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