begins cooling slowly underground forming large crystals, but then is forced to the surface where the remaining molten rock (lava) cools quickly.

Picture 3 shows another extrusive igneous rock texture referred to as pyroclastic. This texture results from a very explosive eruption which sends not only lava flying through the air, but also fragments of the volcano itself. All airborne volcanic fragments, referred to as pyroclasts, eventually fall to the surface where they weld together due to their high temperature, or simply compact together to form rock. Pyroclasts come in many sizes: the smallest are called ash, slightly larger are lapilli, and the biggest are called blocks or bombs. The rock in picture 3 is composed mostly of ash, but also contains some lapilli-sized pyroclasts. Picture 4 shows a large pyroclast called a volcanic bomb. Note its streamlined shape, complete with a head and a tail. This shape is developed as still-molten lava flies through the air after being ejected from a volcano. Bombs can inflict severe damage if they strike your person.

Picture 5 illustrates the vesicular texture which develops when lava containing some gas bubbles cools and solidifies before the gas escapes into the atmosphere.

Picture 6 shows the highly vesicular or frothy texture which forms as gas-charged lava cools very rapidly. This forms the extrusive rock called pumice, which is so light that it floats on water.

Picture 7 shows the extrusive rock called obsidian which is characterized by its glassy texture. Such natural glass was used by native Americans to fashion spear and arrow tips for hunting, and is still used today by surgeons because it holds a very sharp edge for a long period of time.

Pictures 8 and 9 illustrate textures that develop as a flow of lava cools. Picture 8 shows a ropey texture called pahoehoe which forms as the surface of a very hot and fluid lava flow begins to cool, but underlying lava continues to flow. Picture 9 illustrates the angular, chunky texture that can develop as an entire lava flow cools and loses gases. This causes the lava to thicken and to break apart as the flow continues to move downslope. Such a texture is referred to as a-a, probably because of the pain inflicted on a person trying to walk across such a flow.

PICTURES Click once on a picture to enlarge it. Click on the "back" button to revert to normal picture size.

1	2	3	4	5
fine-grained texture	porphyritic texture	pyroclastic texture	volcanic bomb	vesicular texture

6	7	8	9
frothy texture	glassy texture	pahoehoe texture	a-a texture