Origin of Hydrothermal Features
To examine life in Lassen’s thermal pools we will need to dive down to the microscopic scale of single-celled organisms, where entire ecosystems span mere millimeters and inhabitants traffic in molecular currencies. But to understand why hot springs are hot at all, we must first broaden our view beyond the park borders, beyond the Cascade Range, to encompass the entire planet and its inner geologic machinery. In fact, the same large-scale forces that heat the groundwater beneath the Lassen’s surface and produce the hydrothermal features of the park also built the chain of volcanoes that stretch northward into British Columbia. These forces rely on the dynamic nature of the earth’s surface and can be understood through the lens of plate tectonics.
Image: cutaway of earth’s layers
You may have learned that the surface of the earth is broken up into “plates” that move independently of each other and are involved in shaping the continents and building mountain chains. These plates consist of a solid lithosphere floating on top of a more fluid asthenosphere. Temperature and pressure allow the rock of the asthenosphere to flow like a very thick liquid. This movement permits the plates to drift apart, collide or slide past one another. It is a zone of collision, called a convergent plate boundary, between the North American plate and the much smaller Juan de Fuca plate that produces the ongoing volcanic activity of the Cascade Range.
Image: subduction
Subduction
When oceanic and continental lithospheres collide, the higher density of the oceanic plate causes it to plunge beneath the continent in a process called subduction. As the oceanic lithosphere angles down into the lower mantle, it carries with it volatile gases that lower the melting temperature of the surrounding rock. At a depth of about 150 km these gases bubble out of the subducting plate and partially melt the rock of the surrounding asthenosphere, forming magma. Since this magma has a lower density than the rock above, it rises through fissures in the lithosphere to the earth’s surface, resulting in areas of volcanic activity. The volcanoes that roughly outline the Pacific ocean, known as the “Ring of Fire” were all born of subduction. Thus, the Cascades belong to the same volcanic arc as chains as far away as Japan and South America.
Image: ring of fire
Lassen’s Thermal Features
Much of the volcanism in Lassen is, geologically speaking, quite recent. The majority of exposed rock in the Park is less than 5 million years old and Lassen Peak itself formed only 11,000 years ago. Though presently considered dormant, Lassen Peak erupted most recently in 1917. Magma chambers remain beneath the park at a depth of several kilometers, slowly releasing heat and gases into the area’s groundwater. This water follows a cycle, where rainwater and snowmelt percolate down through the rock until heated by a subterranean magma chamber. This thermally expanded water or steam pushes upwards through fractures in the rock, carrying along carbon dioxide (CO2), hydrochloric acid (HCl), and various sulfur compounds. The rising mixture may be highly acidic and can break down surrounding rock, a process called chemical weathering, further adding dissolved minerals to the solution on its way to the surface.
Where heated water surfaces in liquid form we find hot springs. Hot springs range from slightly warm to over 104° C. Water just above 100° C is prevented from boiling by the minerals dissolved in it, just as adding salt to water raises its boiling temperature. Hot springs often contain a high percentage of clay, taken from weathered bedrock. If the clay content is high enough in a hot spring, it is called a mudpot. Places where the liquid water has completely boiled to emerge from vents as steam are called fumaroles. The temperature of the emergent gases may range from just above boiling to 1000° C!
All of these features may be found side by side in Lassen, notably in the active regions of Bumpass Hell and Sulphur Works in the southwest corner of the park and in Devil's Kitchen in the Warner Valley. Beneath these features lies a large magma chamber, the remnant of the once great Mount Tehama volcano.
Image: exciting thermal feature
It is important to note that these features change over short time and distances scales. Seasonal summer drought saps the area’s groundwater and hot springs dry up, leaving behind mudpots and steaming fumaroles. Spring snowmelt brings an influx of fresh water that lowers the temperature and dilutes chemical concentrations. In addition, a single seismic event may create or destroy whole regions of hydrothermal activity. Conditions also vary within a single hot spring; water may emerge from the vent nearly boiling, highly acidic and full of dissolved minerals but just meters from the source it can cool dramatically and precipitate out much of its mineral content. The dynamic nature of these hot springs environments poses yet another challenge to the microbes that reside there and it is to these that we now turn.
Our tour of life in Lassen Volcanic National Park continues with a look at: