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41002_Ward's World+Pillow Basalts Mini Lesson

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Page 2 Pillow Basalts (continued) How do Pillow Basalts form? Pillow basalts form in environments where hot magma is instantly cooled underwater. This is, by far, the most common in the ocean, as it is the fastest cooling environment for magma on Earth, though it can also occur in lakes or rivers. It is the opposite environment that columnar basalts form in because the cooling rate is as rapid as possible. When the basaltic lava erupts into the ocean water, the water rapidly quenches the lava. The outer rind of lava is cooled first and quickest. Instantaneously, it turns into glass as more lava erupts underneath it. The glassy rind insulates the interior lava, which stays hot and keeps moving as it attempts to move from higher pressure/heat to lower pressure/heat. With enough pressure from the inner semi-sold magma pushing out against the glassy exterior rind, cracks form, and more lava squirts out in the ocean water. The exterior of this lava is then instantly cooled down, and a new pillow begins to form. The process repeats and creates an interconnected tubal network of pillow basalts. Eventually, the inner semi-solid lava cools completely to form the basaltic rock. Why do Pillow Basalts form? Pillow basalts ultimately form because of seafloor spreading. At divergent plate boundaries, tectonic plates slowly move away from each other. During seafloor spreading, two oceanic plates "rift" away from each other, and heat from the mantle's convection currents makes the crust more plastic and less dense. The less dense material rises and eventually cracks, allowing the basaltic lava to erupt. Since most volcanoes occur underneath the ocean floor at these "rift zones," pillow basalts are a common texture to see on the ocean floor. Where are Pillow Basalts found? There are two places to observe pillow basalts. First, using underwater cameras, we can see pillow basalts on the ocean floor. Second, we can see pillow basalts on the Earth's surface because tectonic activity (such as mountain building) uplifted the ocean floor to the Earth's surface without being completely eroded. Therefore, any pillow basalt we see on Earth's surface is hundreds of thousands to millions of years old. Since pillow basalts form in rift zones, we most commonly see pillow basalts on the largest oceanic rift zone on Earth, the Mid-Atlantic Ridge. We observe pillow basalts both on the ocean floor along the Mid-Atlantic Ridge and on nearby land surfaces, including Puerto Rico, Hawaii, and Iceland. While The Mid Atlantic Ridge is the most common place, we also observe pillow basalts on land surfaces that are or used to be underwater near volcanic activity, such as New Zealand, Alaska, and Washington state. Questions 1. Why do we observe a bulbous (pillow) texture in rocks when the cooling rate is rapid? 2. What makes the pillow basalts expand in size before they crack? (Hint: think about the relationship between temperature and pressure.) + ward ' s science

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