MT. ELDEN LANDSLIDE

Mt Elden is a volcano looming over all of east Flagstaff.  It is nearly 2,400 feet tall and has very steep slopes that look like long overlapping tongues.  The steepness is due to dacite lava which is thick and sticky and flows like taffy.

Mt. Elden Google Earth image.

The volcanic dome is also a laccolith which is magma that is intruded between layers of rock strata which are consequently uplifted like a blister. As the blister enlarges it fractures at the top and lava erupts at the surface flowing over and around the uplifted strata.

  

The Mt. Elden laccolith intruded under the red Moenkopi sandstone which is well exposed at the Route 66 road cut just east of the China Star Restaurant. The dacite lava dome became so steep that the layer of sedimentary rock slid off the side of the lava dome in a massive landslide into the area where the Shadow Mountain neighborhood is now and as far as Linda Vista Drive and Jamison Boulevard.

           
                          

There may have been a trigger that started the landslide besides just the steep gradient of the laccolith.  The photo below shows the escarpment left behind after the landslide occurred.

At the apex of the escarpment is a rock formation known as Devil's Throne.  This rock and several smaller extrusions were pushed up along a linear fracture in the laccolith just above the top edge of the Moenkopi sandstone slab.  Their placement and enlargement may have provided the push that sent the side of the mountain down.  Not to worry though, all this happened a few hundred thousand years ago.

  

I'M BACK

I have not written any posts for three years.  I haven't had (or made) time for exploring the countryside or taking photos.  I had a knee replacement, my wife and I both retired, we sold our house in Flagstaff and remodeled an old house in Sedona that we now live in.  I've been busy but now I'm back.

June, 2017

May, 2019

COSMOGENIC DATING

Since the 1990's scientists have been able to date surface rocks by using the cosmic rays emitted by stars. Cosmic rays from ancient exploding supernovas bombard the surface of the Earth constantly. When those rays strike a rock on the surface they shatter the elements within the rock minerals. Magnesium, for example, decays into a particular form of helium. By determining the ration of helium to magnesium the approximate age that rock has been exposed on the surface can be calculated. 

This method can be used to date surface rocks 10 to 30,000,000 years old and can tell when a rock fall occurred, a glacier melted or a volcano erupted. 

Radiometric dating is similar but is used to determine when a rock formed based on the decay or half-life of heavier radioactive elements into lighter elements, such as uranium into lead. Radiometric dating is used for revealing the age of rocks as old as our solar system.

Strawberry Crater - 55,000 years old.

SP Crater - 60,000 years old.

COCONINO NATIONAL FOREST TANKS

Anyone who has driven the dirt roads through Coconino National Forest have seen the water tanks as they are called on maps and occasional sign posts.  They are just small ponds where animals can get a drink during the dry season.  They are all located in low areas where they collect snow melt and runoff from the summer monsoon rains.  Many are located around seasonal water seeps or springs. 

Blue Grade Tank in Rarick Canyon near Beaver Creek

Spring fed Locket Meadow Tank on San Francisco Mountain

Committee Tank, Schnebly Hill at the head of Jack's Canyon

There are literally hundreds of them throughout the national forest and a dirt road leads to every one of them.  It seems there are far more tanks than what are needed to water just the local wildlife so I looked into it.

Most of the tanks were dug in 1916 using horse drawn metal scoops called Fresno Scrapers or dirt scoops.  The federal government financed the project to encourage ranchers to raise cattle on national forest land.  The government would then buy the livestock to feed the US troops fighting World War I in Europe.  Originally dug to support the war effort they are maintained today by the National Forest Service to support wildlife and for grazing cattle and sheep on leased land.

Invented in 1883 by James Porteous in Fresno, California.

Sedona Heritage Museum

Fresno Dirt Scoop in use.

Not all ponds in the forest are tanks.  Some are springs where regional aquifers seep to the surface. How to tell the difference?  All tanks have C shaped earthen berms on the down hill side where sinks will not.  Another clue is tanks generally have opaque, brown water where natural ponds or sinks will be cleaner and may have plants growing in them.  Many of the tanks and sinks are seasonal and will dry up in drought years.

