2How Rocks Change Introduction All rocks change slowly from one type to another, again and again. The changes form a cycle, called "the rock cycle." The way rocks change depends on various processes that are always taking place on and under the earth's surface. Now let's take a closer look at each of these processes.
3Heat & PressureHeatDeep below the earth's surface, temperatures are hot enough to melt most rocks. However, before the melting point is reached, a rock can undergo many changes while in a solid state — changing from one type to another without melting.PressureAn additional factor that can transform rocks is the pressure caused by tons of other rocks pressing down on it from above.Heat and pressure usually work together to alter the rocks under the earth's surface. This kind of change is called metamorphism which results in a metamorphic rock.Click here to see the heat and pressure video:
4Melting & Cooling Melting The high temperatures required to melt a rock are generally found only deep within the earth. It takes temperatures between 600 and 1,300 degrees Celsius (1,100 and 2,400 degrees Fahrenheit) to melt a rock, turning it into a substance called magma (molten rock).CoolingLiquid magma also turns into a solid — a rock — when it is cooled. Any rock that forms from the cooling of magma is an igneous rock. Magma that cools quickly forms one kind of igneous rock, and magma that cools slowly forms another kind. Extrusive Igneous When magma rises from deep within the earth and explodes out of a volcano, it is called lava, and it cools quickly on the surface. Rock formed in this way is called extrusive igneous rock. It is extruded, or pushed, out of the earth's interior and cools outside of or very near the earth's surface. Intrusive Igneous Magma that gets pushed up toward the earth’s surface cools at a much slower rate than lava. The kind of rock formed in this way is called intrusive igneous rock. It intrudes, or pushes, into the earth's interior and cools beneath the surface.Click here to see the melting and cooling video:
5Weathering & Compacting Weathering, such as rain or wind, cause rocks to get eroded or broken down into smaller pieces. These small pieces of rock form the sediment that creates sedimentary rock.CompactingAs these little pieces of sediment get compacted together, they eventually for a sedimentary rock.Click here to see the weathering and compacting video:
7Types of Rocks Rocks are not all the same! The three main types, or classes, of rock are sedimentary, metamorphic, and igneous and the differences among them have to do with how they are formed. SedimentaryFormed from particles of sand, shells, pebbles, and other fragments of material.These particles or sediments accumulate and harden into rock.You can often see sand, pebbles, fossils or stones in the rock.Examples of this rock type include conglomerate and limestone. MetamorphicFormed under the surface of the earthGo through metamorphosis (change) that occurs due to intense heat and pressure (squeezing).Usually have ribbon like layers and may have shiny crystals.Examples of this rock type include gneiss and marble. IgneousFormed when magma or lava cools and hardens.When lava cools very quickly, no crystals form and the rock looks shiny and glasslike.Sometimes gas bubbles are trapped in the rock during the cooling process,leaving tiny holes and spaces in the rock.Examples of this rock type include basalt and obsidian.
8What to Look For:Here's a chart of some of the key characteristics that can help you identify the rocks within these three main classes.Sand or pebbles Individual stones, pebbles, or sand grains visible in the rock.Fossils Imprints of leaves, shells, insects, or other items in the rock.Gas bubbles "Holes," like Swiss cheese, in the rock.Glassy surface A shiny and smooth surface, like colored glass.Ribbonlike layers Straight or wavy stripes of different colors in the rock.Crystals Small, flat surfaces that are shiny or sparkly, like tiny mirrors.SedimentaryIgneousMetamorphic
9Let’s ReviewTo take the quiz hit the start button. If you don’t feel ready to take the quiz and need to look over the information more, press the back arrow.
10Which type of rock is this? metamorphicsedimentaryigneous
11Click this button to try again: WRONG!Click this button to try again:
12Correct!You can tell that this rock is igneous because of its shiny, glasslike surface.
13Which type of rock is this? metamorphicsedimentaryigneous
14Click this button to try again: WRONG!Click this button to try again:
15Correct!You can tell that this rock is sedimentary because it has large chunks of other rocks in it.
16Which type of rock is this? metamorphicsedimentaryigneous
17Click this button to try again: WRONG!Click this button to try again:
18Correct!You can tell that this rock is metamorphic because of the rippled layers.
19Which type of rock is this? metamorphicsedimentaryigneous
20Click this button to try again: WRONG!Click this button to try again:
21Correct!You can tell that this rock is sedimentary because you can see fossils embedded in the rock.
22Hopefully you have learned a lot about the rock cycle! The End!Hopefully you have learned a lot about the rock cycle!Information and videos from: