Serpentine rock

Serpentine is the official state rock of California…and it’s a good pick since it’s unique to subduction zones that exist in much of California.

Serpentine is a type of rock that forms deep underground when certain heavy, dark rocks from Earth’s mantle, like peridotite, come into contact with water over long periods of time. This process, known as serpentinization, causes the original rock to change chemically and take on new minerals—mainly chrysotile, lizardite, and antigorite—that give serpentine its unique look and feel. This transformation usually happens where tectonic plates meet, such as at ocean ridges or subduction zones, where water can reach deep into the Earth.

Serpentine rock is usually green, ranging from light to dark shades, and often has a smooth, waxy, or slightly soapy texture. It sometimes shows streaks or veins of different colors, which can make it quite beautiful. Because of this, serpentine has been used for centuries as a decorative stone in buildings, sculptures, and countertops. Its rich color and polished finish make it a popular choice for both art and architecture.

The formation of serpentine involves a chemical reaction between the original mantle rock, which is rich in minerals like olivine and pyroxene, and water. When water seeps into cracks in the rock, it reacts with these minerals in a process called hydration. This reaction adds water molecules to the crystal structure of the minerals, changing their composition and creating new serpentine minerals. As a result, the rock becomes less dense, and hydrogen gas can be released during the process. This chemical transformation not only changes the rock’s appearance and texture but can also influence its physical properties, such as making it softer and more flexible than the original, unaltered rock.

Below is the chemical reaction that forms serpentine (from olivine and silicon dioxide). Notice that water is needed; it is present because the rocks were subducted from layers under the ocean.

3 Mg₂SiO₄ + SiO₂ + 2 H₂O → 2 Mg₃Si₂O₅(OH)₄

In the plant world, serpentine soils are toxic to many plants, however Manzanita and some other California natives can handle it just fine.

Resources

• 3D model you can rotate.
• More on Golden Gate NRA website.
• More on Wikipedia.
• Deep dive into the chemistry of the formation of serpentine.
• Video: Serpentine quarry on Angel Island.