Saginaw Sink

November, 2012

Quarry Sink

July, 2015

Rocky Hole Tank (actually a sink)

November, 2012

PILLOW LAVA

Pillow Lava is a bulbous shaped rock that forms when a volcanic eruption occurs under water or when hot lava flows into water.  This is currently happening in Hawaii.  Arizona being a land-locked state for the last 1.75 billion years and 350 miles from the Pacific Ocean one wouldn't expect to find such rocks, but I have located four exposures in Central Arizona thus far.   

On interstate highway I-17 from Phoenix to Flagstaff north of Black Canyon City is one location.  A road cut in the north-bound lane at mile marker 249 is a hill of pillow lava between 8 and 16 million years old.  The Pacific ocean was hundreds of miles to the west at the time so this must of occurred as lava flowed into a lake.

I-17 is heavily traveled so stopping here can be an unnerving experience.

The bulbous nature of the rock is obvious.

One canyon SE of Jerome is Mescal Gulch where there is another exposure of pillow lava.  A marine volcano erupted about 1.75 billion years ago, was buried by later lava flows then covered with sediments.  The pillow lava bed was heated and compressed and mildly metamorphized.  Around 8 million years ago the land was lifted along the Verde Fault line creating the Black Hills overlooking Verde Valley.

Mescal Gulch

Jerome is 3 miles to the right and the pillow lava exposure is just out of the photo on the left.

The pillow lava was metamorphosed so the bulbs are

compressed together.

Lower Gulch Road and FS Road 413

Forest Service Road 413 starts from the Lower Gulch Road in Jerome. It is a 3.3 mile walk to the pillow lava, or from Cottonwood it is a 9.5 mile drive.  A 4 wheel drive high clearance vehicle is necessary.  Take Mingus Avenue past the airport.  The pavement ends and turns to dirt.  In less than a mile is a big dirt parking lot.  The road leaving the parking lot is Forest Service Road 493 although there is no sign.

The big dirt parking lot

In about a mile the road splits.  Take the 

road to the right which is FS Rd 493.

A rough section in the road.

In about 6 miles FS Rd 493 joins up with FS Rd 413.  

Turn right to go to the pillow lava.  If you turn left 

you can go all the way to the top of Mingus Mountain.

Mingus Mountain

Verde Valley

The best exposure of pillow lava I have found is also the easiest one to get to.  Just 2.7 miles west of interstate I-17 on highway SR-169 is a road cut 20 feet high and 900 feet long through a hill of basaltic pillow lava.  It flowed into a lake between 8 and 16 million years ago.

There is little traffic and a wide shoulder to pull to the side.

The bulbs are clearly distinct.

The lava is extruded into water so it cools quickly in layers from the outside inward.

This makes layers like those of an onion that peal off in thin flakes as they weather and erode.

Recently I came across another site I believe to be pillow lava although it's an unusual type that I have not seen or read about before.  It is basalt, bluish in color and made up of indistinct bulbs of cobble sized gravel.  The mass is only partially welded together so it seems the lava was relatively cold when it erupted.  Though I wonder why it was gravel and why did it erupt at all if it was already solidified?  Is it even pillow lava?   

It is located on Forest Service Road 689 which runs parallel to Interstate I-17 between Beaver Creek Road and Stoneman Lake Road.   On topographical maps it is identified as 'Blue Grade' through this part of the canyon for obvious reasons.

The Blue Grade

The road cut exposed the blue gravel lava.  It was mined and used as fill and the base layer in construction of the road.    

    

The quarry next to the road.

The pillows are indistinct but still visible.

The best preserved pillow

Years ago I read a geologic report there is an exposure of pillow lava in Black Canyon somewhere in the ophiolite sequence (oceanic crust and mantle) located there.  It did not say where.  It is a big area and as yet I have not been able to find them.  There are many black dikes in Turkey Creek Canyon and I would think pillow lava would be found at the top of some uneroded hill.  It's very rugged country and inaccessible to me.

Eroded dikes