Joints are natural cracks that form in rock. There is
usually little if any movement along a joint, which is what
distinguishes a joint from a fault. Joints form from changes
in pressure, stresses, placed on the rocks. These stresses
include stretching, compression, and relaxation. The patters
of joints in a rock can be used to determine what kind of
stress formed the joint.
When a rock is pulled apart, it will eventually fracture and
form a crack perpendicular to the direction of pull. This would be
similar to pulling on the ends of a piece of paper until it ripped
down the middle. Here at Joshua Tree, rocks would be pulled apart
as magma pushed its way up through the surrounding rock.
A couple of geologic processes that would cause compression
stresses include mountain building and the cooling of magma.
Mountains are built as masses of rock collide and push up against
each other. This kind of stress forms two sets of joints that
intersect at either 60 or 120 degree angles. These cracks are
called conjugate (together forming) joints, because two sets of
joints are created at the same time. These same kind of joints can
form as rock cools. As the rock cools it shrinks slightly causing
the compression stresses that form conjugate joints.
Relaxation joints form from the removal of stress. Rock deep
underground has the weight of all the rock above it pressing down
on it. As erosion removes the rock from above, the weight of the
rock is reduced. This reduction in the weight on the rock allows it
to expand, similar to the way a squished sponge would expand after
the weight is taken off of it. However, rocks are much more brittle
than a sponge, so they fracture in lines parallel to the ground
All of these forces have occurred at Joshua Tree
sometime throughout its history, so there is a complex set of
intersecting joints running through the rocks. Not all of the
joints formed at the same time and some are more eroded than
others. The scale of the joints range from the cracks in the
rocks in front of you to large joint systems recognizable in
satellite photos. The large scale joint system of the
Wonderland of Rocks can be seen in the aerial photo to the
Send me a note with :
- The text "GCZ5XR Joints of the Wonderland of Rocks - Joshua
Tree NP" on the first line
- The number of people in your group.
- Look at the rocks, you should be able to see 3 sets of joints.
Two sets going up and down (vertical)and some what perpendicular to
eachother and another set somewhat level with the ground surface.
Do your best to get the compass direction of the two vertical
sets of joints. Send your measurements to me in an
The following sources were used to generate this
- Trent, D.D. & Richard W.
Hazlett, Joshua Tree National Park Geology, Joshua Tree National
Park Association, 2002
- Google Earth
Placement approved by the
Joshua Tree National Park