Ward's World Activity Guides

Page 1 Pillow Basalts + ward ' s science Recommended Grade Level(s): Appropriate for: High School Earth Science to College Introductory Geology Teaching Topics & Concepts: This "mini-lesson" connects geology and chemistry to teach how basaltic lava behaves in the opposite geologic environment from columnar basalts and forms into pillow basalts. I. Key Concepts a. Basalt b. Temperature, pressure, and rock formation c. Pillow basalt texture d. Seafloor spreading II. Content a. The mantle is the semi-solid layer of Earth between the hot and dense inner core and the thin outer crust. During volcanic activity, a plume of magma from the inner core impinges upon the outer crust. The most common place for basaltic magmatic eruption is the bottom of the ocean, which is also one of the coldest environments on Earth. b. When the basaltic magma erupts on the ocean floor, the cold water immediately absorbs the heat, quenches the lava, and cools it instantly. The resultant formation is a pillow basalt. c. In three dimensions, pillow basalts look like an interconnected network of stacked tubes, but in two dimensions, they look like a bulbous pillow with a rind on the perimeter, indicating the first place the cold water hit the lava. d. While pillow basalts are common on Earth on the ocean floor, it is rare to see them in the continental crust. (i.e., the land's surface). Modern places to observe pillow basalts are land surfaces near the Mid-Atlantic Ridge, the most significant locations of seafloor spreading on Earth, and on nearby land surfaces like Puerto Rico, Iceland, and Hawaii. III. Lesson What are Pillow Basalts? Basalt is a fine-grained igneous rock that is typically dark in color. Most vol- canoes on Earth are underneath the ocean floor. Due to frequent volcanic eruptions over Earth's history, basalt makes up a two- to four-kilometer-thick layer of ocean crust that covers 50% – 60% of the Earth's surface. Pillow basalt is a round and bulbous igneous rock with a glassy rind on its exterior. Pillow basalts are typically seen in groups and can individually range from 30 – 60 centimeters in length. In three dimensions, pillow basalts look like an interconnected network of tubes and appear almost worm like. We can observe this formation with underwater cameras. On land, we typi- cally see pillow basalts in two dimensions because the rocks at the bottom of the ocean floor were tectonically uplifted and exposed. These outcrops appear bulbous, like giant sacs or pillows. "Pillow" is a textural description of these basalts. Typically, there is evidence of an oceanic environment in the rock record surrounding pillow basalts. Ward's Geologist Amelia standing by an eroded pillow basalt near Ponce, Puerto Rico. Pillow lava on the ocean floor of Hawaii. Credit: OAR/National Undersea Research Program (NURP)

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