The Rock Cycle

In this unit, students use what they know about the relationship between energy and matter to investigate phenomena of how energy powers the cycling of Earth materials. They begin with this lesson on modeling the Earth processes that form different kinds of rock. This page provides an overview of this lesson.

Science Background for Teachers:

Science background gives teachers more in-depth information on the phenomena students explore in this unit. Below is an excerpt from the science background section on the rock cycle.

Relationship Between Energy and Matter on Earth

Understanding Earth begins with the materials that make up Earth. Geologists need to have a good understanding of the relationship between energy and matter, and how energy changes matter, because none of the changes that take place on Earth’s surface can occur without energy.

Rocks are some of the most common materials on Earth. Most of the Earth’s ocean’s floors are made up of a dark, finely grained rock called basalt. Most of the continents are made up of granite, a coarse rock that often looks pink with black specks.

Both granite and basalt have something in common. Like all rocks, they are made up of mixtures of minerals that have been pressed together by heat and pressure. A mineral is a naturally occurring, inorganic solid with a crystal structure and its own chemical formula. Minerals are made of different elements that join together. Both granite and basalt have large amounts of the elements silicon and oxygen.

The matter that makes up rocks is never created or destroyed. Instead, it is constantly changed by processes that happen on Earth’s surface and deep within Earth’s interior. Heat, pressure, and time reshape and re-form one kind of rock into different kinds. The processes that form, break down, and re-form rock from one category to another are called the rock cycle.

Energy from Earth’s Interior

Some of the energy that shapes Earth’s surface comes from deep within Earth’s interior. Earth is divided into four layers. The inner core is Earth’s hottest layer, and it is made up of a mixture of solid iron and nickel. It is surrounded by an outer core that is made up of a less dense mixture of liquid nickel and iron.

Surrounding the inner and outer core is the mantle, which is mostly molten, semi-solid rock called magma. The mantle makes up almost two thirds of the Earth's volume and is about 2,900 km (1,802 miles) thick. The mantle is in constant motion, powered by the transfer of energy between materials.

Remember that heat is energy that has transferred whenever two substances are at different temperatures, and it always flows from faster-moving atoms (warmer substances) to slower-moving atoms (cooler substances) until both substances reach the same temperature.

Because of Earth’s structure, heat is continuously flowing from Earth’s center outward. Heat from Earth’s core warms the lower part of the mantle through conduction (heat transfer that occurs when molecules collide). When rocks near the core are heated, their particles move faster.

Earth Cross-Section

As the particles move faster, they become more spread out and less dense than the cooler, upper mantle rocks, whose particles are much slower. The cooler particles clump together, becoming denser. The warmer rocks rise while the cooler rocks sink, creating slow currents within the mantle. This tumbling motion describes a form of heat transfer called convection, which occurs in fluids (liquids and gasses).

The motion of convection in Earth’s mantle plays a major role in shaping Earth’s surface because Earth’s surface is fragmented into drifting slabs of solid rock, called tectonic plates. As magma moves beneath the crust (Earth’s crust is the hard rock layer of the planet that supports the continents and oceans), it pushes the tectonic plates toward or away from each other. The places where Earth’s tectonic plates meet are called fault lines.

Convection Powers Tectonic Plates

The tectonic plates are made up of Earth’s outermost 100 kilometers, made up of the crust and the coolest, strongest part of the upper mantle. This is called the lithosphere. The lithosphere floats on a zone of weak, melted rock called the asthenosphere.

The plates move extremely slowly, no more than a few centimeters a year, although the different plates move at varying speeds and in different directions. Scientists continue to study the complex behaviors of the plates.

Scientists know that as plates come into contact with each other, they transfer tremendous amounts of energy that cause various geological processes to happen at their boundaries. Their movements cause mountains, valleys, and oceans to form. These collisions between the plates are also the cause of earthquakes and volcanoes. Earthquakes are the shaking of the ground caused by a sudden release of energy when two tectonic plates suddenly slip past one another. Volcanoes are structures formed around a hole in Earth’s crust that release magma. When magma reaches the surface during an eruption, it is called lava.

Because of how they are formed, earthquakes and volcanoes are not found randomly across Earth’s surface. Instead, most are found along plate boundaries: on the edges of continents, near island chains, or underneath the ocean.

