In the broad sense, tectonics refers to the large scale internally-driven processes responsible for constructing features on the earth's surface.  Most tectonic activity occurs along or in proximity to the margins of giant tectonic plates(fig 1). Mountain belts, rift valleys, mid-oceanic ridges, and volcanic arcs are all tectonic in origin. Tectonic activity is driven by thermal energy, which causes rocks in the mantle to flow and sets the lithospheric plates above in motion.

 Tectonic deformation creates the large-scale variations in topography and geology defining the geologic provinces.  Provinces that are flat and have little relief lie beyond the buckling and warping activity causing mountains to rise along plate margins.  The Pacific and Columbia provinces are realms of igneous activity created by plate subduction or rising subcrustal plumes. The Basin and Range Province, wherein lies Death Valley National Park, contains hundreds of parallel, north-east trending mountain ranges and valleys pulled and sheared apart by the interactions of the Pacific and North America Plates. The elevated Colorado Plateau Province, forced up recently by tectonic activity to the west, contains thousands of feet of sedimentary rock deposited on a stable platform that prevailed for nearly 500 thousand years.  Unraveling past and present tectonic settings is key to understanding park geology.

Figure 1. The earth's lithospheric plates.  The earth's rigid outer layer (lithosphere) is broken into several mobile plates that grind together and tear apart creating mountains, earthquakes, and volcanic activity.  The most stable regions of the earth's surface (i.e. Interior Plains and Atlantic Province) lie within the interior of the plates.    (Image source: This Dynamic Planet, url: http://pubs.usgs.gov/gip/dynamic/ Vigil.html)

Plate Tectonic - In depth

Plate tectonics is the theory that the earth's rigid outer layer, the lithosphere, is broken into several plates, like a cracked shell. Movement of lithospheric plates is driving by the Earth's internal heat that sets the solid mantle into a state of slow convection. Plates converge along the descending and diverge along ascending limbs of vast convection cells.  Presently there are twelve major lithospheric plates that float over the soft rocky layer beneath called the asthenosphere(USGS). (See Inside the Earth form the USGS.) Most tectonic deformation occurs were plates interact along their boundaries (fig. 2). Where they converge rocks are folded into mountains and intruded by molten rock, where they diverge stretching produces fault-bound basins and creates oceanic crust, and where they slide past each other linear faults scar the landscape.  Escaping heat drives the convection that sets these plates in motion. They travel and grind against each other at rates of 1-6 cm/year. With few exceptions plate activity controls the distribution of earthquakes, mountain, and volcanic activity.  

Over geologic time, continental landmasses merge and rift apart. (Read supercontinent cycle.) Consolidation of land masses produce supercontinents, such as Rodinia (Paleos), assembled during the Precambrian between 1.7-1.1 billion years ago, and Pangea, constructed between 450-250 million years ago.    Rocks recording the formation of Rodinia are exposed in the inner gorge of the Grand Canyon, the mountains surrounding Death Valley, and in the rugged peaks of the Rocky Mountains.  Events leading the the formation of Pangea are recorded in rocks seen in parks along the Appalachian Mountains, such as Acadia National Park in Maine, and Shenandoah National Park  and the Blue Ridge Parkway in Virginia and North Carolina. Pangea began splitting apart 200 million years ago. Its dispersed fragments form the present continents. (View animation by Christopher Scotese.) 

Outlined below are four major tectonic settings defined by a region's location relative to plates boundaries (fig. 1), the breadth of the area affected by plate motion, and the occurrence of thermal plumes (hotspots).  Investigate these further by reading Understanding Plate Motion (USGS).

1. Plate margin (boundary) settings:  Deformation most commonly occurs in narrow linear belts that parallel plate margins (fig. 2). As shown in Table 1, relative plate motion (red) and crustal composition (bold) are used to classify plate margins.  Both of these attributes determine plate margin processes, such as subduction and sea-floor spreading.  Collision involving oceanic crust always results in subduction.  However, if both colliding plates are of buoyant continental crust they will overlap causing immense crustal thickening but no subduction.  Tectonic events leading to the formation of a mountain belt are called orogenies.   An orogenic belt may be ancient and deeply eroded like the Appalachains or modern and still rising like the Hymalayas. (View animations.) They are all evidence of colliding plates, past or present.  The largest orogenic belts are created by the collision of plates containing continental crust that can't subduction but thickens and overlap building huge contiental mountain belts.  Over time the plates will suture together, the moutain belt will erode and then 10s to 100s of million years later rift apart creating a new ocean basin rimmed by subsiding continental margins. Sea-floor spreading, the product of plate divergence, creates new oceanic lithosphere.
   
