Introduction to the
Mojave National Preserve
& Ancient Lakes
Weathering & Erosion
Faults & Active Tectonics
Pediments & Alluvial Fans
& Older Surfaces
Sand Dunes & Dust
3D Geology Tour
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
|These three cinder cones are in the Cinder Cones and Lava Beds area.
Eruptions in this volcanic field occurred over the last several million
years. Successive eruptions and flows have blanketed older flows.
In some cases, lava flowed on the surface for as much as 10 kilometers
from the eruption site.
|A nearly perfect cinder cone in the Cinder Cones and Lava Beds
Natural Landmark Area in the north-central portion of the Mojave
National Preserve. Notice rills and gullies that are slowly degrading
|A lava tube forms where the surface of a lava flow cools, but lava
continues to flow below the surface. In this example in the Cinder
Cones and Lava Beds area, the surface layers of basalt lava have
collapsed, creating an entrance to the lava tube. More cinder cones
are in the distance. Note the rubbly, rough lava flow surface.
|View of the inside of a lava tube in the Mojave National Preserve.
Natural skylight shines on the dust-covered floor and provided reflected
light throughout the short passage of this lava tube.
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.
|View looking north toward the volcanic rocks (consisting of ash
beds and lava flows) of Hole-In-The-Wall (left foreground) and Table
Top Mountain (distance).
|Tafoni is a type of surface weathering caused by wetting
and drying of porous rocks that gives these cliffs of rhyolite tuff
a "cheese-like" appearance (located in the canyon below
the campground at Hole-In-The-Wall).
| Weathering and erosion of tuff forms hoodoos (irregularly-shaped
rock spires) along Wild Horse Canyon near Hole-In-The-Rock.
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
hem, forming narrow channelized canyons. This can be seen in drainages
from the New York Mountains through Lanfear 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
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.