The Rock Cycle: Conservation of Mass & Changes

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we'll review the rock cycle. Then, we'll get into how the law of conservation of mass in physics applies to the rock cycle. As we do this, we'll also cover the physical and chemical changes occurring in the rock cycle.

What Is the Rock Cycle?

Picture a rock. Pretty boring, huh? Now, envision a rocky seacoast in the Pacific. Slabs of sandstone rocks jut out of the ocean, forming tall cliffs. Fine particles of rocks form sand and pebbles on the beach. Mountains above the shore are thick slabs of black rock formed from volcanic eruptions many years ago. As you can see, a rock isn't just a rock. There are many types of rocks and they form from different processes. But, all rock formations are connected through the rock cycle. In the rock cycle, natural pressures like weathering, erosion, heat, and compression change the type of rock present. There are three main types of rock in the rock cycle: igneous, sedimentary, and metamorphic.

Types of Rock

Igneous rock is formed when magma, or liquid rock, leaves the Earth and cools, such as during a volcanic eruption. Over time, wind and water break down the igneous rock into tiny pieces called sediments. Sometimes, these pieces can be crushed small enough to form the sand we find on beaches. Eventually, the sediments fall into layers and are compressed over time, forming sedimentary rock.

Sedimentary rock was carved out by wind and water to form Antelope Canyon in Arizona
sedimentary rock

Over time these layers will be pushed deeper and deeper into the Earth as the sedimentation process continues. As the rock is compressed and heated, it starts to form crystals in the Earth, and eventually becomes a type of rock called metamorphic rock. The metamorphic rock is melted by the intense heat inside the Earth and forms magma to start the rock cycle again.

The rock cycle
rock cycle

Conservation of Mass

So, now that you know some basics about the rock cycle, think about these questions: Does the Earth ever make new rocks? Where does rock come from?

Sometimes it's easy to think that because the rocks are changing form, new rock must be materializing. But, this actually is false. All of the rock in the Earth is recycled and accounted for during the rock cycle. This principle applies to other areas of science as well and is known as conservation of mass, which says that mass cannot be created or destroyed but is cycled between different forms.

You can think of conservation of mass in the rock cycle as rock recycling. Let's say we have two 20-ounce plastic cola bottles. They serve their purpose and we drop them in the recycling bin. With water, pressure, and heat, the recycling company melts the bottles down to make new bottles, maybe milk jugs. The same amount of plastic still exists, it's just in a different form.

Physical and Chemical Changes in the Rock Cycle

There are two main changes that drive the rock cycle, physical and chemical. In a physical change, the composition of the material stays the same, it may just change how it looks. However, chemical changes occur when a substance undergoes a chemical reaction that changes the actual makeup of a substance. Let's look at how this affects each of the three types of rock.

Igneous Rocks

Igneous rocks are formed when molten rock inside the Earth is ejected onto the surface and cools. Inside the Earth, the extremely high pressure and high temperatures keep the rock in a liquid phase. All the atoms and ions inside the rock are disorganized at this temperature, floating around freely. As the magma leaves the Earth, the rock undergoes a physical change, cooling to a solid.

However, chemical changes are also taking place at this time. As the magma cools, silicate minerals, compounds made of silicone and oxygen, start to combine. Different minerals are formed through different chemical reactions depending on the makeup of the magma and how fast it cools.

Basalt rock is a type of igneous rock formed inside the Earth
igneous rock

Sedimentary Rocks

Igneous rocks go through the physical change of erosion, where the rock is broken apart by wind and rain. The resulting sediments are deposited on the ground or ocean floor and start to be compressed into sedimentary rock.

Sediments are compressed to form sedimentary rock
compression

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