Home / National Park Service Exhibits / Sunset Crater Volcano National Monument (SUCR) / Virtual Trail Guide - Lava Flow Trail 29
The virtual trail guide mimics the paper trail guide and benchmarks you would experience in person when walking the Lava Flow Trail. The text derives directly from the Lava Flow Trail Guide currently used by the National Park Service at Sunset Crater Volcano National Monument, and from interpretive stations positioned along the trail itself.
Introduction to the Trail
A sculpture garden of extraordinary shapes and forms awaits you on this walk through the lava flow and cinder fields.
This moderate (1 mile, 1.6 km) trail has rough surfaces and takes you through lava flows and cinder barrens to the base - not to the top - of Sunset Crater. In person, you need water and sturdy footwear. There is also an alternate 1/4-mile (0.4 km), easy, wheelchair accessible paved loop.
Throughout this virtual trail guide, you will find benchmark pages and interpretive station pages. The benchmarks are those numbered in the guidebook, and the interpretative stations are text and photo displays placed along the trail itself.
Our Dynamic Earth
Alpine slopes, forests, and grassy parks disguise the fiery, molten, and often explosive history of this region’s landscape. But here, amidst the Bonito Lava Flow and Sunset Crater’s cinder fields, the land’s volcanic origin is revealed in stunning clarity.
As you walk the trail, use this guide to explore the powerful forces – rapid and violent, slow and patient – that continually shape our planet. These forces all affect our lives and provide for life in ways we may not realize.
Xenolith
Patterns often reveal forces otherwise invisible to us. As you cross the bridge, look to your right for a small light-colored rock embedded in the dark basalt rock. This is a xenolith (zee-no-lith), a rock fragment foreign to the body of rock in which it occurs. When magma rose to the surface, it brought up pieces of limestone from 700 to 1,000 feet below us. Uplift and erosion have exposed this rock (known as Kaibab limestone) elsewhere: at the rims of Grand Canyon and nearby Walnut Canyon and in cliffs at Wupatki National Monument.
The Birth of a Mountain
A Complicated History
Sunset Crater rises before you, a nearly symmetrical cone. Its perfect shape suggests a simple eruption history, but that was not the case. In fact, it wasn't until the 1980s that scientists began to understand the complexity and extent of the eruption.
First, molten rock (magma) and gasses pushed up to the earth’s surface along a six-mile-long weak spot or fissure in the earth’s crust. Volcanic fragments, called pyroclasts, shot upward along the fissure in a “curtain of fire” as gasses escaped violently. Small cinder cones formed along this fissure before the magma became focuses and erupted as a lava fountain from a primary vent. A large cone (Sunset Crater) grew as the shower of cinders and ash piled up around the vent.
Curtain of Fire
Meanwhile, lava broke out of the base of this side of the cinder cone forming the Bonito Flow all around you. The lava pooled, trapped by surrounding older volcanoes, and accumulated to perhaps 100 feet (30 m) thick during at least three separate flows. On the opposite of the cone, lava flowed more than six miles (10 km), filling a narrow valley. When the volcano coughed out its last cinders – after several months or perhaps several years – they were colored by the oxidation of iron in the magma. Similar to the way metal rusts, the magma came in contact with water-rich gasses emitted during the final stages of the eruption. These red cinders rim the top of the cone.
- Eruption date: sometime between 1040 and 1100
- Height: 1,000 feet (305 m)
- Elevation at summit: 8,029 feet (2,447 m)
- Diameter at base: 1 mile (1.6 km)
- Diameter at top: 2,250 feet (868 m) from rim to rim
- Depth of crater: 300 feet (91.4 m)
- Extruded material: approximately 1 billion tons
- Extent of ashfall: approximately 800 square miles (2,072 sq km)
Life and Landscape Transformed
As Powerful as a Volcano
Volcanoes
One Volcano Amid Many
At nearly 1,000 years young, Sunset Crater volcano is a geological infant, the latest development in a series of eruptions that have taken place here over the past 6 million years. It is only a small part of the impressive San Francisco Volcanic field of northern Arizona.
From this point, three different types of volcanoes are visible. What makes a volcano tall or short, steep or gentle, or an eruption more or less violent? Mostly the magma’s stickiness or viscosity, which is determined by its chemical composition and gas content.
Cinder cones, like Sunset crater and most of the 6000 or so volcanoes in this field, tend to be cone shaped with sides not steeper than 33 degrees. A cinder cone is literally a pile of loose fragments. It is easily eroded and will change shape, becoming less steep as it ages.
O'Leary Peak
Look for a dome volcano (O’Leary Peak) on the horizon in front of you. Dome volcanoes tend to have steep sides and rounded shape.
San Francisco Peaks
Composite or stratovolcanoes have sharp peaks or less steep sides. Mount St. Helens, Mount Fuji, and the San Francisco Peaks, seen here in the distance, are examples of this classic volcano type.
Frozen in Time
Ropes and Clinkers
You are now on the southern edge of the Bonito Lava Flow. Magma, periodically relieved of gas pressure, squeezed out of the base of the cone as glowing liquid lava, creating a structurally complex flow covering 2 sq miles (5 sq km).
Lava flows tend to form either jagged blocks, known as aa (ah-ah), or a smooth, ropey surface of pahoehoe (pa-hoy-hoy). Flows usually start as pahoehoe, thin and runny. As the lava cools and becomes more thick and pasty, it can change into an aa flow. The Bonito Flow is mostly aa lava. When aa is forming, cooled, hardened blocks - sometimes called clinkers - are rafted along the surface of moving lava, making clinking noises as they tumble into each other.
Although its structure is complicated, the flow’s composition is uniform throughout. The lava and cinders around you, whether black or red, ropey or jagged, are basalt.
Dating Debates
What if scientists always agreed, never argued, or changed their minds?
Clues in Buried Homes: Until archeologists discovered Sinagua-style pithouses beneath the cinders, scientists did not suspect Sunset Crater was so young. Knowing the age of the pithouses from tree-ring dates and pottery types found in these homes, they concluded the eruption occurred after 1046 and before 1071.
Patterns in Tree Rings: Trees near an erupting volcano, if they are injured but continue to live, show a growth disturbance in their rings. Based on the growth pattern seen in three wood specimens from nearby Wupatki Pueblo, scientists hypothesized that the eruption occurred between the growing seasons of 1064 and 1065. But, this evidence is limited and inconclusive.
Magnetism in Rocks: Geologists have taken more than 100 core samples from the Sunset Crater lava flows for paleomagnetic studies. Using both paleomagnetic dating and stratigraphic evidence, geologists currently restrict the Sunset eruption to sometime between 1040 and 1100.