Weathering Rocks

Once students understand Earth’s structure and how Earth’s features are formed by the movement of tectonic plates, they study rocks, which are mixtures of minerals. In this lesson, they analyze how matter is re-shaped and re- formed in the rock cycle, focusing on the geologic process of mechanical weathering.

• Rocks are mixtures of different kinds of minerals, and are categorized into one of three categories (igneous, metamorphic, and sedimentary) depending on how they formed. 
• The rock cycle refers to the processes that form, break down, and re-form rock from one category to another. It is a cycle because it is always happening, changing one type of rock into another. 
• Weathering can be either chemical or mechanical, but both forms result in rocks being broken down into sediment. 
• Weathering and erosion can be understood by looking at interactions among Earth’s systems (sets of connected, interacting parts that form a more complex whole).
• Interactions among these four systems (biosphere, geosphere, atmosphere, and hydrosphere) shape the planet.
• Rocks, tectonic plates, and minerals are part of the geosphere, which includes Earth’s landforms, as well as its rocks and soil. The other systems interact with the geosphere to cause weathering and erosion.
• Rocks have different properties because they are made up of different combinations of minerals, which are made up of different kinds of elements. An element is entirely made up of one kind of atom. A rock’s properties are determined by the number and kind of atoms that make it up.
  
  

• How are different properties useful for describing different rocks?
• What properties do all igneous rocks share?
• How are the properties of sedimentary rock connected to how this category of rock forms?
• How does the rock cycle support conservation of matter, which states that matter is never created or destroyed?
• How is igneous rock that reaches Earth’s surface re-shaped as part of the rock cycle?
• How is mechanical weathering different from chemical weathering?
• Why is abrasion considered a form of mechanical weathering?
• What is the relationship between the solid rock layers of Earth and sediment?
• Why is weathering by wind, water, or gravity an example of mechanical weathering?
• How can wind change mountains through weathering and erosion?
• How do Earth’s systems interact when rocks are weathered and eroded?
  
  

Misconception: Weathering and erosion refer to the same process.

Fact: Weathering and erosion are two different processes that often work closely together (weathering breaks down rocks and erosion transports the sediment).

Misconception: Volcanoes and earthquakes are located randomly around Earth.

Fact: Volcanoes and earthquakes result from the movement of Earth’s tectonic plates, so their locations usually correspond to plate boundaries.

Abrasion :  a type of mechanical weathering that occurs when rocks and sediment grind against each other

Chemical Weathering :  a kind of weathering that occurs when chemical reactions change the composition of rocks, rearranging the atoms or molecules into new substances with different properties

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

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

Mechanical Weathering :  a kind of weathering that occurs when rocks are broken down by physical force without any change in their chemical structure

Metamorphic Rock :  a category of 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

Rocks :  mixed mineral matter that makes up the surface of Earth and other terrestrial objects

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

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

System :  a set of connected, interacting parts that form a more complex whole

Weathering :  the breakdown of rock into smaller pieces from exposure to wind, water, changes in temperature, and/or biological forces; there are two kinds: mechanical and chemical

Igneous Rock

Many diamonds are found in a kind of rock called kimberlite. Kimberlite rocks are generally only found in very old parts of Earth’s crust. Kimberlite is a kind of igneous rock. Igneous rocks are often found around volcanoes because they form when hot semi- solid or liquid rock (either magma or lava) from Earth’s mantle cools into solid form. The magma either cools slowly below Earth’s surface or is released as lava in volcanoes.

When magma spews from a volcano, it cools quickly when it is exposed to the cooler temperature of Earth’s oceans or atmosphere. The cooled lava hardens into igneous rock. Rocks formed this way are fine-grained or glassy.

However, not all magma erupts out of a volcano. Some magma is pushed slowly toward Earth’s surface over many years. This magma will cool more slowly than magma erupting from a volcano. Igneous rocks formed this way are coarser.

Categories of Rock

Sedimentary rock forms from layers of sand, soil, clay, gravel, and other sediment that built up in one location over time. Sediment is a collection of particles and pieces of living and nonliving things. It collects in layers over time. Over thousands of years, the pressure of more and more top layers of sediment compresses the bottom layers of sediment into solid rock. The oldest sediment forms the bottom-most layers of the rock. Newer layers replace older layers at the top.

Metamorphic rock is a category of 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. The movement of the tectonic plates can cause rocks to get pushed down deep into the crust. There, the heat and pressure of all the weight on top of it cause chemical reactions in the rock to change it into a new rock with different properties. In a chemical reaction, the molecules of a substance are rearranged into a new substance with new properties. The word “metamorphose” means “to change or to transform.” It can take millions of years for a metamorphic rock to form.

The Rock Cycle

Rocks do not remain the same forever. The matter that makes them up is cycled and transformed into different categories over time. For example, rocks on Earth’s surface break down into sediment. That sediment can collect in layers. Over time, heat and pressure can compress the layers of sediment into new sedimentary rock.

Or the sediment can get pushed deep into Earth’s crust by the movement of tectonic plates. There, it can undergo chemical reactions that change its properties, turning it into metamorphic rock. Eventually, any rock pushed deep into Earth’s interior will melt into magma.

If magma reaches the surface, it will cool and harden into rock again. The processes that form, break down, and re-form rock from one category to another are called the rock cycle.

Weathering Rock

All rocks on Earth’s surface are broken down over time through a natural process called weathering. Weathering is the breakdown of rock into small pieces from exposure to wind, water, changes in temperature, and/or biological forces. For example, when kimberlite rocks reach the surface, they are immediately weathered. Sometimes weathering wears down the rock so much that the diamond becomes exposed. There are two kinds of weathering: mechanical and chemical.

Mechanical weathering takes place when rocks are broken down by physical force without any change in their chemical structure. Mechanical weathering is a physical change because it does not affect the chemical structure of a substance. Whenever rocks and sediment grind against each other, they mechanically weather surfaces. This kind of weathering is called abrasion.

Chemical weathering occurs when chemical reactions change the composition of rocks, rearranging the atoms or molecules into new substances with different properties. Chemical weathering generally occurs gradually over time.

Eroding Rock

Wind, water, or gravity then transports the sediment to new locations in a process called erosion. Kimberlite rocks containing diamonds have been found in areas away from volcanoes, eroded by wind, water or gravity.

Weathering and erosion usually work together. For example, wind and water carry pieces of sediment that grind against rock and wear it away. Water can also seep into cracks between rocky particles and expand, making cracks larger. over it. Over time, the rock wears down from the inside and crumbles into sediment.

Weathering and erosion are both processes that can occur rapidly or over many years. For example, it can take many years for some rocks to be broken down into smaller pieces of sediment and to then be transported through erosion to a new place. However, erosion can also occur rapidly, such as through landslides caused by flooding.

For the hands-on activity in this lesson, students figure out how the science phenomena of mechanical weathering affects the breakdown of different rock categories. Students experiment to compare the amount of sediment mechanically weathered from each type of rock. Then, students use the data they gathered in the experiment to explain whether some rocks are more vulnerable to abrasive mechanical weathering than others.