   As outlined in Table 1, the three major boundary types defined by relative motion are convergent, divergent, and transform (animations).  Any further division of these into oceanic-oceanic, continental-continental, and oceanic and continental is based on the type of crust separated by the boundary.  No subdivisions exist for transform boundaries because  they are not influenced by crustal composition.  
   

Table 1. Types of plate boundaries (map)

Convergent Boundaries	Divergent Boundaries	Transform Boundary
a. oceanic-continental (subduction zone and volcanic arc) b. oceanic-oceanic (subduction zone and volcanic arc) c. continental-continental (alpine mountain belt)	a. continental-continental (continental rifting) b. oceanic-oceanic (sea floor spreading and formation of oceanic ridges and rises) animation	(no subdivisions)



2. Plate boundary zones: Occasionally a boundary may splinter into a network of interacting pieces covering a broad region, rather than a narrow linear belt.  Such regions are plate boundary zones and can extend up to 1000 miles or more in width. A broad convergent plate boundary zone is reflected in the complex zone of mountains and plateaus created by India's collision into Asia. In North America, the Basin and Range is an example of an incipient divergent plate boundary zone.  This western region, extending from the Sierra Nevada Mountains in California to the continental divide in the Rockies, is being tectonically stretched and torn apart.  Thinning and fracturing of the crust throughout much of the the West is evidenced by the earthquakes, lava flows, and the fault-block mountains and basins so typical of the province.  
   
3. Stable intraplate or within-plate setting: Intraplate settings are notably stable.   Plate interiors are shielded from the crumpling and faulting produced by activity along plate margins. The scarcity of earthquakes and complete lack of volcanic activity in the Interior Plains, Laurentian Uplands and Atlantic Plain is attributed to the location of these provinces within the center of the North American Plate.  For several hundreds of millions of years the prevailing geologic processes affecting the stable interiors is erosion on land and deposition in lakes and shallow seas.  Long term erosion destroys mountains, and deposition buries irregularities. Both processes make stable areas flat and reduce their relief.   The dominance of deposition over erosion is controlled by changes in baselevel--a concept that will be discussed in more detail later when we move on to the Colorado Plateau.  
   
   Mountain belts found within a plate are sutures left behind by past collisions. The Appalachian Mountains rose along a convergent zone that became inactive with the final assembly of Pangea. The mountains built, which would have rivaled the Himalayan Mountains in their grandeur, are now eroded to their roots. Rifting of Pangea didn't occur along the thickened crust of the mountain belt, but to the east, leaving behind the Appalachians within the newly modified North American plate. Within-plate, seismically-inactive stable mountain belts, such as the Appalachian Mountains, and Ural Mountains in Russia, are continental sutures where past continents collided and were joined. Since their creation, erosion has removed miles of rock from their surface, exposing their metamorphic and igneous core.
   
4. Intraplate hotspot setting:  Scattered throughout the earth are hot boils just beneath the lithosphere or deep within the mantle that send plumes of magma to the surface.  These hotspots (fig. 1) can occur along a plate boundary (e.g. Iceland) or within a plate (e.g. Yellowstone).   They may actually be responsible for continental rifting.  When a plate moves over a hotspot a trail of volcanic features is left behind.  Along the entire hotspot trend the only active region lies at the the end presently over the plume. (c.f. the Hawaiian-Emperor seamount chain, animation). Hawaiian Volcanoes National Park sits over a plume beneath the Pacific Plate. Craters of the Moon National Monument and Yellowstone National Park  are the product of hotspot activity beneath the North American Plate.

In summary, if there is a rising mountain range (e.g. the Alps) or seismically active trough (e.g. East African Rift System) you can be sure that it marks a plate boundary. The further a region is from a plate's edge the more stable it's likely to be. For over a billion years the Midwest has been in the center of the North American Plate, far from any plate margin.  Predictably its sedimentary cover is undeformed and its landscape flat. Landscapes also record past tectonic activity. Seismically inactive, deeply-eroded mountain ranges, like the Appalachians in the eastern U.S., record ancient plate collisions, or orogenies, and the location of a past boundaries now sutured.  Understanding the landscape of a national park necessitates consideration of both its present and past tectonic setting.