Creating New Land

The movement of the tectonic plates causes materials from deep within Earth’s interior to erupt onto Earth’s surface through volcanoes. When an eruption causes hot molten magma from deep within Earth’s interior to reach the surface, it cools, forming a category of rock called igneous rock. The word igneous means “from fire.” Basalt and granite are both forms of igneous rock.

As the lava 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.

When magma 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. Igneous rocks formed this way are fine-grained or glassy, such as basalt and obsidian.

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, which results in coarser rock, such as granite.

As volcanoes spew lava onto the surface, where it cools into igneous rock, it produces new land on Earth’s surface. Because of this, the ongoing eruption on Kilauea has added 500 acres of new land since it first started in 1983. Because of this process, this is now some of the newest land on Earth. Like all matter, it came from already-existing Earth materials, and through various Earth processes, it was transformed into new land.

The Cycling of Earth Materials

There are other ways that geoscience processes cause Earth materials to change. For example, when two tectonic plates collide with one another, one plate sometimes is pushed beneath the other plate, back into the mantle. 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. Because of the extreme temperatures, much of the rock from the tectonic plate melts, forming magma.

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.

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.

If rock gets buried deep inside Earth, heat and pressure will deform it, changing its external structure. Or the heat and pressure will cause chemical reactions that change the chemical structure of the rock, changing its properties.

Weathering and Erosion

When rocks on Earth’s surface are exposed to changes in temperature, wind, water or biological forces (such as plants or animals), they experience weathering.

Weathering is the result of interactions among all of Earth’s systems. For example, one of the gasses in the atmosphere is carbon dioxide. When water falls to Earth’s surface as rain, it carries some of this carbon dioxide, making the water slightly acidic. This slightly acidic water causes chemical weathering of the rocks on Earth’s surface. Chemical weathering occurs when chemical reactions break down the bonds holding the rocks together, causing them to fall apart, forming smaller and smaller pieces.

This is an interaction between the hydrosphere and the geosphere. The chemical weathering breaks down the rocks, transforming the matter into new substances with different properties, including salt and other minerals. Chemical weathering generally occurs gradually over time.

Wind and water also cause mechanical weathering. Mechanical weathering takes place when rocks are torn apart by physical force without any change in their chemical nature. The constant freezing and thawing of water is one of the most common types of mechanical weathering.

Rocks are penetrable enough for water to seep into the cracks between particles. During cold months, the water turns to ice and expands, putting force on the rock. This is an interaction between the geosphere and the hydrosphere.

Rocks are also weathered by plants and animals. Plant roots that grow into the cracks between rocks weaken the structure. Earthworms and other burrowing creatures have a similar effect, spreading out rock particles and allowing water to reach deeper into the soil.

The Rock Cycle

As rock is weathered, it is broken down into sediment. That sediment is then transported by wind, water, and gravity to new locations, a process called erosion. The friction of erosion hastens weathering. Rocks grind against one another as the wind blows small bits of sediment against whole rock surfaces. Moving water currents wear down sharp cliff edges and drag loose rock to new locations along riverbeds. Sediment formed from weathering and erosion slowly accumulates in layers in oceans, lakes, and valleys through a process called sedimentation.

Heat and pressure compact the layers of sand, soil, clay, gravel, and other sediment that build up in one location over time into sedimentary rock. The layers represent a specific period of time. The oldest sediment forms the bottommost layers of the rock, while the new layers form at the top.

The processes that drive the rock cycle happen all the time and interchangeably. Wind and water can break sedimentary rock into smaller pieces of sediment that become new sedimentary rock. Igneous rock is the newest rock, formed from molten magma deep inside earth or cooling lava. Sedimentary rock can sink into Earth’s interior, where heat and pressure will turn it into metamorphic rock. Energy is the primary driver behind the rock cycle, breaking down and reforming rocks through heat and force.

Supports Grade 7

Science Lesson: Exploring 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.

Science Big Ideas

  • 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.

Sample Unit CTA-2
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Science Essential Questions

  • 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?

Common Science Misconceptions

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.

Science Vocabulary

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

Lexile(R) Certified Non-Fiction Science Reading (Excerpt)

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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.

pattern
 
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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.

Hands-on Science Activity

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.

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Science Standards

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Standards citation: NGSS Lead States. 2013. Next Generation Science Standards: For States, By States. Washington, DC: The National Academies Press. Neither WestEd nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.