Plate Tectonics

The thin, outermost layer or shell of the earth is called the crust. The crust is made up of solid rock and is found below the oceans as well as across the continents. On average, continental crust is much thicker than oceanic crust, about 30 to 35 kilometers as opposed to about 5 km beneath the oceans. Below the crust is the mantle. With increasing depth into the earth and distance from the surface, the mantle material gets hotter and behaves more like a plastic than solid rock. This portion of the earth is known as the asthenosphere, and the rocks in this zone can stretch and fold, deform without fracturing, and even flow very slowly. But the upper portion of the mantle, like the crust, is solid. Together, the crust and the upper, solid part of the mantle form the lithosphere. Because the lithospheric materials are solid, they are more likely to break or fracture when compressed or stretched. In the mid-20^th century, geologists recognized that the lithosphere is not one uniform sheet around the earth but is broken into a number of different, thick slabs they called tectonic plates. These tectonic plates are made up of relatively light rock, in contrast to the heavier materials deeper in the earth, and they are able to float on the dense, plastic, flowing asthenosphere. There are about a dozen or so major tectonic plates and several minor ones. Through time, convective forces deeper within the earth have caused the tectonic plates to move, slowly reconfiguring the location, size and shape of oceans, continents and other land masses that we see today. https://geomaps.wr.usgs.gov/parks/pltec/index.html#crust

The tectonic plates under the world’s oceans are composed of different and generally heavier rock types than the plates that carry the continents (although, most tectonic plates include portions of both continental and oceanic materials, as seen on the figure). The North American plate, for example, is comprised not only by the North American continent, Greenland, and portions of other land masses, but also a significant portion of the Atlantic and other oceanic basins. The tectonic plates move independently; they may collide with one another at a convergence zone, move apart at divergence zones, or move past each other at transform zones. The Grand Wash Cliffs are some distance away from where the North American continental plate meets the Pacific plate, or any other lithospheric plate boundary today. But the geologic record contained in rocks of the Parashant, from both the Colorado Plateau and the Basin and Range provinces, has always been directed by the movement of distant tectonic plates. The geologic history of this area can only be understood by knowing how tectonic plates move and what happens at plate boundaries.

 

Convergent Plate Boundary

For graphic representations of these plate boundaries see this US Geological Survey website here.

When two tectonic plates are moving toward each other and ‘collide,’ (if indeed that term is appropriate for two objects that may cover 6 inches per year at their fastest pace, but more likely 1 to 2 inches per year) the denser of the two plates ends up being subducted, or pulled beneath the less-dense plate. These collisions may involve an oceanic plate and a continental plate, such as in the northwestern United States and part of western Canada, where the oceanic Juan de Fuca plate is being subducted below the North American plate. Or collisions may occur between two lithospheric plates beneath oceans, as occurs at the boundary of the Caribbean and South American plates. For either scenario, because the subducted plate is pulled down into the hot asthenosphere, trapped water and other gases are released, migrate upward and melt overlying rocks of the mantle. The resulting hot magma generated in the mantle rises upward into the lithosphere. Although most of this molten rock cools and solidifies far below the earth's surface, forming large intrusive masses at the core of great mountain ranges like the Sierra Nevada, some will reach and break through the earth’s surface. The resultant volcanic activity may involve huge explosions of gases, lava, and ash, and over time repeated eruptions will also build mountain ranges, such as the Cascades of the northwest. As will be discussed later in this report, subduction zone tectonics has played and continues to play a major role in shaping the geologic and geomorphic landscape of the Monument.

A third convergence boundary involves two continental plates. Even though both plates will have similar densities, one of them will inevitably end up being pushed and pulled beneath the other. However, both continents are relatively light and buoyant, and the subducted plate will not sink to the depths needed to volatilize the entrapped water and gases, so volcanic activity is less common. Instead, the solid rock of both plates gets smashed together, folded and faulted, with huge chunks kilometers wide broken and thrust on top of one another. The Himalayan Range is evidence of the mountain-building power associated with continental plate collisions.

Divergent Plate Boundary

Since the size of the earth isn’t changing, if new plates are converging in one area there must be spreading or divergence along plate boundaries in another area. Almost all the earth's new crust forms at divergent boundaries, which usually lie deep beneath the oceans but can also form within continents. These are areas where rifts in the crust allow magma from the mantle to rise up and solidify as new crust while pushing two plates away from each other.

As hot magma moves slowly upward from the lower mantle, convection currents move the plastic and semi-fluid materials within the asthenosphere. As the currents approach the surface they diverge, and the heat and tension of the moving magma weakens the overlying lithospheric plate. The solid lithospheric plate eventually breaks apart, and the two pieces or sides move away (diverge) in opposite directions. Magma fills the weakened areas between the plates and is quickly cooled by sea water, which solidifies and becomes new oceanic plate material. This ongoing process creates volcanic chains, such as in Iceland along the Mid-Atlantic Ridge spreading center, and rift zones such as within the Gulf of California, where rocks of Baja, part of the Pacific plate, are slowly being pushed away from the North American plate.

Transform Boundary

At transform boundaries, plates grind horizontally past one another. This type of boundary separates the North American plate from the Pacific plate along the San Andreas fault, a famous transform plate boundary that's responsible for many of California's earthquakes. Most transform faults are found on the ocean floor but some, such as the San Andreas fault zone, occur on land. The San Andreas, which is about 1,300 km long and in places tens of kilometers wide, slices through two thirds of the length of California. Along it, the Pacific Plate has been grinding north past the North American Plate for 10 million years, at a rate of about 5 cm/yr. (USGS Webpage at https://pubs.usgs.gov/gip/dynamic/understanding.html). Although not close to the Parashant, development of the San Andreas transform fault system significantly affected this part of the western United States and initiated a transformation into a Basin and Range landscape.