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 surface.
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 right.
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 e-mail.
The following sources were used to generate this cache:
- 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