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The Rock Cycle: Processes That Change One Rock Type into Another

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Author: Janice VanCleave

Volcanic rocks and fire rocks are common names for igneous rocks. These solidified masses are, as their names imply, the results of great temperatures within the Earth. Igneous rock is one of a trio of rock types–sedimentary, metamorphic, and igneous. Through different processes, each rock type can be changed into one of the other types in the trio. This process of change is called the rock cycle.

In this project, you will study and model the texture of different igneous rocks. The metamorphism of porphyritic rock (a kind of igneous rock) into foliated metamorphic rock will be demonstrated. You will also examine the relation between the three rock types and model their transformation from one type to the other.

Getting Started

Purpose:   To model the difference between a porphyritic rock and other types of igneous rocks.

Materials

  • two walnut-size pieces of blue modeling clay
  • two walnut-size pieces of red modeling clay

Procedure

The Rock Cycle: Processes That Change One Rock Type into Another

  1. Break one red clay piece into four relatively equal size pieces.
  2. Roll the four small pieces into balls.
  3. Repeat steps 1 and 2 with one blue clay piece.
  4. Lay the eight small balls in two rows next to each other, alternating the colors of the balls in the rows.
  5. Gently press the clay balls just enough so that they stick together but retain as much of their shape as possible.
  6. Break the other large red clay piece in half. From one half, form two relatively equal size balls, and from the other half form four relatively equal size balls.
  7. Repeat step 6 with the remaining large blue clay piece.
  8. Lay the twelve small balls in two rows next to each other, alternating the colors (and sizes) in the rows.
  9. Repeat step 5.
  10. Compare the appearance of the two clay rolls (see Figure 13.1).

Results

One of the clay rolls has large balls of clay pressed together. The second has large and small balls.

Why?

Rock is a firm, coherent aggregate of one or more minerals. Rocks produced by the cooling and solidifying of molten rock are called igneous rocks. Magma (molten rock under the Earth's surface) at great depths cools slowly, and during this cooling process, large mineral crystals form. Igneous rocks that form within the crust and contain large uniform interlocking crystals are called intrusive igneous rocks. The texture of rocks is determined by the size of the mineral grain (hard particles) making up the rock. Intrusive igneous rocks are coarse-grained (having large hard particles). In this experiment, the clay roll made with large clay balls represents a coarse-grained intrusive igneous rock.

In porphyritic rock, like other types of intrusive igneous rock, large crystals form from magma cooling at great depths beneath the Earth's surface. However, during the formation of this rock, the magma is pushed to the surface before it completely hardens. There the final cooling occurs rapidly, producing small crystals. Thus, porphyritic rock contains two or more different sizes of interlocking crystals and can be said to have varied grain sizes. The clay roll with the large and small clay balls represents a porphyritic rock.

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