Rock Talk By Larry Larason

Hike the Magic!

Two Cinder Cones near Flagstaff

October is the magical hiking time in the Four Corners. It's not too hot, not too cold; there's little chance of rain; there's still plenty of daylight; and none of those horrific springtime winds.

Red Mountain and Strawberry Crater, both near Flagstaff, provide wonderful opportunities for day hikers who enjoy great views and a chance to learn more about geology. Either of these hikes are suitable for taking children five years old or more with you.

Both cinder cones are part of the San Francisco Volcanic Field (SFVF), so first let's consider the SFVF. Eruptions began about 6 million years ago near the location of Williams, Arizona and migrated northeast to cover 1800 square miles with volcanic ash and lava. The most recent eruption was at Sunset Crater, just east of Flagstaff, in 1064 C. E. The SFVF includes one breached stratovolcano [the San Francisco Peaks] and at least 600 cinder cones of various sizes. The migration of volcanic activity has been about half an inch per year, roughly equivalent to the rate of continental drift. Some geologists see this as an indication that the SFVF was caused by North America sliding over a hot spot in the Earth's mantle, similar to the situation in the Hawaiian Islands. However, if that is true then the SFVF is still potentially active and might erupt sometime in the future.

Cinder cones have a typical life cycle. First, magma rises along a fault or joint in the rocks to erupt in what is called a fissure eruption, or wall of fire. During the initial eruption, lava fills and hardens in the fissure except for one major vent, which continues to erupt, building a cone. As the magma expends the gases it contains, the eruption becomes quieter and a lava flow may break out of the cinder cone. These flows often breach the crater and carry away parts of the wall. For example, Sunset Crater was breached, but subsequent eruptions "healed" the break and restored its near-symmetrical appearance. At Strawberry Crater you can see portions of the cone that were rafted, or floated, on the lava flow.

Red Mountain Geologic Area is located about 35 miles north of Flagstaff on US 180; a sign marks the turnoff to the trailhead near Milepost 247. Red Mountain erupted in successive stages about 740,000 years ago, which makes it one of the older cinder cones in the SFVF. [A sign at the trailhead says the eruptions were a million years ago, but the lower figure is based on more recent studies of the lava.] The trail leads to an "amphitheater" gouged out of the cone, most likely when water seeping into the still hot cinders exploded into steam and blew away a good chunk of the mountain.

The ash in Red Mountain has been cemented, as opposed to welded, probably by steam or rain water percolating through the ash and dissolving minerals then redepositing them. It is iron in the cement that gives Red Mountain its color.

Don't mistake the amphitheater for the volcanic vent. That was located farther west and was breached by a lava flow, which rafted away the western wall of the crater leaving a horseshoe shaped cone when seen from above.

The hike from the parking area is gently uphill on the way in and easy Round trip distance is about 3 miles. In a couple of spots you have a nice view toward the east of Mt. Humphey, one of the San Francisco Peaks and the highest point in Arizona. The only problem on the trail is an awkward short ladder that you climb to get into the bowl of the amphitheater.

Stubby brown-black hornblende crystals used to be found all around Red Mountain, but now they are rare along the trail. I suppose everybody who hiked here took home a pocketful. If you watch carefully you might find a few where the trail follows the bed of the wash that drains the amphitheater. There is also white or clear feldspar glittering on the ground like mica, but it is mostly shards too small to pick up. Both types of crystals formed in the magma and were shattered during eruptions. Look for hexagonal prisms of hornblende embedded in lava inside the amphitheater.

Entering the bowl you may begin thinking of hobbits and trolls because of the fanciful eroded hoodoos and columns. A friend of mine called it a "magical place." As you look around at the magic, notice that the amphitheater provides a cut-away section of the cinder cone and allows you to see layers of ash displayed in bands that represent the sequence of eruptions. Take water and a snack with you so you can linger and enjoy this magic realm.

Strawberry Crater sits in a wilderness area of more than ten thousand acres. It is seldom visited, so you can find some solitude here. Despite its name, Strawberry Crater is not as red as Red Mountain. However, there are blobs of lava here and there with a strawberry hue. This cinder cone erupted from a single vent and built on top of two previous lava flows. After the cone was formed, lava containing less gas breached the eastern wall, rafting parts of it away. These rafts can be seen as you climb up the north side of the cone. They appear as mounds of layered lava near the base of the volcano. The outer walls of the cone are mantled by welded spatter [agglutinate], indicating that a short-lived, frothing lava lake occupied the bowl before it was breached. The agglutinate gives the mountain a frosted appearance, although in places it has slumped or fallen away.

Strawberry Crater is an exception to the general cycle of eruption in that lava seems to have come out a single isolated conduit, rather than beginning with a wall of fire event. Lava spewed here between fifty and one hundred thousand years ago. After this cinder cone became quiet, it was dusted with ash from eruptions of nearby Sunset Crater. The final plug at Strawberry Crater is composed of a rock geologists call dacite vitrophyre. This is black with spots of white feldspar crystals and a semi-glassy texture. You can see a small outcrop of it east of the trail at the bottom of the bowl.

The trail makes a loop to the top of Strawberry Crater then back down. When you hike I suggest that you take the left [north] fork at the junction. The trail is mostly on loose cinders and is rather steep in part. If you have kids along, don't let them run on the trail! Take your time and look around at the volcanic landscape. As you approach the top of the cone watch for twisted, contorted blobs of lava that were hurled into the air and cooled enough to retain their shape when they fell. From high on the cone you can see the Painted Desert, which gives you an idea of what the surrounding region would look like if the SFVF had not occurred.

The rest of the loop trail goes up the inside of the crater wall, then steeply down the south side on a treacherous slope with extremely loose cinders; you may want to return the way you came, especially if you have children along. This is a short hike, probably one and a half miles, but because of the slopes it will require about two hours.

To get to Strawberry Crater drive US 89 north out of Flagstaff about 16 miles. About four-tenths of a mile beyond Milepost 434 turn east onto FS 546. This road is not marked! Where this road turns south, continue east on FS 779 to the trailhead. You can get directions or maybe a brochure and map in the Visitor Center at Sunset Crater.

Turn your trip into a weekend outing by hiking one of the cinder cones in the morning, then visiting Sunset Crater and Wupatki National Monuments in the afternoon. There's plenty to see at both places.