There is nothing like a good mystery to spark intrigue and interest into our boring lives. Whether that comes in the form of a suspenseful thriller movie, a good novel like Sherlock Holmes, that disturbing CSI show you can’t stop watching, or even just trying to figure out who stole your doughnut at work. But I bet you never picked up a rock to fulfill that same intrigue and mystery you yearn for. Each rock holds the most complex mystery you could ever imagine. Where did this rock come from? Where on earth (or underneath the earth) has it been? What minerals and elements make up this rock? If we are willing to listen, these rocks contain all of the clues to solving their great mysteries.
Last week I attempted to relate the three types of rocks to three types of people you would find at a neighborhood party. The scenario plays out as follows:
“Anybody who is anybody gets invited to the rock party. But once you have attended the rock party, you know that there are really only three types of people who attend the rock party. First you have the sedimentary people who are rough, rigid, and not afraid to tell you about the storms they have weathered (and they will probably use some expletives while they’re at it). Next you have the igneous people who are bold, beautiful, and generally the life of the party with their gaudy crystal jewelry and their lightweight pumice handbags. Finally you have the metamorphic people. These people are quiet at first, they keep all their heat and pressure inside of them rather then flaunting it like the igneous and even the sedimentary people do. But once you get to know the metamorphic people, you will find that the heat and pressure they have been enduring has made them delicate, refined, and beautiful. Once you dig even deeper past their crystal banded personalities, you will be shocked to discover they were just like the igneous and sedimentary people until they went through big life changes.”
Today I will describe in greater detail each type of rock party-goer from the rough and rugged sedimentary rock to the bold and boisterous igneous rock to the refined metamorphic rock. As I have already spoken briefly about igneous and metamorphic rocks (See my posts: “science snapshot igneous and metamorphic rocks”), I will focus mostly on sedimentary rocks today.
First a recap of the party animals known as igneous rocks.
Igneous rocks are the most diverse type of rock as far as physical characteristics are concerned. They can be smooth and glassy, bulky and filled with crystals, or rough and filled with holes. They may have multiple complex physical traits, but their method of formation is the same: the cooling of molten rock.
There are two main locations in which they form. They can form inside the earth or outside the earth. If magma cools inside the earth, igneous rocks with mineral crystals form. The slower the cooling process means the bigger the mineral crystals.
If lava cools on the surface of the earth, then we can either get smooth and glassy igneous rocks or rough and holey igneous rocks. This has to do with the way the lava reached the surface of the earth. If it was a volcanic eruption with mostly slow moving lava and no gas-filled discharge, then we get smooth, glassy igneous rocks like obsidian. If it was a volcanic eruption with a mix of lava and gas-filled discharge (either a gentle or explosive eruption), then we get a hole-filled, rough igneous rock like pumice. The gases get trapped in the igneous rock as it cools, thus producing holes.
Picture your favorite soda. Mine is Mountain Dew. Now do the crazy thing no soda addict would ever do in a hundred million years, shake up your soda and open the bottle. The smooth, rich syrup at the bottom of the bottle would form an igneous rock similar to obsidian while the gas filled soda hurling out of the bottle would form an igneous rock similar to pumice.
Igneous rocks have many uses in humanity. We use them to make your beautiful granite kitchen counter tops. We use them in cheap construction materials like the sheet rock that forms the walls of your home. We even use them for beauty products! Pumice is used by some people as an exfoliation scrub, to wipe away dead skin.
Next a recap of the dynamic metamorphic rocks.
Metamorphic rocks are the most exotic and complex in formation. They can form rocks with beautiful crystal bands or even rocks with smooth, sleek crystalline finishes. They form from intense heat and pressure deep inside the earth. I am not talking about the heat needed to melt rock (that’s igneous rocks’ territory), but just enough heat to make the rock a malleable, almost plastic-like material. The characteristics of these metamorphic rocks is based on how much pressure and stress they were under, and what type of rock they were before receiving the pressure. Metamorphic rocks are the butterflies of the rock world!
Here are a couple of examples of my favorite metamorphic rocks and how they formed…
Quartzite: First you start with a rather drab, gritty rock known as sandstone and after some intense heat and pressure, you end up with the beautiful quartzite. The boring quartz sand grains metamorphose into large, intergrown quartz crystals after being buried deep in the earth and receiving large amounts of heat and pressure. A common characteristic of metamorphic rocks is the distinguishable bands or crystals layers (sometimes called foliation) within the rock. However, some metamorphic rocks are not foliated. Quartzite contains random crystal patterns similar to igneous rocks, the difference is that quartzite did not form from melting and cooling, but from heat and pressure. I love this rock because of the stark difference between the ‘before’ and ‘after’ rocks. It really is a motivational story, the Cinderella of the metamorphic rocks.
