Volcanoes

User Rating: 5 / 5

Star ActiveStar ActiveStar ActiveStar ActiveStar Active
 

 

By Scott Schmitz

As one drives east on Interstate 8 through Walker Canyon, it is hard to miss the hulking black mass of Table Mountain dominating the eastern horizon.  It is even harder to miss Round Mountain as you approach DeAnza Springs Resort.  It’s the dark gray mountain south of Interstate 8 looming high enough to be seen over the rocky hills that border the resort’s southern side.  And if you drive into Jacumba Hot Springs, you may also notice also the darkness of the rocks on several of the hills dotting Jacumba Valley.   Even the presence of the hot springs seems to cry out that this is volcano country.

Table Mountain
Table Mountain

While the lighter-colored rocks of the La Posta and Indian Hill plutons never made it to the surface and remained trapped beneath a two-mile high mountain range that has since eroded away, while the Jacumba volcanoes were made from magma that managed to breach the surface.  The La Posta pluton stopped growing 96 million years ago and the Indian Hill pluton stopped growing 89 million years ago as the shrinking Farallon Plate ceased producing the melts that supplied these plutons.  And things remained quiet for 62 million years, an era that was dominated by erosion.  Mountains disappeared and their rubble was spread out over a wide plain, forming the coarse conglomeratic sandstones of the Anza formation.

 And then 27 million years ago volcanoes began to sprout all up and down the California coast. These California volcanos can be divided into three age and geographic groups;  a mid‐Tertiary group with ages 27–22 Mya located primarily in central California; a mid‐Miocene group with ages 18–12 Ma primarily in southern California; and a post‐mid‐Miocene group with ages younger than 15 Ma showing a northward age progression from older eruption in central California to younger eruptions in northern California. 

  These eruptions coincided with the western edge of North America overriding the East Pacific Rise.  Each of these eruptions follow a pattern of early hypersthene andesite eruptions followed by a period of olivine eruptions and ending with eruptions of siliceous andesite.  The early eruptions contain melt from oceanic crust, then, as the volcano moves directly over the mid-oceanic ridge, the lava consists of mostly olivine originating in the earth’s mantle.  Finally, as more of the continent moves over the mid-oceanic ridge, the lava begins incorporating more continental crust.  The higher siliceous content increases viscosity and the later lavas are more prone to plugging the volcanoes, thereby ending the eruptions.

Lava Cliff
Lava Cliff

 The arrival of the East Pacific Rise in the San Diego area was first announced by the eruption of Mount Calavera between Carlsbad and San Marcos 22 million years ago.  But then the East Pacific Rise was overridden at first by the Western Peninsular Batholith and then by the Eastern Peninsular Batholith which included the La Posta pluton.  The spreading center had to content itself with attempting to stretch apart and fracturing the strong granitic cores of these plutons but it was not very successful until the Rise encountered the La Posta pluton.  20 million year ago the spreading center at last found a pluton weak enough to pry apart and the Jacumba Valley graben began to open up.  But the northern end of the pluton proved to be stronger than the southern part thanks to the underlying Indian Hill pluton.  However by this time the damage was done and the heat of the East Pacific Rise was finally close enough to the surface to unleash its fury.

 Round Mountain was the first volcano to erupt.  The initial eruptions of oceanic basalt spread across the valley and formed a cap on top of the coarse conglomeratic sandstones of the Anza formation, forming the deep aquifer that supplies the resort with its water.  Round Mountain then began alternating between explosive eruptions of cinder and ash and quiet eruptions of basalt.  The resulting volcano was large in area, extending past Jacumba Hot Springs south and beyond the Interstate 8 ramps to the east.  The volcano covered DeAnza Resort.  You can still find lava rocks scattered along Squaw Ridge.  The hills south of the resort are remnants of the volcano, as are the lava bluffs seen west of Round Mountain.  And if you look carefully, you can see the line of lava rocks on Gray Mountain that outline how high the volcano reached up the sides of the horst.

 Eventually the lava became contaminated with encroaching continental crust and formed a plug.  Most of Round Mountain that you see today consists of that volcanic plug.  Once the eruptions stopped, the erosion started.  Round Mountain had the misfortune of forming at the head of Carrizo Gorge.  The resulting gradient meant the volcano eroded rather swiftly, its cinders being flushed down the gorge and out onto the inland lake that would become the Carrizo Badlands.

 Cerro La Miel (Honey Hill) was the next volcano to erupt.  It rises just east of Jacumba and bestrides the international border.  Evidently its vent did not develop properly because it erupted next to the Gray Mountain Sill.  The sill constricted its flow, and perhaps its growth was further stunted by the last gasps of Round Mountain.  Its cone did not come close to obtaining the size of Round Mountain.

 In the middle of the Gray Mountain Sill a small vent opened up and produced the Black Boulder, but it does not appear this vent produced much.

 But on the other side of the Gray Mountain Sill a crack opened up that gave birth to a monster.  Table Mountain is a truly impressive pile of volcanic rock.  There is evidence of extensive alternating basaltic and pyroclastic flows.  Presently Table Mountain is 4,092 feet high, 329 feet higher than Gray Mountain and 722 feet higher than Round Mountain, although one could imagine Round Mountain as once having comparative size and height.  But given Table Mountain’s location and younger age, it was more protected from erosion than its older brother.

