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41002_Ward's World+Pillow Basalts Mini Lesson

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+ ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com Find materials for this activity at wardsci.com. Discover more free activities at wardsworld.wardsci.com Pillow Basalts (continued) II. Teacher's answer key 1. Why do we observe a bulbous (pillow) texture in rocks when the cooling rate is rapid? Since lava explosively exits a volcano and the ocean water rapidly cools it down, there is no time for a uniform or geometric texture to form. 2. What makes the pillow basalts expand in size before they crack? (Hint: think about the relationship between temperature and pressure.) The outer glassy casing of the pillow basalt insulates the hot, inner semi-sold lava inside. As the volcano continues to outpour lava into this insulated shell, the heated lava expands outwardly and causes external pressure on the glass rind. When enough pressure is reached, the glassy rind cracks and allows the inner lava to move from a higher pressure/temperature environment to a lower one, repeating the cycle. 3. Draw a 2-dimensional profile view of pillow basalts. Include labels. How do these two-dimensional bulbs form into three-dimensional, interconnected tubes? One pillow basalt forms the next as lava seeps out from the cracks and hits the ice-cold water. [SEE IMAGE BELOW] 4. Most of the basalt we see on Earth is not columnar or pillow in texture. Why is this? Two acceptable answers: (1) Pillow basalts represent the fastest cooling environment for lava, whereas columnar basalt represents the slowest cooling environment. Most lava erupts in temperature settings between these two end members. (2) Most of the rocks we see on Earth have eroded and may have lost their original texture. Fresh pillow lobe with chilled glassy rind Quenched and shattered glassy beccia Quenched rind ruptures, inner skin stretches, and the next pillow lobe advances.

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