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Geology of Sonoma County Vernal Pools

Vernal pools are ephemeral. They don't last long and only form when the conditions are right, but they are an important haven for rare plant and animal species in California. Sonoma County is dotted with vernal pools each winter and spring, creating a patchwork of wetlands, when there has been sufficient precipitation. Vernal pools are not limited to Sonoma County, or even California, but here the unique geology allows them to form and the climate makes them critical for rare species.

The Mediterranean climate of Sonoma County brings rain in the winter and spring seasons, and drought conditions in the summer and fall months. The word vernal is Latin for "pertaining to or occurring in spring", and is also used to describe something as "spring-like, fresh, or young". In the wetter months the vernal pools form and in the dry season the surface waters evaporate away, the pools dry out & the plants die. The seeds of these hardy plant species have adapted by remaining dormant for years, until the conditions are right to germinate again.


Geologically Sonoma County is a network of faults that are part of the San Andreas Fault tectonic plate boundary, which causes hills and mountains to rise above the adjacent valleys. This patchwork of undulating elevations allows low lying basins to form, within which pools of water can accumulate, if one more geologic ingredient is present, clay. When clay accumulates in the basins it prevents the water from draining quickly. The topic of clay formation is complicated and there is a vast variety of ways that it can form. For the purposes of this article, I will focus on an important chapter in Sonoma County's geologic history, volcanism. As the San Andreas plate boundary formed over the last ~20 million years, it caused tears in the crust, through which magma rose causing volcanoes to erupt in this region. One of the most common minerals formed by the cooling of magma and lava is feldspar, which is also the most common mineral in Earth's crust. When feldspar is exposed to the atmosphere and water it breaks down into clay. Volcanism of the past created the minerals that became the abundance clay in the region. Vernal pools that support rare species unique to this region exist because volcanoes erupted here millions of years ago & ongoing faulting (& the earthquakes that result) forms the depressions where we can find these unique and ephemeral features.

If you want to know more about this topic, join me on a geologic field trip starting summer 2020.

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