Ask WTA: How Do Columnar Basalt and Andesite Form?
If you’ve ever seen cliffs made of towering columns, it’s hard to forget that gorgeous, dark rock, fitted together in a near-perfect repeating pattern. It’s mesmerizing and it inevitably raises the question — how on Earth did that happen? The story starts millions of years ago, deep in the Earth. ...
If you’ve ever seen cliffs made of towering columns, it’s hard to forget that gorgeous, dark rock, fitted together in a near-perfect repeating pattern. It’s mesmerizing and it inevitably raises the question — how on Earth did that happen?
The story starts millions of years ago, deep in the Earth. The columns began their journey as magma, far below the Earth’s surface. That molten rock was forced to the surface, where it cooled. The outer layers began to cool first and, as they cooled, they shrank and began to crack. The rocks are made of relatively uniform material and during the cooling process, spots known as “centers” formed.
When those centers are evenly distributed, as they are in columnar basalt and andesite, as the rock cools, it pulls in around those centers, forming a repeating pattern. (A hexagon is a common shape, but others are possible.)
These columnar rocks often showcase tall, straight columns. But sometimes, they also show up as bent columns that curve off at wavy angles. Those columns can form when the lava encounters water or another substance that distorts it and causes uneven cooling.
Sometimes, you’ll see columns that make a straight line, but the whole line is at an angle. That likely means the crust of the earth itself has been bent since the rock formed — in the Cascades, for instance, as the mountains were created by plate tectonics and volcanic activity.
In Washington, you’re likely to see columns made up of basalt or andesite, which are similar rocks. Scientifically, they’re both extrusive, igneous rocks, which simply means they are formed from melted rock that solidified after being forced to the surface. The main difference between the two is the percentage of certain minerals that each rock is made up of.
You’re most likely to see basalt in the eastern part of the state, south of the Columbia River toward Oregon and also along the Columbia River all the way to the Pacific Ocean. Most of the basalt is around 13–17 million years old. The Frenchman’s Coulee area and anywhere along the Columbia River are particularly good spots to see columnar basalt.
Andesite is more likely to be found in the Washington Cascades. For instance, a spectacular example is visible along the South Puyallup Trail.