The Rock Cycle

In this lesson, students model the processes that cycle Earth’s materials, evaluating how energy from deep within Earth’s core and the sun powers the constant reshaping and re-forming of the materials that make up the planet. As Earth’s materials are transformed by heat and pressure, matter is never created or destroyed. Instead, it is changed from one form to another.

• The movement of Earth’s tectonic plates, powered by energy from Earth’s interior, is responsible for some of the processes that cycle Earth materials throughout the planet.
• In addition to energy from Earth’s interior, energy from the sun also plays a role in shaping Earth’s materials on the surface.
• Scientists divide Earth into smaller systems to better understand the processes that change Earth’s surface because the planet is so complex and so massive.
• Understanding the different parts of Earth’s systems, how those parts interact together, and how the systems interact with one another can help scientists better understand the processes that have shaped Earth over time.
• The processes that form, break down, and re-form rock from one category to another are called the rock cycle, and occur because of interactions among Earth’s four systems.

• How would you describe Earth’s structure?
• What is the relationship between Earth’s mantle and the crust?
• How can the movement of tectonic plates cause rocks to melt?
• What happens to matter when it melts? Why is energy needed for this process to occur?
• How does the sun’s energy power different processes that shape Earth’s materials on the surface?
• How do weathering and erosion cause Earth’s materials to change?
• Why is the geosphere a system?
• How can changes to Earth’s surface influence all of Earth’s systems?
• How does igneous rock form?
• Why is the rock cycle a cycle, and why are the steps interchangeable?

Misconception: Earth’s surface is constant and unchanging.

Fact: Heat and pressure are constantly reshaping Earth’s surface.

Misconception: Geological processes happen in human time frames, and any change will happen within a person’s lifetime.

Fact: Many of the changes to Earth’s surface occur over thousands or millions of years. Other changes can be very rapid.

Misconception: Earth’s systems interact independently of one another. 

Fact: The geosphere, atmosphere, biosphere, and hydrosphere are constantly interacting with one another, and a change in one system will affect the other systems.

Convection: heat transfer in fluids (liquids and gasses) where warmer, less-dense fluid rises, allowing cooler, denser fluid to take its place; causes a tumbling motion in the fluid

Erosion : the transport of sediment by wind, water, or gravity

Igneous rock : rock formed when hot liquid rock (either lava or magma) cools into a solid

Metamorphic rock : rock formed in chemical reactions where one type of rock is changed by pressure or heat into a new type of rock with different properties

Radiation:heat transfer that occurs without contact between the heat source and the object heated

Rock : mixed mineral matter that makes up Earth’s continents and oceanic crust

Rock cycle : the processes that form, break down, and re- form rock from one category to another

Sedimentary rock : rock formed from layers of sand, soil, clay, gravel, and other sediment that built up in one location over time

Weathering : the breakdown of rock into smaller pieces from exposure to wind, water, gravity, changes in temperature, and/or biological forces

A Scientific Record in a Volcano Myth

Don Swanson is a scientist who studies volcanoes. He is a former director of the scientific observatory that overlooks an active volcano in Hawaii called Kilauea. Volcanoes are structures formed around a hole in Earth’s crust that release magma (hot, molten rock).

One evening, he was reading a book of Hawaiian chants that told the legend of the goddess Pele. Islanders believed she was the goddess of Kilauea. According to the legend, Pele was violent and jealous. One time, she set fire to a forest because she was angry with her sister. That set off a chain of events that ended with the sister digging furiously in the ground, sending rocks flying in the air.

As he read, Swanson realized that the legend was actually providing a description and timeline of the two largest volcanic events that had happened on the island. First, the burning forest was most likely a lava flow from Kilauea in the 15th century that lasted for 60 years. This lava flow was so massive that it covered almost 430 square kilometers of the island of Hawaii, dramatically reshaping the land. Secondly, the rocks flying in the air were likely the result of a collapse of part of the volcano that resulted in a volcanic feature called a caldera. A caldera is a special kind of volcanic crater.

The reference to these two events dramatically changed how scientists viewed Kilauea because it offered a different timeline for when these events took place from what scientists had previously believed. Scientists are constantly looking for clues in Earth’s surface to figure out what happened in the past. They were excited to discover that the chants hold clues to Earth’s changing past, and are now digging deeper into the chants for more clues.

Volcanoes Connect Earth’s Systems

Scientists study volcanoes to learn more about Earth’s interior and some of the natural processes that have shaped the planet over time. For example, the volcano in Hawaii, Kilauea, has been constantly erupting since 1983. As a volcano erupts, it spews lava. It also releases gasses and ash into the atmosphere. Scientists are still trying to figure out exactly why the volcano has been so active, and where exactly it gets its heat source.

