Article

Geology of Serpentine Hot Springs

A shallow pool of water is surrounded by algae and a grassy meadow.
Geothermal pool at Serpentine Hot Springs.

NPS Photo - Savannah Glasscock

Serpentine Hot Springs consists of two thermal areas (Serpentine Hot Springs proper and Arctic Hot Springs) located on Hot Springs Creek. The waters of Serpentine and Arctic Hot Springs are likely fed by the same source because they have virtually identical chemical compositions and similar maximum subsurface temperatures of 127+/-3°C (260+/-5.4°F). Based on hydrogen and oxygen isotopes, the thermal waters are primarily meteoric (derived from precipitation) in origin. The thermal waters also contain a saline component, interpreted to be a product of bedrock leaching or a ~7% seawater component.
Map of hot springs and granitic plutons on the Seward Peninsula.
Figure 1. Map of hot springs and granitic plutons on the Seward Peninsula.

NPS

Most of the geothermal features in northern Alaska, including Serpentine Hot Springs, are not associated with recent volcanism, but are instead associated with granitic plutons.(Figure 1. Map of hot springs and granitic plutons) 33 of the 36 thermal springs north of the Alaska Range occur within 5 km (3 mi) of a granitic pluton margin. Serpentine Hot Springs occurs above a 70 km2 (27 mi2) Late Cretaceous (69.2 ± 2 to 80.2 ± 3 million years ago) biotite granite stock called the Oonatut Granite Complex. Based on the geochemistry and geology of northern Alaska thermal springs, the heat is being derived from deep circulation of meteoric water through fractures in the granitic plutons. As the water percolates down through the fractures, it heats up due to the geothermal gradient (increase of temperature with depth), and then moves back up to the surface along the fractured and faulted margins of the plutons. Subsurface temperature estimates for Serpentine Hot Springs as well as 25 other thermal springs in west-central Alaska indicate that the thermal waters circulate to depths of 3.3-5.3 km (2.1-3.3 mi). This circulation depth was determined using a geothermal gradient of 30 °C/km to 50 °C/km; however, if magma underlies the springs and is contributing heat to the system these depths could be shallower.
Granite tors around Serpentine Hot Springs. Below is the same photograph, with red dotted lines drawn to highlight joints in the granite, which are controlling the morphology of the tors.
Figure 2. Photograph of granite tors around Serpentine Hot Springs. A. Unannotated photograph. B. Same photograph, with red dotted lines drawn to highlight joints in the granite, which are controlling the morphology of the tors.

NPS

Oonatut Granit Complex

The Oonatut Granite Complex is the largest of seven tinbearing granite intrusions that form a 170 km- (105 mi-) long belt across northwestern Seward Peninsula. These intrusions have tin lode or placer deposits associated with them, and in the Lost River area tin was mined commercially. K-Ar radiometric dates of the granites range from 80.2 ± 3 to 69.2 ± 2 million years ago, with the Oonatut Granite Complex being the youngest of the seven intrusions. The Oonatut Granite Complex is composed of biotite granite and has distinct zones defined by changes in crystal size, petrology, and geochemistry. The outermost zone grades from fine-grained equigranular (crystals all the same size) granite to coarse-grained porphyritic (crystals are two distinct sizes) granite. This zone differs in terms of petrology and geochemistry from the rest of the tin-granite belt and is instead much more similar to the Darby pluton (found in the Darby Mountains). The rest of the Oonatut Granite Complex is split into three textural facies that can be found in all the other tin-bearing granites on the Seward Peninsula: medium- to very coarse-grained seriate (crystals are different sizes) biotite granite, porphyritic biotite granite, and fine- to medium-grained equigranular to subequigranular biotite granite.
Three diagrams showing different models of tor formations.
Figure 3. A. Bedrock is molded by chemical weathering deep underground, then exposed by the removal of sediments. B. Intermediate model involving a combination of chemical weathering under shallow ground and preferential weathering after the removal of sediment. C. Tors form as rock is exposed.

NPS

Tors

Around Serpentine Hot Springs, the Oonatut Granite Complex forms freestanding spires of rock called “tors”. The term “tor” is derived from the Celtic word for hill, and refers to an exposed mass of rock (usually jointed granite) that rises abruptly from the surrounding landscape (Figure 2). There are several models that explain tor development (see Figure 3 for a description), but all models involve rock making up the tors weathering more slowly than surrounding rock. For example, tors in Scotland have been found to preferentially develop in coarser-grained granite with wider-spaced jointing. It is unknown if grain size or joint spacing are driving the formation of tors in the Oonatut Granite Complex, but the differing textural facies makes this a possibility.

Bering Land Bridge National Preserve