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Wells Fargo Bank Building - Location on the Walking Tour of Urban Geology in Downtown Boise, Idaho

Introduction

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The Wells Fargo Bank building is located at the northwest corner of the Grove Plaza. Built in 1988, this triangular shaped high rise utilizes a dark granite rock near the building base and as trim for large panels of a lighter granite rock. The darker granite, known as Carnelian Granite, comes from Millbank, South Dakota. It is about 1 billion years old. The lighter granite, known as Town Mountain Granite, comes from Marble Town, Texas. It is about 1.1 billion years old.

Backstory and Context

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All rocks are classified into one of three main types, which include sedimentary, igneous, and metamorphic. Granite is an igneous rock. All igneous rocks form through the cooling and solidification of magma (which occurs below the surface of the earth) or lava (which occurs above the surface of the Earth). A close-up visual scan of the lighter Town Mountain granite reveals various minerals that are commonly found in granite. These include pink potassium feldspar, light-gray quartz, white plagioclase feldspar, Black biotite flakes, and needle like crystals of greenish-Black amphibole. As molten magma slowly cools deep within the earth, these minerals crystalize out of the magma in a predictable sequence that is related to the temperature of the magma. The sequence at which specific minerals crystalize was elucidated by Norman L. Bowen. Bowen’s work revealed that potassium feldspar is one of the first minerals to crystallize as magma cools. Minerals that crystalize first have ample space within the magma to form complete geometric crystals. As can be seen in the Town Mountain Granite, the pink potassium feldspar forms large, blocky, often rectangular crystals. The Town Mountain Granite exhibits some exceptionally large potassium feldspar crystals up to 3.5 inches in length. Minerals that crystalize out of the magma at a later, lower temperature are forced to occupy the remaining spaces between previously formed crystals. This often results in smaller, irregularly shaped crystals that are less well formed and squished into the remaining available space.

Sources

Wilsey, Shawn. Geology Underfoot in Southern Idaho. Edition 1st. Mountain Press, 2017.

Additional Information