Rock Texture

In classifying rocks, it´s important to describe the texture. Fine-grained means the grains are too small to identify without a hand lens (sometimes you need a microscope!). Coarse-grained means the grains are large enough to identify without a hand lens.

Glassy texture means the rock has no grains, not even under a microscope. If a rock has glassy texture, it´s volcanic, meaning it cooled from liquid magma, at or very near the earth´s surface, and it cooled too quickly for individual mineral grains to form.

Crystalline Igneous Rocks

If a rock is made of randomly oriented crystals of different sizes, it´s probably a plutonic igneous rock, one which cooled slowly far below the earth´s surface.

Granite and granitic rocks (light-colored plutonic igneous rocks) often are made of three or more different minerals, so they are often a mixture of clear, white, pink, light gray, and a black minerals grains.

Diorite and other "intermediate" (not light, not dark) rocks are often made of clear, white, and dark minerals - the dark minerals are usually a higher percentage of the total rock than you find in granites.

Gabbro (dark-colored plutonic rock) will appear to be mostly dark minerals.

Rock Classification

Rocks are classified based on composition (which will affect the color - is it a dark-colored rock? a light-colored rock?) and texture (size and arrangement of mineral grains that make up the rock - can you see individual minerals? are the minerals deformed?).

When classifying rocks, first decide whether it´s igneous, metamorphic, or sedimentary. Then classify it according to composition and texture to find the actual rock name.

Sedimentary Rock Identification I

If a rock looks like it´s made of sand that has been cemented together, that´s a sedimentary rock with clastic texture (sandstone). If it looks or feels like cemented gravel mixed with sand, or cemented silt or mud, that´s also a sedimentary rock with clastic texture (conglomerate, siltstone, mudstone, shale).

Metamorphic Rock Identification I

If the minerals in a rock seem to be layered into dark and light bands, and the bands are somewhat folded or faulted, it´s a metamorphic rock (gneiss).

If the rock doesn´t have layers, and if the crystals in it all seem to be shiny and flat (mica) then it´s probably a metamorphic rock (schist, phyllite).

If the rock isn´t very shiny, is mostly one color (for example, dark gray) and breaks into very hard, flat sheets it is probably a metamorphic rock (slate). (If the sheets are flat and crumbly, a soft rock, it´s probably shale, which is sedimentary, not metamorphic.)

Metamorphic Rocks I

If the rock you are classifying has deformed fossils in it, it´s a metamorphic rock.

If the minerals are arranged in bands or streaks, and the streaks are folded or faulted, it´s a metamorphic rock. If the rock appears to be made of colored bands or streaks that aren´t folded or faulted, it might not be metamorphic - it might be an agate.

If it made of a lot of small, shiny crystals so that the whole rock looks like it´s made of glitter, it´s a metamorphic rock that has a lot of mica in it.

Meteorites

Where do meteorites come from? While they obviously come from many different places, both the Moon and the planet Mars have been identified as sources for meteorites that land here on earth. Less than 20 have been identified as coming from either site, though.

Sedimentary Rock Identification II

If a rock seems to be made of layers of crystals that effervese or fizz when you carefully drop acid on them, it is a sedimentary rock (limestone).

If you have to scratch the rock first before dropping acid causes it to fizz, that´s still a sedimentary rock (dolostone).

Texture - Coarse vs. Fine

Igneous rocks are classifed by their composition (basically, the elements that make up the minerals, which make up the rock) and their texture. Texture, in this case, isn´t how it feels, but rather, it´s a way to describe the shape, size, and arrangement of the mineral grains that make up the rock.

If you can see individual grains without a hand lens or microscope, the rock is coarse-grained, or phaneritic. If you cannot, the rock is fine-grained, or aphanitic, and will look like a rock that is pretty much all one color, for example, dark gray.

Rock Classification - Fossils

Some (not all!) sedimentary rocks have fossils. If a rock has fossils, it is definitely not an igneous rock (since igneous rocks begin as liquid magma). If the fossils in the rock are not deformed, it is a sedimentary rock.

But if the fossils *are* deformed, it is classified as a metamorphic rock (it used to be sedimentary, but has been subjected to pressures that deformed the fossils).

Glassy Rocks

If a rock seems to have a "glassy" texture, that is, you´re pretty sure there´s no mineral grains in it, it is probably a volcanic igneous rock.

