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- Why are Volcanoes Here?
Most volcanoes are found along tectonic plate boundaries where continents and seafloors collide or move apart. We are far from a plate boundary; consequently, geologists debate why volcanoes occur here. One theory proposes that the North American plate is moving over a stationary “hot spot.” But this theory does not explain why there are young volcanic fields all along the southern margin of the Colorado Plateau and then east to New Mexico and Oklahoma. Another more accepted theory focuses on a disruption in the flow of Earth’s mantle at the boundary between the Colorado Plateau, where the Earth’s crust is very thick, and the Basin and Range region (to the south and west), where the crust is very thin. At this transition, heat from the mantle rises and melting occurs. Ancient faults, activated by stretching of the North American plate (which has created the Basin and range and is extending into the Colorado Plateau), serve as pathways for magma to move to the surface. - Taking Root
When the lava flowed and the cinder cone formed, all plant life within a several mile radius was destroyed. The oldest ponderosa pine trees here today are about 400 years old and are probably not the first generation to return. Unlike Hawaii where abundant moisture promotes plant growth soon after eruptions, precipitation here averages only 16 inches (41 cm) a year. Furthermore, soil is sparse in this dry climate where water limits plant life and organic material breaks down slowly. Notice the massive trailing root system of this dead tree. Trees growing on these cinder slopes face extraordinary challenges in finding anchor in loose, unstable cinders and in capturing water as it drains rapidly through them. Notice also how this tree spiraled as it grew. Ponderosa pines grow either straight or dextrally (spiraling to the right). Spiral growth reduces the overall strength of a tree but increases its flexibility. The spiral grain also provides an even distribution of water from a single root to all of the needles and of nutrients from a single branch to all the roots. In a straight tree, wind-damaged branches or damaged roots would deprive water and nutrients to the roots directly below and branches directly above. By spiraling, this tree was better adapted to the stresses of its environment: lack of water, high winds, and occasional heavy snow. But it wasn't able to remain anchored when its roots were exposed by erosion from foot traffic. - Miniature Volcanoes
On the slope below you is a small spatter cone. Spatter cones, or hornitos (“little ovens” in Spanish), form when lava is forced up through an opening in the cooled surface of a lava flow. They are “rootless,” fed by the underlying flow rather than a deep magma conduit. Can you picture fluid fragments of liquid spurting upward, flattening, congealing, and mounding around the opening? Can you imagine approaching an erupting spatter cone? Unique artifacts found nearby – corn casts in lava rock – suggest people did. Experiments conducted in Hawaii demonstrated that “corn rocks,” like the one on display in the visitor center, can form when ears of corn are covered by fluid blobs of spatter. It appears people intentionally ventured close to an active hornito, maybe this one, to leave corn – perhaps as an offering. More than 50 rocks with corn casts have been found in homesites attributed to the local Sinagua cultural tradition. - Undercurrents
Beneath the lava you now stand on are perhaps dozens of caves. As lava flowed, the top surface cooled first due to contact with air. A hardened crust formed. Meanwhile, rivers of lava continued flowing below, periodically breaking through the crust to form the spatter cones you saw earlier. When the flows ceased, the remaining lava drained out the downhill end of the crusted-over channels, leaving behind tube-like caves. Most of these caves were sealed with hardened lava and have no openings to the surface. A collapse created the opening you see here. - Survival in Extremes
Notice the different patterns of vegetation around you. Volcanic eruptions alter plant communities, destroying some, creating new habitat for others, and changing growth rates for those remaining. On the lava flow in front of you most soil has come from wind-blown material that collects in cracks and holds water. Consequently, soil and moisture vary dramatically across the flow. And, since seeds blow in, colonization is quite random. Classic succession with lichens breaking down rock into soil, and plants returning in predictable sequence is not occurring here. As plants return to the barren landscape, so do the animals that use the plants for food and shelter. Many animals that live here are nocturnal. Some take shelter in the lava flow. Others blend in so well with their surroundings that they are difficult to spot. A careful observer may see evidence such as tracks or droppings. - 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.