In this lesson, we will discuss the three main types of rocks and how they are formed. The lesson also gives an introduction into how matter locked in rocks can be cycled through the earth.
'Welcome to Earth Science TV! We are back once again to everyone's favorite show, What a Great Life! I am your host, Geo Logy. We have an all new participant this week, Ms. Crystal Rockena. Crystal, you have had an eventful life working your way through the rock cycle. You have met some interesting formations and been in some exotic places. Can you start by filling our viewers in on what the rock cycle is?' 'Sure, I'd be happy to!'
The rock cycle can take thousands of years to complete
When the earth was first created, the materials that it made were so hot from the contractions that formed the planet and from the heat generated by the core that the outer layers were mostly liquid. Over time, it slowly cooled, and a hard crust formed on the outer surface. This was much like the thin layer of ice that first forms over a pond when the temperature dips below freezing. And just as there is liquid under the layer of ice, there is still liquid magma flowing under the crust. As this happened, a cycle of events began to be put into place that causes changes in the rocks that exist on the earth. This cycle is known as the rock cycle. The rock cycle is a model used to describe the creation, alteration, and destruction of the rocks that form from magma.
It is important to note that new matter is never created on the earth. All the matter that the earth contains was present when the earth formed and still is here today. It may be in different forms, but it is still present. The rock cycle is the earth's way of recycling the matter used to make up rocks from formation to destruction and back to formation.
'And you participate in that cycle - isn't that right, Crystal?' 'Yes, Geo. All rocks, whether solid or liquid, are located at some point on that cycle.' 'How long does the whole cycle take?' 'It can take a long time, sometimes thousands of years.' 'Well, we won't ask your age or anything, but you still look great! So, Crystal, do you remember this voice?' 'Hi Crystal, remember when we both came up and were formed and spent the beginning of the cycle together? Those were some of the best times!' 'Wow! It's my good friend Iggy Neous! We started the cycle at the same time! We were igneous rocks because we were rocks that formed directly from magma or lava.'
Granite is a common igneous rock
That's right, Crystal. To begin the process, magma cools and hardens either under the ground or on the earth's surface. As the rock cools, the minerals that make up the rock take shape. All magma that solidifies becomes one of two types of igneous rock: either extrusive rocks, rocks that form on the earth's surface, or intrusive rocks, rocks that form under the earth's surface. Hawaii is made up mostly of extrusive rock called basalt that came from the volcanic activity common to the area.
The most common type of intrusive rock is granite, recognized by its pink or grey color flecked with light- and dark-colored mineral specks. Igneous rocks are constantly being formed on the earth whenever magma leaks to the surface through cracks or erupts in volcanoes.
'Ah, those were good times. That is where I got my name, you know, because the flecks of color you often see in igneous rocks are crystals of minerals!' 'That's lovely! After growing up in the relative safety of a boulder of granite, your life was headed for a big change, wasn't it, Crystal?' 'It sure was, Geo. It was a very difficult time. My home was broken down by the constant exposure to the elements, and then one day it happened. I was broken apart from my family and friends in a small bit of rock.' 'How traumatic!' 'Yes it was, but then I was on to the next phase of my life, learning to be sediment.' 'But don't forget about me!' 'My good friend Sandy Grain! If it wasn't for you, I never would have found my home!' 'Yes, we were all a bit gritty, but we learned how to stick together, right?' 'Absolutely!'
Limestone is a kind of sedimentary rock
Yes, Crystal, rocks that are exposed to the weathering nature of the elements can be broken apart. Small grains of exposed rock can be broken off by the action of wind and water or broken apart as water freezes in cracks. This can affect any type of rock, not just igneous. As time passes, these grains of rock accumulate in the slow currents in rivers or river mouths, low areas, or other quiets spots.
As sediment piles up, the first sediments get buried deeper and are subjected to increased heat and pressure. Sometimes water will flow through the sediment, dissolving and redepositing glue-like substances to bind the grains together into layers. Rock made from eroded material broken off other rocks and cemented together is called sedimentary rock. One of the best examples of sedimentary rock is the layers that are seen in the Grand Canyon.
There are many kinds and types of sedimentary rock. Sandstone forms mostly from quartz, or silica dioxide, the most common mineral found in sand. Limestone is another very common type of sedimentary rock often used to make gravel, and it forms from the calcium carbonate substances from dead sea animals or from ocean water. Fossils often can be found in sedimentary rock, as they get buried and trapped.
'You know, Geo, those were some challenging decades, but we stuck together through thick and thin. Eventually, those weathering processes uncovered us and tore us apart again. This time, I was deposited, but I was covered up very deeply. I knew it was time to change. I had been the same type of crystal my entire life. I needed to be transformed into something new.' 'Yes, and change you did. What happened next?' 'Well, as a rock, you never know when you'll be forced back into earth to be re-melted and start all over. But I wasn't ready for that yet. From all the heat and pressure in the situation I was placed in, I gradually felt all my physical properties changing. I was being remade!'
The metamorphic rock marble begins as limestone
'I became the third type of rock, called metamorphic rock. The word metamorphic can be broken down into its roots 'meta,' meaning 'change,' and 'morphos,' meaning 'shape.' Metamorphic rocks are rocks that are transformed into new substances under intense heat and pressure. In certain conditions, igneous or sedimentary rocks can be changed into different rocks under intense heat and pressure. These rocks literally change into new substances very different from their old ones.
This usually happens as rocks are buried deep within the crust. One of the most common metamorphic rocks is marble. Marble begins as the sedimentary rock known as limestone, and then, because of intense heat and pressure, it changes into marble, known for its many uses in sculpture and architecture.'
'And this is where you currently are now, in a beautiful piece of marble, correct?' 'Yes, Geo. But rather than being in a work of art on display, I am in a kitchen countertop, scuffed with knives and covered with chicken juice. At least I clean up nice, right?' 'You are lovely, Crystal.' 'One day I will make my way back to the earth's interior and be remelted. I will be all young again, but I always have great patience because the process is quite slow. I'm content to stay put right now.' 'Well, let me see if I have this all straight.'
All three rock types - igneous, sedimentary, and metamorphic - make up a portion of the rock cycle. Igneous rocks are rocks that formed directly from magma and can form either in the earth or on its surface. Igneous rocks can weather and be compressed to form sedimentary rocks, rock made up eroded material broken off of other rocks and cemented together, and both of these can be transformed into metamorphic rock. Metamorphic rock is rock that is transformed into new substances under intense heat and pressure. Any one of these rock types can be forced back into the mantle and re-melted to emerge again and cool as igneous rock. The process starts over again.
'You have it down, Geo!' 'Well, thank you so much for sharing the cycle with us, and you certainly can say 'What a Great Life!' about Crystal's progress through the rock cycle!'
After watching this lesson, you should be able to:
- Define rock cycle
- Identify and describe the three types of rocks that make up the rock cycle, as well as how each is formed
- Differentiate between extrusive and intrusive rocks