 Eventually Table Mountain succumbed to the same fate as Round Mountain and was plugged.  But you won’t find a crater on top of Table Mountain.  The mountain is aptly named as most of the top of Table Mountain is flat.  Two promontories rise above the ashen tableland, a crescent rim of rock that is the mountain’s highest point and which may be remnants of a crater’s rim, and Squaw Tit, which may be a plug but is offset to the south of the mountain’s highest point.  Looking around the tableland from the top of both of these pinnacles gives a sense that the later eruptions of Table Mountain were in the form of massive pyroclastic flows.

Stone Rainbow
Stone Rainbow

 More volcanoes erupted further east and north.  One heavily eroded volcanic center lies just southwest of Ocotillo, broken up by the faulting that occurred when the Salton Trough opened up.   Another up in Anza-Borrego, the Andersen Andesites, stretches across the Volcanic Hills and along the north side of the Coyote Mountain Domelands.  The Andersen Andesites are poor in olivine, an indication that the hot mantle magma of the East Pacific Rise was having trouble reaching the surface.  With the shutdown of this segment of the East Pacific Rise around 16 million years ago, volcanic activity ceased and this section of the North American Plate began to adhere to the underlying Pacific Plate.  Soon the drag of the Pacific Plate would begin to stretch the land to create the San Andreas Fault and the Gulf of California.

 The lava left behind by Round Mountain is easy to visit.  The Lava Cliff section of the Peyote Trail comes very close to one of the early basalt flows which now forms a distinctive cliff along the south side of the resort.  Look closely for a fissure in the cliff that has formed a small cave.  More adventurous hikers can step off the Peyote Trail and follow Pumice Canyon up to the Mystery Mine Trail.  This rugged route passes several dry falls gouged out by flash floods and carved into bowls and swirls.  Be sure to spot the occasional piece of pumice that gives Pumice Canyon its name.  Most of the rock in the canyon are cinders and lava bombs that have eroded out of the debris of Round Mountain’s former cone.  As you approach the Mystery Mine Trail, the canyon narrows and passes through a lava dike where basaltic lava broke through the cinders of the cone to form a narrow wall.

 You can climb to the top of Round Mountain’s lava plug.  Head south on Carrizo Gorge Road to where it heads under Interstate 8.  Bushwhack across Carrizo Creek here and head south up the valley between Round Mountain and the lava bluffs to the west.  Eventually you will come to the saddle between Round Mountain and the bluffs.  Here the real work starts.  The mountain’s southern ridge is steep but climbable, avoiding the jagged cliffs that nearly ring the mountain.  Once you’ve reached the true summit, head down to where the Jacumba Hikers have erected an American flag.  There is a beautiful view of the resort from here.  There is a register at the top.

 Table Mountain is a mysterious place.  Table Mountain is one of the four sacred mountains of the Kumeyaay.  They called it mat-kusiyaay, Mountain of the Shaman, and no one but the spiritual leaders of the Kumeyaay were allowed upon its slopes.  If you are in tune with the spirit world you can almost feel their presence.  But tread lightly, for the spirits deal harshly with the unwary.

 The best way up to Table Mountain is heading out east on old Highway 80 to Carrizo Creek Road.  Follow Carrizo Creek Road to where it tees with the Interstate 8 frontage road and turn left.  Almost immediately you come to Mica Gem Road which goes under the freeway.  It’s a rough road so regular vehicles should proceed up this road with caution.  At the first junction of dirt roads there is a confusion of BLM signs.  Look for TH020.  This will take you to the cinder quarry where you can park.  A steep road on the south side of the quarry leads to an abandoned road that accesses the top levels of the quarry.  Turn right and be on the lookout for a trail that angles off to the left, heading for a low saddle.  Beyond the saddle you enter the mountain’s tableland.

 Squaw Tit is to the left, the mountain’s high point is to the right, but head straight west to the old abandoned road that runs down the middle of the tableland.  Use this road as a marker.  You can follow the road north to a spur that takes you to the high point cliff with spectacular views.  The abandoned road continues on to end on top a high lava cliff overlooking the rough desert landscape west of Jacumba Mountain. To the west are more lava cliffs and views of the pyroclastic flows that make up Table Mountain’s western flank.  Somewhere on the northwestern side of the tableland lies a Kumeyaay medicine wheel, a geoglyph following the basic pattern of having a large stone at the center, surrounded by an outer ring of smaller stones, with "spokes" (lines of rocks) radiating from the center to the cardinal directions (east, south, west, and north). They are known to be associated with religious ceremonies, so it was probably here the gathered shamans communed with the spirits.  Look but do not touch if you manage to find it.

  There is still heat somewhere down beneath the volcanoes of Jacumba.  But the heat no longer emanates from magma chambers.  Those have long since cooled.  It is with some irony that Jacumba Hot Springs owes its heat not from the volcanic history of the area but to a more recent source.  The thermal waters in the area are heated by the friction of the area’s faults.  Like two sticks rubbing together, the scrape of rock upon rock, maybe Miocene basalt rubbing against Cretaceous granodiorite, enough heat is generated to warm the groundwater to 98 degrees.  It’s the perfect temperature to soak sore muscles after spending the day climbing extinct volcanoes.

DeAnza Springs Logo
DeAnza Springs Logo

1951 Carrizo Gorge Rd.

Jacumba, CA 91934

619-766-4301

Stay@DeAnzaSprings.com

1951 Carrizo Gorge Rd.

Jacumba, CA 91934

619-766-4301

Stay@DeAnzaSprings.com

Site designed and hosted by:

One Stone Web 

Site designed and hosted by:

One Stone Web

AANR Logo
AANR Logo
TNS Logo