Volcanic Rocks and Associated Landforms

Examples of landform features associated with modern and ancient volcanism. A volcano forms at an site where erupted material builds up (including lava flows, cinders, and ash). Over time, weathering and erosion break down and strip away surficial materials, leaving behind remnants of volcanic rock that chilled below the surface (including plutons, dikes, sills, and laccoliths). A pluton is a deep-seated igneous intrusion. A stock is a remnant of the vent of a volcano or plutonic body with an areal extent less than 40 square miles (or 100 square kilometers). Far below the surface, a large magma chamber will slowly cool to form small plutons and large batholiths. A dike is a place were molten material cooled in a vertical crack. Sills form when molten material squeezes between horizontal layers. An escarpment that forms when erosion exposes a sill is call a palisade. A laccolith is a blister-shaped intrusion.

Evidence of past volcanism can be seen throughout the Mojave Desert region. Within the Mojave National Preserve, two notable areas that display volcanic rocks and associated landforms include the Cinder Cones and Lava Flows National Natural Landmark Area (which encompasses many Quaternary-age volcanoes) and the Hole-in-the-Wall area (a more ancient volcanic area). Other areas of extensive volcanic rocks are the Piute Range and Pinto Mountain.

Cinder Cones and Lava Flows National Natural Landmark Area

Volcanic eruptions have occurred many times throughout the Mojave National Preserve in the geologic past; the most recent eruption in the region was about 8,000 years ago. Many of the youngest volcanic features in the Cinder Cones and Lava Flows area have changed very little since lava last erupted. Plants communities have not yet become established on younger cinder cones and lava flows. Recent volcanic cones and lava flows are easy to recognize. The cooled black lava rock is called basalt. In many places, the surface of these flows still preserves the fluid texture created by flowing lava. Blocks and pieces of frothy lava rock (called cinders) piled up around places where molten material reached the surface, producing cinder cones.

The Hole-in-the-Wall Volcanic Area

Evidence of older volcanic eruptions is found throughout the Preserve. Massive eruptions, larger than perhaps any eruption recorded in historic times, occurred in the vicinity of Hole-in-the-Wall. Geologists have determined that massive eruptions began in that region around 18.5 million years ago, and continued for several million years. Massive explosive-style eruption blanketed the region with molten or near molten debris. In places where these rocks are still preserved, individual beds (consisting mostly of rhyolite tuff) approach several hundred feet in thickness. Rhyolite is a volcanic rock that has a mineral composition similar to granite, but most crystal grains are too small to see. Tuff is a textural name for a volcanic rock formed from an agglomeration of volcanic particles or rock fragments that may be tightly cemented or "welded" (due to heat from the eruption). Clouds of volcanic ash from these eruptions probably blanketed the entire Great Basin region and beyond. With time, however, most of the volcanic material ejected from these eruptions has long since eroded away.



Weathering and erosion of tuff forms hoodoos (irregularly-shaped rock spires) along Wild Horse Canyon near Hole-In-The-Wall.

In the past, massive explosive volcanic eruptions (called ignimbrites) produced thick blankets of volcanic tuff that covered large portions of the Mojave region. In places these ignimbrite deposits and other more fluid lava flows mantle topography, preserving past landforms. A pediment is preserved at Cinder Cones, and ancient hills and pediments are preserved near Hole-In-The-Wall. Ignimbrite deposits are dense and tend to resist weathering and erosion. As a result, they tend to focus stream flow around them, forming narrow channelized canyons. This can be seen in drainages from the New York Mountains through Lanfair Valley to Fenner Valley. This channel development pattern can also be seen along lava flows in the Cinder Cones area. Dry water falls occur where streams drain coming off the volcanic flows into the more easily eroded alluvial sediments surrounding the volcanic field.

In contrast, cinder cones degrade relative quickly compared to the lava flows. In a study of erosion rates of volcanic materials in the Cinder Cones region, Dohrenwend and others (1984) found that the oldest remains of cones were only about 1 million years old, whereas the older flows were about 3 million, and the remains of stock edifices were as old as 7 million years.

Next > Faults & Active Tectonics

Also see:

Cinder Cones National Natural Landmark