Ward's World Activity Guides

Page 1 Columnar Basalt Recommended Grade Level(s): Appropriate for: High School Earth Science to College Introductory Geology Time Requirements: Activity Time: 30 minutes Teaching Topics & Concepts: • Basalt • Columnar jointing • Cooling rate • Low energy geometry Background: This "mini lesson" teaches students how to connect geology, physics, geometry, and chemistry to understand the rare and spectacular phenomena of columnar basalts. a. The mantle is the semi-solid layer of Earth between the hot and dense inner core and the thin outer crust. A plume of magma from the inner core impinges upon the outer crust during volcanic activity. When volcanoes outpour high volumes of thick and heavy basaltic lava at a slow speed, the rare opportunity for columnar jointing occurs. b. When magma slowly and uniformly cools from liquid to solid rock, it contracts and creates tension cracks. The rocks will crack in the lowest energy geometry when given an extended time to cool. The lowest energy geometry is at 120-degree angles, forming a hexagon. This is what gives columnar basalts their unique and exquisite shape. c. Columnar basalts are regular and systematic looking. They are typically in a hexagonal shape and produce a series of repeated columns meters high. d. Columnar basalts are rare and only found where there have been floods of basaltic lava. Modern places to observe columnar basalts are Iceland, Ireland, and California. Lesson: What are Columnar Basalts? Basalt is a fine-grained igneous rock that is typically dark in color. It is one of the most common volcanic rocks on Earth and is also found on Mars and the moon. While basalt is very common, columnar basalts require extremely specific conditions to form. Columnar basalts are a particular type of basalt that form in columns. The columns vary from three to thirty meters high, a few centimeters to a meter in diameter, and are straight. They stand parallel to one another and occur in a regular and systematic repeated formation. Texturally, columnar basalts are typically hexagons (6-sided) but occasionally polygons (3 – 8 sides). How do Columnar Basalts form? Columnar jointing describes the specific fracture pattern, or breakage in rocks, creating roughly hexagonal basaltic columns. Magma, like all matter, contracts as it cools. Molten basaltic magma initially cools from the outside edges towards the center because the outer surfaces are in contact with cool air compared to the inside of the compacted rock and can't easily release heat. If the liquid basaltic magma inwardly cools down at a slow and uniform rate, the top and bottoms of the newly formed rocks will crack under the tension. Therefore, columnar jointing is dependent on the slow cooling rate (see fig. 1) of liquid magma into solid rock. + ward ' s science Ward's Geologist Amelia standing on top of a columnar basalt in Iceland.

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