17 Rocks and the Rock Cycle

Rock Cycle Formative Assessment

Types of Rocks

Rocks are classified as three types: Igneous, Sedimentary, and Metamorphic.

Igneous Sedimentary Metamorphic
  • formed when magma cools
  • formed when many small particles called sediments are compacted together over time
  • formed when existing rocks undergo extreme heat and pressure
  • The early Earth was made of liquid magma that then cooled to form solid rocks, so this was the first type of rock.
  • Fossils are always found in sedimentary rocks.
  •  The word metamorphic means to transform or change shape; you may have heard the term metamorphosis–the process in which a caterpillar transforms into a butterfly.
Intrusive Igneous Rocks Extrusive Igneous Rocks
When magma cools underground, it cools more slowly. Since it is inside Earth, it is well protected and forms larger crystals. This is called an intrusive rock. When magma cools in water or on the surface of Earth, like after a volcano erupts, it cools more quickly and forms smaller crystals. This is called an extrusive rock.

Rock Cycle

The key processes in the rock cycle are heat & pressure and weathering & erosion. Each type of rock can undergo different changes that affect its form and the type of rock it is.

Igneous rocks:

  • Can undergo heat and pressure to become a metamorphic rock
  • Can undergo weathering and erosion to become a sedimentary rock
  • Can melt and become magma again. Then, it will cool and re-form as an igneous rock.

Sedimentary rocks:

  • Can undergo heat and pressure to become a metamorphic rock
  • Can undergo weathering and erosion, thereby breaking apart into sediments. Then, these sediments can be compacted again into sedimentary rocks.
  • Can melt and become magma or igneous rock

Metamorphic rocks:

  • Can undergo heat and pressure to transform into another metamorphic rock
  • Can undergo weathering and erosion to become a sedimentary rock
  • Can melt and become magma and igneous rock
The Rock Cycle
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A geode forms when a cavity forms in a rock, which can occur in different ways. One way a cavity can form occurs when a bubble of carbon dioxide and water vapor forms in flowing lava. As the molten rock cools and the gas dissolves, an empty space is left behind. Another possibility in which a cavity can form occurs when lava solidifies under water. Sometimes the outside of the melted rock cools before the inside, which becomes brittle and breaks a little bit due to the weight of the liquid inside.

However, cavities can also form in sedimentary rocks, such as limestone and sandstone, which produce the geodes we find in Iowa. In these particular rocks, the leading theory is that after deposition there was a period of erosion. In this time, the believe is that a more dense object carved out the cavity from the inside, like a shell moving around and gouging out the hollow space.  Then, a period of deposition occurred where mineral infused water penetrated the permeable surface and began filling the geode with quartz. In other cases, organic material, like a piece of wood, gets buried in the sediment and eventually weathers over time, leaving behind empty space. Once these rocks are hollow, or at least semi-hollow, various minerals are able to seep in through the rock’s microscopic pores, creating crystalline structures over long periods of time. Different minerals form different types and colors of crystals inside the geodes.

Archimedes’ Principle

Archimedes’ principle says that the weight of the displaced liquid is equal to the weight of the object. In class, we apply Archimedes’ Principle to figure out how dense a geode is. A geode that is less dense will have more crystals inside, which is usually the kind of geode you see cut open.

Watch the video, below, for an explanation of Archimedes’ Principle.

Video credit: “How taking a bath led to Archimedes’ Principle” by Mark Salata/TED-Ed is licensed under CC BY-NC-ND 4.0 

Like the Archimedes’ crown, a geode is an irregular shape so it is hard to measure the volume. We can use Archimedes’ principle to figure out whether a geode is solid or hollow. To do this, calculate the specific gravity of the geode.

Specific gravity

  • Formula for specific gravity: W(air) / (W(air)-W(water))
    • Weight of displaced liquid = weight of the object
  • The specific gravity for a geode is 2.7.
    • If the formula equals 2.7, it is likely the geode is solid quartz.
    • If the formula equals less than 2.7, the geode will be more hollow and have more crystals inside.

Weathering, Erosion, and Deposition

Weathering is the breakdown of rocks on Earth’s surface. There are two types of weathering:

  1. Mechanical: also known as physical weathering, rock is broken down into smaller fragments due to water, wind, or other conditions such as temperature and pressure changes
  2. Chemical: chemical reactions change the molecular structure of the rock

After the rocks are broken down through weathering, erosion can occur. Erosion is the process by which the small bits of rock are transported to a new location. Finally, deposition occurs when the particles are added to or deposited at a new location. These three processes act as a cycle, continually breaking down and building up different parts of Earth’s landscape.

Key Takeaway

Weathering is the making the mess and Erosion is cleaning it up.

K-6 Standards

2-ESS2-1. Compare multiple solutions designed to slow or prevent wind or water from changing
the shape of the land.

4-ESS2-1. Make observations and/or measurements to provide evidence of the effects of
weathering or the rate of erosion by water, ice, wind, or vegetation.



Sand is any rocky material that is bigger in size than silt and smaller than gravel. Sand is created when rocks are weathered, or broken down, in one of two ways: by water or by wind. When wind or water continually passes over a rock, it breaks it down into smaller and smaller pieces and sand is formed. Sand that was formed from weathering by wind tends to be pitted and frosted in appearance because other grains of sand have constantly been pelted against the rock. Sand that was formed from weathering by water tends to be smooth and polished because the water has continually passed over the rock.

Where did this sand come from?

Type of Sand Location Characteristics
Weathering by wind Dunes in the desert Scratched or frosted, pitted, uniform in size
Weathering by water Rocks near water Rounded, polished, smooth

Like fossils, sand can also tell a story. Based on where it is found in the world, sand is composed of different materials. Thus, it can also come in a variety of colors such as black, white, green, red and pink. Black sand, for example, is made from lava that has cooled to form an igneous rock; one place it can be found is near volcanoes in Hawaii.

Hawaii Black Sand Beach” by Ryan Keene is licensed under CC BY-NC-ND 2.0




5-ESS2-1. Develop a model using an example to describe ways the geosphere, biosphere, hydrosphere, and/or atmosphere interact. [Clarification Statement: Examples could include the influence of the ocean on ecosystems, landform shape, and climate; the influence of the atmosphere on landforms and ecosystems through weather and climate; and the influence of mountain ranges on winds and clouds in the atmosphere. The geosphere, hydrosphere, atmosphere, and biosphere are each a system.] [Assessment Boundary: Assessment is limited to the interactions of two systems at a time.]
MS-ESS2-1. Develop a model to describe the cycling of Earth’s materials and the flow of energy that drives this process. [Clarification Statement: Emphasis is on the processes of melting, crystallization, weathering, deformation, and sedimentation, which act together to form minerals and rocks through the cycling of Earth’s materials.] [Assessment Boundary: Assessment does not include the identification and naming of minerals.]




Second grade

ESS2.A: Earth Materials and Systems


fifth grade

ESS2.A: Earth Materials and Systems


middle school

ESS2.A: Earth’s Materials and Systems



middle school

Cause and Effect

Energy and Matter

Stability and Change



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