Rock Cycle: Igneous, Sedimentary, and Metamorphic Rocks

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  • 0:09 Introduction
  • 0:37 Rock Cycle
  • 2:23 Igneous Rock
  • 4:00 Sedimentary Rock
  • 5:55 Metamorphic Rock
  • 7:12 Lesson Summary
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Lesson Transcript
Instructor: Peter Jaeger

Pete currently teaches middle school Science, college level introductory Science, and has a master's degree in Environmental Education.

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!'

Rock Cycle

The rock cycle can take thousands of years to complete
Rock Cycle Diagram

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.'

Igneous Rock

Granite is a common igneous rock
Igneous Rock Granite

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!'

Sedimentary Rock

Limestone is a kind of sedimentary rock
Sedimentary Rock Limestone

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.

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