17 Rocks and the Rock Cycle

Rock Cycle Formative Assessment

Types of Rocks

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

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

Geodes

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.

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.

Sand

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.

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.

NGSS

DCI

Second grade

ESS2.A: Earth Materials and Systems

• Wind and water can change the shape of the land. (2-ESS2-1)

fifth grade

ESS2.A: Earth Materials and Systems

• Earth’s major systems are the geosphere (solid and molten rock, soil, and sediments), the hydrosphere (water and ice), the atmosphere (air), and the biosphere (living things, including humans). These systems interact in multiple ways to affect Earth’s surface materials and processes. The ocean supports a variety of ecosystems and organisms, shapes landforms, and influences climate. Winds and clouds in the atmosphere interact with the landforms to determine patterns of weather. (5-ESS2-1)

middle school

ESS2.A: Earth’s Materials and Systems

• All Earth processes are the result of energy flowing and matter cycling within and among the planet’s systems. This energy is derived from the sun and Earth’s hot interior. The energy that flows and matter that cycles produce chemical and physical changes in Earth’s materials and living organisms. (MS-ESS2-1)

crosscutting

middle school

Cause and Effect

• Cause and effect relationships may be used to predict phenomena in natural or designed systems. (MS-ESS3-1)

Energy and Matter

• Within a natural or designed system, the transfer of energy drives the motion and/or cycling of matter. (MS-ESS2-4)

Stability and Change

• Explanations of stability and change in natural or designed systems can be constructed by examining the changes over time and processes at different scales, including the atomic scale. (MS-ESS2-1)