Gneiss: All the rock puns you can think of aside, Gneiss is actually a really ‘nice’ rock! It forms from a rock known as granite which contains large, chunky, randomly dispersed crystals. Granite undergoes a hefty dose of heat and pressure to form Gneiss. Granite became malleable from tons of heat, then the stress from the pressure caused the crystals to form bands. Imagine stomping on a granite rock harder than the incredible Hulk has strength and you would cause those randomly dispersed crystals to line up into distinct bands. Hopefully you now made the connection and realize Gneiss is an example of a foliated rock. For gneiss to form, granite rock needs to undergo an almost unfathomable amount of heat and pressure. Other metamorphic rocks, like slate or schist, do not need quite as much heat and pressure. Next time you are in a sauna room feeling like you may die from the heat, just imagine how ‘nice’ it is not to be gneiss who has to suffer through way more heat and pressure than you do ever in your life!
Finally a description of the coarse and gritty sedimentary rocks.
The type of ways that sedimentary rocks can form is arguably the most diverse of any of the three rock types. Though their appearance is almost always gritty and rough, a sedimentary rock always has a unique and interesting story to tell. Some sedimentary rocks may have been a beautiful mineral before becoming a sedimentary rock. Some sedimentary rocks may have been a dinosaur or a tree before becoming a sedimentary rock. Take a closer look at the three ways a sedimentary rock can form.
- Broken particles of pre-existing rocks and minerals: Take any fragment of a rock, whether it is a mineral like quartz, a pebble from a stream, a grain of sand, or even a glob of mud. Now add various ‘weathering’ processes to that fragment of a rock. This could be any combination of wind, rain, erosion, a retreating glacier, and so forth. Next pile on multiple layers of different rock fragments. Maybe that means sand on top of sand on top of sand. Or multiple pebbles from a dried up stream bed. The final ingredient is time. Time for more water to seep into the clump of rocks and more weathering to occur causing them to cement together. Mix all that together and you get a sedimentary rock like sandstone, conglomerate, and shale. Sandstone is made from a bunch of sand clumped together from this process, conglomerate is made from a bunch of pebbles, and shale is made from a bunch of mud.
2. Remains of organisms’ growth processes and skeletons: Trees that die and become buried, microscopic organisms that die and sink to the bottom of the ocean, and even your pet dog you buried in the backyard all have two things in common. One, they’re dead. Two, they will eventually, after many many years of compaction and weathering, become part of a sedimentary rock. Even dinosaur fossils are considered sedimentary rock. Other common sedimentary rocks that fit the profile above include coal, which is formed from plants in a swamp being buried for a long time; chert, which forms from microscopic organisms called diatoms being compacted at the bottom of the ocean; and limestone, which is formed from living marine organisms in a coral reef.
3. Dissolved materials crystallized from water and deposited in solid form: I once found a rock hiking in the mountains. It is my favorite rock. It had holes all throughout it and beautiful while crystal bands within it. I immediately thought it was metamorphic due to the crystal bands. Then I thought it was igneous because of the holes and the crystals, upon closer inspection, appeared more chunky. Years later I came to the conclusion that it was actually a sedimentary rock. Mostly due to the location of this rock, I figured out that the crystallizing patterns in this rock were formed from water cutting into the rock for many years, evaporating, and leaving behind salt crystals within the rock. Take a walk into a cave and you will see a beautiful sedimentary rock lining the ceilings and walls known as limestone. Walk along the salt flats and you will see a beautiful sedimentary rock glistening in the sun known as rock salt or rock gypsum.
Even the rough and tumble sedimentary rocks have significant purpose within our world. We use them as a preservative for foods (rock salt) or to power our vehicles and heat our buildings (coal) or as a window to the past (sandstone monuments, cliffs lined with sedimentary rock, and fossilized organisms). Indeed, common sedimentary rocks appear dull at first, but these rocks hold immense value when it comes to energy, history, and even shaping life itself. The great coral reefs are formed from the sedimentary rock known as limestone and the coral reefs are perhaps one of the most beautiful and biologically significant structures in the world.
Conclusion: Say what you will about the crazy rock party goers, but no one can argue their importance in our lives. We use them for building materials, as an energy source, and as beauty supplies. We even wear them around our necks, on our ears, and around our wrists. But looking beyond their practical uses, rocks have played a key role in solving mysteries of earth’s past. Fossils tell us a lot about what life was like many years ago. All three types of rock tell us much about the formation of the earth and its ongoing life. Hopefully the next time you pick up a boring old rock, you will actually pause and try to solve the mystery that lies within that rock. Where did that rock come from? What minerals and conditions aided in the formation of that rock? What type of rock is it currently? Each and every rock has an unbelievable story to tell if we will listen.
If you read this whole blog post, you are my hero! Thanks for reading. Please feel free to leave me a comment and feedback below. Let me know what your favorite rock type is and why. Also, let me know if there is any particular science topic you would like me to look at “through green tinted glasses” and I will write about it.
**All of the photos in this post were taken by me, myself, and I. I have an obsession with taking pictures of rocks and bugs… Also most of the information collected for this blog post were personally given to me by the amazing Natural History Museum of Utah. 10 stars out of 5, I highly recommend you visit that museum if you are ever in the area!