Scientists study volcanoes to learn more about Earth’s interior and some of the natural processes that have shaped the planet over time. For example, the volcano in Hawaii, Kilauea, has been constantly erupting since 1983. As a volcano erupts, it spews lava. It also releases gasses and ash into the atmosphere. Scientists are still trying to figure out exactly why the volcano has been so active, and where exactly it gets its heat source.

Scientists who study volcanoes need to have an understanding of Earth’s different systems, and how those systems interact together. Remember that a system is a set of connected, interacting parts that form a more complex whole. Earth is so massive and so complex that scientists study smaller systems to better understand how everything works. There are four main systems: the geosphere, atmosphere, hydrosphere, and biosphere.

The volcano itself is part of the geosphere. The geosphere is the Earth system made up of Earth’s solid materials, including its interior and surface features, such as landforms including mountains, valleys, rocks, and soil. Volcanoes release gasses into the atmosphere—the mixture of gasses, dust, water vapor, and other molecules above Earth’s crust. The hot lava and ash can be deadly for the biosphere, which is the Earth system made up of all living things.

Earth’s Surface Changes Over Time

Scientists know that Earth’s surface is constantly changing, and volcanoes are just one natural process that change Earth’s surface over time. Scientists study Earth’s processes to better understand the scale of the changes. Remember that scale is the size, extent, or importance (magnitude) of something relative to something else. For example, some natural processes occur rapidly. Others occur much more slowly, and can take place over millions or even billions of years. These are time scales.

At the same time, the changes to Earth’s surface can be small, such as when wind or water carries small bits of rock or other material to another location. Other changes are much more massive, including the uplifting of land to form a mountain range. These are spatial scales.

The changes to Earth’s surface are caused by interactions among all of Earth’s systems. For example, when rain falls to the surface, it involves the interactions of two Earth systems: the hydrosphere and the atmosphere. The hydrosphere is made up of all of the water on Earth, including ice, liquid water, and water vapor. As the addition of energy from the sun causes water molecules on Earth’s surface to heat up, they evaporate and turn into water vapor, which is held in the atmosphere. Water is constantly cycling between the hydrosphere and the atmosphere.

The Cycling of Earth’s Materials

As tectonic plates move, they change Earth’s surface and materials in different ways. For example, all of the rocks that are on Earth today are made of the same matter that existed when dinosaurs roamed. But rocks do not stay the same. The matter is reshaped and re-formed over millions of years into new rocks with different properties.

Most of the rocks found on Earth today started out as magma deep within Earth’s core. Over millions of years, the magma hardened, changed form, wore down, and re-formed into new kinds of rock. The processes that form, break down, and re-form rock from one category to another are called the rock cycle.

Heat and pressure are the primary causes of these changes. As far as 200 kilometers below the Earth’s surface, temperatures are hot enough to melt most rocks. It takes temperatures between 600 and 1,300 degrees Celsius (1,100 and 2,400 degrees Fahrenheit) to melt rock.

When two tectonic plates collide with one another, one plate sometimes is pushed beneath the other plate, back into the mantle. Because of the extreme temperatures, much of the rock melts, becoming magma again.

At these temperatures, rock can also become deformed without melting. For example, when tectonic plates collide, they compress the materials making up Earth’s crust. The result is that the crust becomes shorter and thicker, building mountain ranges. When two tectonic plates move away from each other, they stretch the crust, causing it to become thinner. Deformation is a very slow process, taking millions of years.

When magma cools into a solid, it forms a category of rock called igneous rock. Basalt and granite are both forms of igneous rock. As the magma cools, different-sized crystals are formed at different temperatures. This means the atoms are neatly organized to form a repeating pattern. This process is called crystallization, and it can occur rapidly or slowly. Slow cooling produces larger crystals than quick cooling.

For example, when lava spews from a volcano, it cools very quickly when it is exposed to the cooler temperature of Earth’s oceans or atmosphere. This produces small crystals. In contrast, some magma is pushed slowly toward Earth’s surface over many years. This magma will cool, but at a much slower rate than magma erupting from a volcano. This produces much larger crystals.

Another category of rock is formed as a result of the tremendous heat and pressure of Earth’s interior. Metamorphic rocks are rocks formed in chemical reactions where one type of rock is changed by pressure or heat into a new type of rock with different properties. For example, the heat of Earth’s magma and the pressure of the rock layers above turn soft limestone into hard marble.

In this lesson, students develop visual models to describe the cycling of Earth’s materials, driven by Earth’s internal energy and energy from the sun. Students analyze their models to explore the geosciences phenomena that shape Earth’s surface and change it over time, discovering different time and spatial scales for these processes.