Cerro La Jara Stop 4

South Mountain, viewed from the Cerro La Jara Trail.

NPS

Look straight down the path. Cerro La Jara is to your right and South Mountain is directly in front of you. Both of these formations are volcanic domes. Volcanic domes are formed by the extrusion of lava that piles up in a dome shape. Cerro La Jara and South Mountain were formed about 700,000 years after the eruption that formed Valles Caldera. They are both made of a rock called rhyolite. Rhyolite lava has a lot of silica in it – which makes it thick and resistant to flow. This property helps form the steep-sided domes you see here and in other areas of the preserve.

There is a boulder to your left that will be visible if it is warm and there isn’t deep snow covering it. It is covered with lichen and is gradually eroding away. It seems to be sitting in a depression or indentation. How might this have happened? We don’t have a perfect explanation, but it is interesting to think about.