Peperite at the Two Sentinels EarthCache

Two sentinels located in Kirkwood, CA is at an elevation of 8665' above sea level. It is accessible via a clearly navigable trail. Parking for the trailhead is off route 88,approximately two miles west of Kirkwood Meadows drive. Look for a dirt parking area and the Thunder Mountain trailhead.

The Two Sentinels formed during the Miocene, and is part of the Mehrten Formation. The Miocene Mehrten Formation comprises a voluminous, lithologically and compositionally variable but dominantly andesitic, including basaltic andesite and trachyandesite, volcaniclastic sequence formed from subduction-related volcanism along the present crest of the Sierra Nevada mountains in California. Near the crest, the formation is dominated by primary volcaniclastic rocks, including numerous proximal vent-filling and near-vent sequences, with decreasing grain-size and increasing degree of reworking by sedimentary processes (dominantly fluvial) down the flanks of the Sierra. Lava flows and domes are not commonly preserved, even in vent-proximal areas.

When you reach the Two Sentinels, take a moment to enjoy the spectacular view of Kirkwood valley below you. When you are ready, examine the Sentinels themselves. For this earthcache, we are going to look at peperite within the Mehrten Formation.

Although disagreements exist as to the usage of the term peperite, many current classifications of peperite may be summarized by the definition outlined by White, et. al. (2000):

Peperite (n.): a genetic term applied to a rock formed essentially in situ by disintegration of magma intruding and mingling with unconsolidated or poorly consolidated, typically wet sediments. The term also refers to similar mixtures generated by the same processes operating at contacts of lavas and other hot volcaniclastic deposits with such sediments.

Peperite forms when hot, fluid magma (or volcaniclastic) of any composition ranging from felsic to intermediate to mafic intrudes into or flows over and intimately mingles with an unconsolidated or poorly consolidated sediment, giving rise to magma break-up. This processes forms juvenile clasts of volcanic material and often results in the destruction of original host sediment structures.

The host sediment may be of any composition and may be cogenetic with magma emplacement but more commonly is not. Juvenile clasts within the peperite deposit may range from a few millimeters to meters (megapeperite of Coira and Perez, 2002) in size. Peperite deposits or domains range in volume from isolated, millimeter or centimeter sized within associated deposits to extensive deposits of up to several cubic kilometers. Frequently, peperite deposits are located at the margins between the host sediment and an igneous body such as domes , dikes, sills, lava or ash flows or conduits. Such margins are typically irregular and may be glassy. Contacts between the sediment, peperite domains and magma source may be baked or display haloes of hydrothermal alteration.

One theory for fluidal peperite development hypothesizes that a non-explosive fuel-coolant interaction (MFCI) takes place between the wet, host sediment and the magma. As the magma intrudes into wet sediment, it generates a thin, unstable, insulating vapor film. In order for the vapor film to form, there must be sufficient quantities of pore water and temperatures must be greater than the Nukiyama critical temperature. At or below the Nukiyama temperature, vapor films are unstable and rapid heat exchange between the water and magma occurs. The rapid exchange of heat allows the water to flash to steam and explosive eruptions occur. Above the Nukiyama temperature, the temperature of the fuel (magma) is much greater than the vaporization temperature of the coolant (water). However, the vapor film insulates the water from the magma and prevents the pore water from immediately flashing to steam. Instead, transfer of heat energy from the magma to the liquid water within the sediment raises the temperature of the water to a metastable state. Spontaneous nucleation of this metastable fluid forms the vapor film, which rapidly expands. As the film expands, heat is transferred to the surrounding sediment. Sediment particles act as heat sinks for some of the energy. When the vapor film has cooled sufficiently, it condenses and collapses, transferring energy to the surrounding fluid. This energy, coupled with an influx of heat energy from the magma gives rise to a new vapor film. At the same time, some of the thermal energy is converted into kinetic energy at the vapor film locale, creating distortions in the magma-sediment interface. If there is insufficient coolant to transfer and redistribute the heat from the magma, an explosive event may occur, but if peperite is formed, it is by definition non-explosive.

In order to log this cache, you must:
1.Take a photo of an example of peperite. Describe some of the characteristics of the rock that makes you believe that you are seeing peperite. Is your photo of blocky (sharp, angular clasts, often with jigsaw fit textures) or fluidal (round, globular, elongated) peperite?
2.Look for evidence of the processes that might have occurred during peperite formation. Remember what conditions are necessary for peperite formation. In particular, look for evidence of hot fluid flow. What do you see?
3.Bonus: Remember that the Mehrten formation is primarily andesitic in composition. Can you identify any of the minerals that you see?

The Two Sentinels is easily accessible via the Thunder Mountain trail. Bushwacking is not necessary and is strongly discouraged. Collection of rock samples is also highly discouraged. Please adhere to the tenets of the Leave No Trace principles.