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Mt Calavera volcanic plug

Mount Calavera (22 million-year-old volcanic plug)
Carlsbad, California
Coordinates: 33°10'6"N 117°17'2"W
View from the road   


Mount Calavera is 513-ft volcanic plug that was created 22 million years ago in what today is now Carlsbad, Ca. A volcanic plug, also called a volcanic neck or lava neck, is created when magma hardens within the vent of an active volcano (plug). However, when the ‘hot-spot’ which created the volcano moves on because of the conveyor belt system of plate tectonics, the volcano becomes extinct leaving a plug behind. Mount Calavera is only one of three volcanic plugs in Southern California and can easily be seen from either northbound Interstate 5 at Cannon Rd or westbound on Lake Blvd. Mount Calavera is located within The Lake Calavera Preserve offering county maintained trails that lead directly into the ancient volcano, making for great hiking adventures. 

Geologic Background

Along the western edge of North America, extensive subduction occurred during the early Mesozoic to early Miocene time (200 to 19 million years ago). Oceanic crust, equaling the size of the entire Pacific Basin subducted underneath the North American plate during this time. Subduction occurs at convergent boundaries where the heavy Oceanic tectonic plate moves under the lighter Continental plate sinking back into the mantle - this process creates a subduction zone; often creating volcanoes (see figure below).

However, the ‘hot-spot’ which formed underneath Mount Calavera most likely was the result of natural fracturing and faulting. As the warm, buoyant spreading center was subducted under North America, crust was uplifted, fractured, creating zones of weakness where magma could ascend to the surface and become lava.

Geology of Mount Calavera

Mainly, there are three rock units ranging from Cretaceous to Miocene in age (oldest to youngest):

1. Green Valley Tonalite (Cretaceous) - The oldest formation around the volcanic plug is the Green Valley Tonalite. As part of the Peninsular Ranges, it consists of intrusive igneous rock that was cooled slowly at depth, uplifted, eroded, and eventually exposed. It is mapped as a Tonalite (see Geological Rock types map). Tonalite has phaneritic texture (medium to coarse-grained). 

2. The Santiago Formation (Eocene) - very soft sedimentary rock deposited in a beach setting during the Eocene time (34-56 MYA). The sandstone is fine to medium grained, poorly cemented, poorly bedded, and is interbedded with siltstone and claystone. The Santiago Formation was renamed the Scripps Formation in the mid-1980s.

3. The Volcanic Plug (Miocene): Along the walls of the caldera you can see what’s called “columnar jointing” where rocks cooled quickly into geometric columns (see picture below). The rock is “mouse gray” in color, aphanitic texture (fine grain), and dacitic in composition. A dacite is between a rhyolite and an andesite in composition (see below).

Geologic map of the Calavera Hills area. (Figure courtesy of U.S.G.S, open File Report 96-02.)
  • The Cretaceous Green Valley Tonalite (Kg (gv)) is shown in redish-pink
  • Eocene Santiago Formation (Tsa) is shown in tan
  • The Miocene Dacite Volcanic Plug (Tda) is red and in the middle of the figure
Dating a Volcano

There are two main ways to determine the geological age of a volcano, radiometrically (absolute age dating) or relative age dating techniques. Radiometric dating determines rock age by measuring radioactive decay. Relative age dating is the science of determining the relative order of past events (i.e., horizontal layers, index fossils, etc) in relation to each other) estimating age.

Mount Calavera has never been radiometrically dated because reliable techniques are still being created to age date this relatively young rock. I would like to be part of that study.

Using relative age dating techniques, the volcanic plus is thought to have formed in the Miocene. Several key field observations helped form this hypothesis:

(1) When the volcanic plug formed it cut through the surrounding sedimentary layers. These layers, or strata, is called the Santiago Formation and was formed in the Eocene time (40-56 MYA). In the early 1900’s when the volcanic plug was quarried for gravel, they exposed the younger volcanic rocks which cut through the older Santiago Formation – a principle of relative age dating (younger cuts through older).

(2) This timing correlates very well with other similar regional volcanic activity that occurred along our coastline during the same timeframe (Ex: Conejo Volcanics - western side of the Santa Monica Mountains north of Los Angeles).

(3) Volcanic ash found in sedimentary rocks of this age exists. However, it has not been determined which volcanic plug specifically produced the ash.