Mountain Building Models and the Geologic History of Virginia
~Introduction to Mountain Building Models~
In the past 1.1 billion years the eastern seaboard of North America has experienced four major mountain building episodes; the Grenville, the Taconic, the Acadian, and the Alleghenian. Virginia has a viable record of each of these events, however, these mountain building events did not create the modern day Appalachian mountain chain. The modern Appalachian mountains, for example, are not "mountains" in a plate tectonic sense; they have not been created by dynamic forces currently thrusting the land upward. The Appalachian mountains are more complex than that. The rocks that make up the mountains are the fragmented, eroded remains of at least four ancient mountains that were between 5 and 10 times taller than the present day chain. Structures in the rocks that give the Appalachian mountains their distinctive shape and size are ancient structures created in a former mountain 300 million years ago.
In contrast, the modern Appalachian mountains are the result of a gentle, almost passive uplifting, followed by erosion that has removed the soft rock in the valleys, and left the hard, resistant rocks at the ridges. "Mountains" in a geologic history describes something more dynamic and powerful than the modern Appalachian mountains. Because there are many different types of mountains in existence it is useful to apply a model for understanding the different kinds of mountains so that we can distinguish them in a geological framework.
~Mountain Building Models~
Mountains that result from plate tectonic processes are typically the types of mountains considered in geology. It is not easy to provide a complete classification here since processes overlap, but mountain-building can be considered from two perspectives: 1) the energy source driving it (heat vs mechanical), and 2) the relationship to plate tectonics (divergent, convergent, and transform boundaries, and within plates). The table below briefly illustrates some of the processes and how they overlap.
It is important to recognize that all mountain building ultimately comes from heat, but more interesting is the immediate cause of the mountain. Very commonly, more than one of these mechanisms may operate in the same place, either at the same time, or sequentially.
~Energy Source Mountain Building: Heat vs Mechanical Energy~
First and foremost, mountain building results primarily from heat
and uplift occurs secondarily because heat from inside the earth heats the overlying lithosphere, causing it to expand, lifting and swelling the surface upward. Often associated with these processes are the formation of volcanoes, including hot spots and subduction volcanoes. Hot spot volcanoes are discussed in the Rifting Model
section. Subduction volcanoes are generated along subduction zones at convergent plate boundaries; they are cordilleran mountains, discussed below.
Second, is mechanical mountain building; that is, tension (pulling apart) and compression (squeezing together). The relief (differences in elevation) changes generated during these processes occur because different blocks of earth move relative to each other, either falling or rising vertically, or thrusting horizontally over one another.
~Plate Boundary Mountain Classifications~
Plate Boundary Mountain Building
Mountain building is associated with all three kinds of plate boundaries: divergent, convergent, and transform. Here, we consider divergent and convergent plate boundaries only and movement along these boundaries. In the classification below, we try to capture the dynamics of processes on earth, that change rapidly and transition from one process to another. Below we discuss only the mountain types written in red in the table above, although clicking on any mountain type will take you to a fuller discussion of it somewhere.
~Divergent Plate Boundaries: Continental Rift Zones~
New divergent plate boundaries are created at continental rifting centers by tension (pulling apart). This mechanism is explored in more detail in the Rifting Model
section. Here we look only at the principles of tensional mountain building.
Tensional mountain building is almost always of the block
faulted type (horsts and graben). Any condition that pulls the earth apart can cause this type of faulting and for this to occur, something must create the tension. There are circumstances where the tensional stresses operate horizontally, parallel to the earth's surface (for example at transform plate boundaries, but many times the tension occurs because the earth is lifted upward from below by heat (i.e., hot spot). Lifting the earth upward is something like blowing up a balloon; the surface stretches.
When the earth stretches, brittle rock cracks into blocks, and a gap may open as the earth spreads apart. Gravity causes one block of to slide down into the gap. The graben is the down faulted block, and as it goes down, a valley is formed, as shown in the diagram to the right. The blocks on either side of the valley that do not slide down are the horst mountains. This process occurs in tens of thousands of tiny steps, each creating a small gap, the sum total of which stretches the earth apart many kilometers.
This type of faulting is called "normal." Note that the faults form a dipping surface, an inclined plane, and that the graben block slides down that plane. It is "normal" for things to slide down inclined planes under gravity; thus the normal fault (if the block moves up the inclined fault plane it is called a "reverse fault"). These faults can be quite large. The photograph to the right is of the East African rift west of Nairobi, Kenya. The "mountains" are horst blocks, and the valley is the graben.
~Convergent Plate Boundaries: Subduction Zones~
Convergent plate boundaries exist when two plates move toward each other. Convergence begins when oceanic lithosphere decouples, that is, breaks at some place and descends into the mantle along a subduction zone. It is always oceanic crust that decouples and descends as continental crust is too light and tends to override the oceanic crust. Subduction zones can form anywhere in an ocean basin, and face any direction. Also, more than one subduction may be active in an ocean basin at the same time. Thus a complex sequence of mountain building episodes are possible, and are not unusual.
Convergent mountain building is also complex in its mechanisms, since although convergence is compressive, mountains may be either heat driven, or mechanically driven, or a combination. But even in the heat driven examples, it is compression that is ultimately generating the heat so we have to understand the relationships and processes operating. A simple classification is possible, however, and is shown in the illustration below. Mountains result from either subduction or collision.
Two kinds of subduction orogenies exist; those within ocean basins (island arc type), and those that descend under continents (cordilleran type). The island arc type orogeny is illustrated by the islands of Japan, and the Aleutian islands. Here uplift is mostly heat driven as magma rises off of the subduction zone. In addition, the volcano itself builds a mountain on top of the swelling.
The cordilleran type mountain building is illustrated by the Andes mountains, or the Cascades. Here also, heat swells the continent upward, and then volcanos build even higher on top of that.
Both of these subduction orogenic types have numerous processes, and generate a wide diversity of rocks and structures. For full details and explanations see the Wilson Cycle Stages E
The second class of convergent plate orogenies are collision orogenies. We recognize two types; continent-island arc type, and continent-continent type. In both cases an ocean basin descends completely below a subduction zone until it disappears completely. These are called "remnant ocean basins" (ROB) since they are only a remnant of their former selves. The two floating blocks on either side of the remnant ocean then collide. The floating blocks may be island arcs, or microcontinents, or continents, or some combination of these.
In the cross sections above, the continent-island arc collision has not taken place yet, but is imminent. The continent-continent collision has occurred and one continent has overridden the other. In both cases the subduction zone acts as a ramp, and one block will slide up and over the other. The overriding block is a hinterland; the overridden block the foreland.
Note that it is not possible to produce a collision orogeny without at least one of the floating blocks undergoing a subduction orogeny first (island arc or cordilleran type); it is the only way an ocean basin can close. As with the subduction orogeny, a collision orogeny can produce a wide diversity of rocks and structures. For more details see the Wilson Cycle. Each of these mountain building types have played important roles in Virginia's geologic history. The 16 Page History explores them in detail.
Contributed by Lynn Fichter