Pumice is a volcanic igneous rock that is full of tiny holes and is so light small samples of it will float on water. Scoria is a glassy igneous rock that is usually dark-colored and full of dark holes.

Obsidian is a volcanic glassy rock that has no holes, and a shiny appearance, as if it were made of glass.

Crystalline Texture

Many rocks are made of intergrown crystals, that is, their texture is crystalline.

Crystalline rocks tend to be somewhat glittery or shiny - if you move them around slowly in good light, the crystal planes in the individual mineral grains will reflect the light.

Texture - Glassy

Some igneous rocks have glassy texture. That doesn´t mean the rock shines like glass (although some might!). It means that the lava cooled so quickly that individual crystals didn´t form at all. In other words, there are no individual crystals, even under a microscope. Glassy igneous rocks often seem to be a solid color, like light gray or black.

Lots of folks get "glassy" confused with "shiny." Obsidian has a glassy texture, and even looks shiny, like glass. But pumice, which also has glassy texture, is a dull rock which doesn´t shine at all.

How do you know if a rock is fine-grained (has individual mineral grains in it, but you can´t see them without a hand lens or even a microscope) or glassy (no individual crystals at all)? You can´t tell, just by looking. But obsidian, pumice, scoria, and tuffs have a glassy texture. Other igneous rocks that seem to be more-or-less a solid color, like basalt, rhyolite, and andesite, have an aphanitic (fine-grained) texture.

Texture - Porphritic

If an igneous rock cools very slowly, it has individual mineral grains that you can see with the unaided eye. If an igneous rock cools quickly, it has very small mineral grains that you can´t see without a hand lens or microscope.

But what if an igneous rock seems to be pretty much all one color (say, light gray) but is scattered throughout with mineral grains, too?

Chances are that rock is a porphyry, that is, it has porphritic texture. (Remember that texture refers to the size, shape, and arrangement of mineral grains.)

A porphry has undergone two stages of cooling - first, it began to cool slowly, and individual minerals began to form (they´ll be called "phenocrysts" in the porphyritic rock that´s going to result). Then something happened - perhaps pressure built up in the magma chamber, and the volcano blew. So the rest of the magma cools quickly into a fine-grained "groundmass".

Result: a fine-grained groundmass that might look like a solid gray or whitish rock, with scattered bits of mineral grains (the phenocrysts) throughout.

Plutonic (Intrusive) Rocks

Igneous rocks form when magma cools. But some magma cools far underground, and it cools slowly. Usually that means individual crystals have time to grow, and the resultant rock is coarse-grained. That means you can see the individual mineral grains without a hand lens.

Igneous rocks that cool far below ground are called plutonic, or intrusive. We wouldn´t find them at the surface at all except that mountain-building processes raise them up, and erosion clears away everything that was on top, leaving the once-buried rock exposed to the atmosphere - and to rockhounds!

Recognizing Igneous Rocks

How to recognize an igneous rock? There are two main types of igneous rocks. Plutonic rocks cool underground, and are later exposed at the surface by erosion. Plutonic rocks will have crystals that are large enough to see with your unaided eye. Granite, for example, typically looks like a bunch of pink, black, white, and clear crystals joined together.

Some igneous rocks aren´t plutonic, but rather are volcanic. Volcanic rocks are ejected from below the surface, and cool rapidly at the surface. That means the crystals that form don´t have time to "grow," and are too small to see without a microscope. Although they can be made of the same individual mineral grains as, for example, a granite, in this case the minerals are microscopic in size, and the rock just looks like a light grey, light pink, or dark black rock. If the rocks are hard, too hard to scratch with a nail, they are probably extrusive igneous rocks (like lava). On the other hand, the solid color rock is softer than a nail, it could be a sedimentary rock, like limestone or shale.

Volcanic (Extrusive) Rocks

Igneous rocks cool from melted material deep within the earth, called magma. Some magma makes its way to the surface (or very close to the surface) where it is exposed to surface temperatures. That means it cools quickly compared to magma that is more deeply buried.

Magma at the surface is called lava, and rocks that cool from lava are volcanic, extrusive, rocks. Of course, you knew that volcanic rocks come from volcanoes!

Volcanic rocks, because they cool so quickly, are fine-grained. That means you probably won´t see a lot of individual mineral grains without a hand lens or even a microscope. Even though the rock is made of different mineral grains, it often looks like a pretty uninteresting, solid-color rock.

So if you know that a rock is igneous, and it seems to be all one color, call it a volcanic rock.