Rocks and minerals are a source of many things we use in our daily lives. For instance, cobalt, which is used to make lithium-ion batteries, is sourced from cobaltite, a rare mineral. Your kitchen counter might be made of granite, an intrusive igneous rock consisting primarily of quartz and alkali feldspar.
What does intrusive mean? Is there a difference between minerals and rocks? How are rocks formed and reformed?
Like many things on Earth, rocks change in a non-linear cycle, aptly called the rock cycle. Let’s explore the rock cycle as well as a few fun rock facts.
The rock cycle
There are three main types of rocks: igneous, metamorphic, and sedimentary.
Igneous rocks are those made of cooled magma. Magma is molten material found primarily beneath the surface of the Earth; when on its surface, magma is known as lava. Igneous literally means “resembling fire.”
Since magma can be found under the crust of and on the surface of Earth, igneous rocks form above and beneath the surface. Igneous rocks that form beneath Earth’s surface are called intrusive (or plutonic) igneous rocks. Extrusive (or volcanic) igneous rocks are those that are made on Earth’s crust.
Grains of matter in rocks can reveal a lot about how they were forged.
Large grains and a coarse texture are often indicative of intrusive rocks that spent a lot of time cooling beneath the crust – enough time for crystals to form in the mineral. Granite is an example of an intrusive rock. Finer textured rocks with smaller grains cooled quicker and are generally extrusive. In some especially fast-forming rocks, such as pumice, the grains are so small they are difficult to see.
Another feature of pumice is its vesicular texture. Vesicular rocks (which are all extrusive) are made from rapidly cooling magma with free gases in it, which leaves cavities called vesicles in the rock. These many vesicles make the rock porous.
Igneous rocks might melt and become magma again. Alternatively, weathering and erosion could turn them to sediment. Or, heat and pressure could transform them into metamorphic rocks.
The term metamorphic is derived from the word metamorphosis, which means “the state of changing form.” Metamorphic rocks form when other rocks are heated and pressurized to a certain point. Similar to igneous rocks, metamorphic rocks are classified into two broad groups based on how they are made: foliated and nonfoliated.
Foliated rocks are those composed of layers created by pressurizing flat or elongated minerals. When pressure is applied to foliated minerals, they align perpendicular to the direction the pressure is applied. Schist and muscovite (mica) are two foliated metamorphic rocks.
Metamorphic rocks that do not contain elongated minerals are called nonfoliated. Marble – metamorphosed limestone – is usually nonfoliate. Nonfoliated rocks form by metamorphism, which happens when lava comes into contact with other rocks, or from pressure applied to sedimentary rocks.
As a part of the rock cycle, metamorphic rocks can melt to become lava (and later, igneous rock), become heated and pressurized again to forge a new metamorphic rock, or break down into sediment. In fact, all three kinds of rock can turn into sediment through weathering and erosion.
Sediment is typically rock fragments, but some sediment is made up of organic material. Deposits of sediment eventually compact and cement to form sedimentary rocks. There are three classes of sedimentary rocks – clastic, chemical, and organic.
Clastic sedimentary rocks form exclusively from sediments of other rocks (clasts). Sandstones are one such sedimentary rock.
Like clastic rocks, organic rocks are made through weathering and erosion. However, they contain organic material in addition to minerals. Bones, plants, and shells are some of the organic materials that might become compacted into organic sedimentary rocks. A common source of energy, coal is an organic rock. Coal is composed of compacted preserved plant material.
Finally, chemical sedimentary rocks are formed from a solution of water and minerals when the water evaporates. Flint, a form of quartz, is a chemical sedimentary rock.
Sedimentary rocks might be heated and pressurized and turn into metamorphic rock, or they may melt into magma and eventually become an igneous rock. Further pressure applied to a sedimentary rock can create new sedimentary rocks.
Rocking rock facts
You might be wondering what the difference, if any, between a mineral and a rock is. There is a difference, although I’m pretty sure I (erroneously) used them interchangeably in this article.
A mineral is an inorganic substance with a definite chemical composition and a crystalline structure, such as olivine. Olivine is a green colored crystal that is known as peridot when polished. Mineral-like compounds that are not crystalline are called mineraloids.
Rocks are made up of one or more minerals, though rocks may also contain organic material. Some rocks are single minerals, but not all. Breccia is a rock, but not a mineral. To be specific, breccia is a cemented clastic sedimentary rock! Oh, and stones are just small rocks.
And speaking of peridot, a gemstone . . . what is a gemstone? Gemstones, or precious stones, are usually polished crystalized minerals. Some gemstones, like the bright blue lapis lazuli, are rocks. Opals are a precious stone made of a mineraloid. And one gemstone, the pearl, is a calcium carbonate deposit, but not really a rock.
Pumice is an extrusive rock, formed when magma cools quickly. This process happens so fast that underwater, giant slabs of pumice are created. Pumice floats, so these “pumice rafts” (which are primarily found in the South Pacific) drift through the sea. Astrobiologists and biologists believe that early organisms used pumice rafts to travel between islands. In the modern day, pumice rafts serve as a hazard to ships.
Also notably, pumice is one of the only rocks that floats on water. It has a very low density, but this is not the only reason it can float! Indeed, pumice floats as a result of surface tension. At a molecular level, there are gas pockets that water can reach. Water surrounds gas molecules, allowing the rock to float. Pumice sometimes sinks in cold temperatures or at night because gas expands in the heat, and hot air rises.
While pumice is created rapidly, other rocks are shaped much slower. Sedimentary rocks especially compact and cement over a long period of time. Coal is an organic sedimentary rock that began forming in the Carboniferous Period – 360 to 290 million years ago – particularly in buried peat bogs and swamps. A fossil fuel, coal is considered a nonrenewable resource because it takes so long to be made.
Additionally, the combustion of coal is required to extract energy from it. But combustion also releases greenhouse gases, which trap heat in the atmosphere. Carbon dioxide (CO₂) makes up a large proportion of greenhouse gases emitted by coal combustion. In 2018, CO₂ emissions from the burning of coal and other fossil fuels made up 93% of all anthropogenic CO₂ emissions in the United States.
Continuing to heat and pressurize a rock will result in a predictable series of rocks. The most prominent sedimentary rock, shale, will first metamorphose into slate, then phyllite, schist, and gneiss. Gneiss can also form from the metamorphosis of granite.
Basalt makes up 90% of all volcanic rock on Earth; a large part of the oceanic crust is basalt. Some island nations, including Hawaii and Iceland, are mostly basalt. Basalt is also common on the moon and Mars!
My favorite gemstone is moonstone. When polished, feldspar-containing minerals demonstrating adularescence become moonstone. Adularescence is a soft glow of light that can be seen on some polished gemstones.
I didn’t think I had a favorite mineral. However, it turns out garnet is a mineral and not just a gemstone. When you think of a garnet, a shiny red gem probably comes to mind. Garnets have a number of uses beyond looking pretty, though. In the United States, garnets are often utilized in abrasives and water filters.
Do you have a favorite gemstone, mineral, or rock? If so, comment below with some information on it!