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Magma On The Move
Along with the faulting, volcanic activity increased across the landscape. Igneous rock is derived from molten layers of magma beneath the Earth’s crust. Under great pressure, the magma rises towards the surface through weak points such as faults. If the molten rock solidifies before reaching the surface, it is intrusive, forming tabular sills, dikes, and large bodies called plutons. These rocks are large-grained from slow cooling. Erosion of less resistant surface rock exposes the plutonic formations. Plutonic rock can be seen all over the Mojave Desert, and Joshua Tree National Park has especially fantastic examples.
Extrusive igneous rock is magma that reaches the surface, in the form of lava and explosive material. Ash, cinder, lapilli, and volcanic bombs are violently expelled by volcanoes and vents when lava is very thick and filled with gas. Cinder cones, calderas, and stratovolcanoes are formed in this way. Shield volcanoes are formed by less viscous lava. Extrusive igneous rock is usually finegrained from cooling quickly on the surface.
The most recent period of volcanic activity in the Mojave Desert occurred only three thousand years ago and was centered at the Ubehebe Crater in Death Valley NP. The hot springs in Black Canyon at Lake Mead NRA and the cinder cones and lava flows found in the Mojave National Preserve are reminders of this active period.
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