• Inform students that the next series of slides are designed to review convection currents within the mantle, connect these currents to the Rock Cycle, and emphasize the formation of soil due to weathering and erosion.

Note: In this Investigation, we wish to take the mantle convection illustration from Investigation 2 and extend it to include the entire Rock Cycle. We will begin, in the following slide, with a review of the mantle’s convection system and how materials deep in the mantle can be brought to and through the crust to add new material (igneous rock) to the Earth’s surface and how the material at the upper reaches of the mantle can be driven deep into the interior. In the third slide, we will see how this is at the heart of the Rock Cycle.

Note: Also in this Investigation, we will emphasize that one of the results of weathering and erosion is the formation of soil. Rock materials that are broken down into sand and small pebbles can serve as an anchoring substrate for the root system of plants. In addition, biological components of the soil function to recycle important nutrients required for plant growth.

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• Remind students that this slide was first presented in the Concepts Presentation for Investigation 2 of Weathering and Erosion and is placed here as a brief review.

• Explain that:

• the diagram shows how convection currents in the mantle lead to the deposit of new material to the crust on the ocean floor,
• the convection current cycle is caused by the hot mantle semi-solid molting rock rising to the surface due to its decreased density on account of its extreme heat,
• as the hotter mantle material reaches toward the crust, it cools somewhat and become denser and sinks, thereby completing the cycle, and
• hotter material again rises toward the crust, cools somewhat, and sinks again, thus perpetuating the convection current cycle.

• Explain that:

• as new crust material, derived from the upward-moving mantle, is added, it forces older crust away from the site of the deposit area,
• this, in turn, pushes the entire plate of crust away,
• the moving crust will then interact with other plates that are in motion from other sites like the one depicted in this slide,
• as a result, entire continents are slowly and steadily moved across the surface of the Earth, and
• the ultimate result of such continental drift is shown over geologic time on the next slide.

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• Inform students that this slide shows mantle convection involvement in the extremely important process known as the Rock Cycle.

Note: This slide is set up with a series of mouse click “animations” for you to walk students through the Rock Cycle. Consequently, we will present your background information here corresponding to that series of mouse clicks.

Click 1:

• Tell students that

• the circled area identifies the region where two moving tectonic plates come in contact with each other,
• at the contact point, the plate at the right is deflected downward and into the mantle, while the plate on the left is stacked into a mountain range due to the force of the collision,
• this is an extremely important event because it is the means by which older rock material is recycled,
• the circle and labels highlighting this area will disappear with the next mouse-click, at which time the Rock Cycle steps will begin.

Click 2: 

• Explain to students that

• we are now at the first step of the Rock Cycle,
• we begin where new material from the mantle cools and solidifies into what is called igneous rock,
• igneous rock makes up about 65% of the Earth’s crust, and
• basalt is the major form of igneous rock.

Click 3:

• Explain to students that existing surface rocks are subjected to weathering and erosion.

Click 4:

• Explain to students that

• as a result of weathering, soil is formed and is sedimented,
• sedimentation results in extremely deep, multilayered deposits that, over extended periods of time, lead to the formation of sedimentary rock,
• sandstone and limestone are examples of sedimentary rock, and
• it is in sedimentary rock where many fossils are found.

Click 5:

• Explain to students that metamorphic rock is formed when any surface rock is buried deep enough under conditions of high temperature (from the mantle) and high pressure (caused by overlying rock layers).

Click 6:

• Explain to students that as metamorphic rock sinks into the mantle along with submerged crust, they both melt together under high temperatures and become part of the mantle.

Click 7:

• Explain to students that through convection, the melted surface material is eventually moved toward the surface as magma (semi-liquid mantle material).

Click 8:

• Explain to students that rock material that was once on the surface and melted into the mantle, once again is pushed to the surface and cools into igneous rock, completing the Rock Cycle.

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• Read the slide to the students.

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• Inform students that this slide presents a high-resolution photograph of a typical soil sample. 

• Ask students what they see in the soil.

Note: Hopefully, students will identify pebbles, smaller particles of sand, and some organic material (stems, etc.)

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• Tell students that in the Lab for this Investigation, they will use sand, pebbles and potting soil (humus) to make their own soil.

• Ask students to consider where these three common components of soil came from.

Note: With three clicks of the mouse, students should be led to suggest the following:

• Sand comes from the weathering of rock.
• Pebbles also come from the weathering of rock.
• Humus comes from decomposing plant and animal materials and is therefore referred to as “organic” material.