Continental Margins and Tectonic Setting

The true edge of the continent is where the submerged portion of the continental margin slopes abruptly into the deep ocean basin (shelf break in fig. 3). There is a tendency for introductory students to assume that plate boundaries follow continental margins.  However this is not the case, as you can observe by looking a the map of lithospheric plates in figure 1.   Most continental margins do not lie along plate boundaries and those that do are quite different in their width, shape, and supply of sediment.  This has led geologist to designate two types of continental margins based on tectonic setting into active margins, lying along plate boundaries, such as the west coast of the Americas (c.f. western continental margin shown in fig. 2), and passive margins, located within a plate, such as the east coast of the Americas (e.g. fig. 3).  Active margins are narrow, straight, and backed by mountains that shed coarse sediment conveyed by numerous short streams to the coast.  Active margin accumulations may be uplifted and deformed within a few million years after their deposition.  In contrast,  passive margins are flanked by broad continental shelves that are covered by thick wedges of mud and sand sediment (e.g. fig. 3).  They are tectonically inactive and receive sediment from long rivers traversing the continent from elevated regions far from the margin.  Most large barrier island complexes and large river deltas are located on passive margins. Because passive margins are stable for hundreds of millions of years they accumulate thousands of feet of marine and coastal sediments that eventually harden into horizontal layers of sedimentary rocks, such as shale, sandstone, and limestone.  Thus, wherever thick sequences of sedimentary rocks are encountered (e.g. Grand Canyon and Glacier National Park), a record of a past passive margin deposition is often recorded.  A marginal sea coast is a passive margin protected from the open ocean by an island arc lying outboard of the continental margin.  For example, the western coast of China is protected by the Japanese Arc. 

Figure 3. Geologic cross-section of the passive continental margin along the East Coast of North America (locally New Jersey).  The thick sedimentary wedge underlying the margin is composed of sediments eroded from the Appalachians (west of image) and deposited by streams over the last 200 million years. The black vertical lines offsetting the basement are normal faults formed during the rifting of Pangea apart. Source the Atlantic Coastal Plain in Geology of the New York City Region (USGS).

Plate Tectonic and the Growth of Continents

Throughout geologic history, continents have been growing and changing.  Nowhere is this concept best demonstrated than in the growth of North America (Laurentia).  Melting of rocks beneath subduction zones produces silica-enriched magma that rises and accumulates along volcanic arcs. These slivers of land are subsequently swept up and added to the margins of continents (animation).   Crustal thickening and accretion also occurs along continental-oceanic subduction zones where partial melting of the hot rock layer beneath produces magma that rises and congeals to the base of the crust--a process know as underplating.  With each collision a continent grows.  Southern and eastern Laurentia was extended by a series of collisions, collectively known as the Appalachian orogenies, that began in the Ordovician Period and ended in the Permian Period (figs 5-6).  When the layered marine and coastal sedimentary rocks of the Grand Canyon were deposited, California and Nevada had yet to be added.   The western North America is relatively young, created over the last 300 million years (figs. 6-8).  Tectonic activity continues to modifying sections of the West Coast.  The Olympic Peninsula is the latest piece of the puzzle.  The glaciated mountains of Olympic National Park contain oceanic basalts and marine sedimentary rocks heaved from the ocean floor onto the Pacific margin.

In addition to expanding over time, North America has traveled thousands of miles across the face of the earth, rotating and shifting latitude (figs. 4-8). Because solar radiation is dependant on latitude such movements will strongly influence climate.  Climate in turn controls weathering, sediment transport and deposition and all other external geologic processes.

Knowledge of our continent's geologic evolution and past geography is being unraveled by scientists interpreting rocks preserved in our national parks and elsewhere. National parks are designated not only retain their beauty but to protect the wealth of information contained in them. The important concept to grasp here is that we can't understand the geology of the national parks if we limit our thinking to their present tectonic and climatic setting.  Study figures 4-8 below.  Observe where North America (a.k.a. Laurentia) has traveled and how it has evolved over the last 500 million years.  Observed also how the climate must have changed as the continent moved across different latitudes, and ponder how these activities are recorded in the rock record.

	Figure 4.  World geography during the Cambrian Period (543-490 ma). By 1.6 billion years the core of ancient North America (called Laurentia) had evolved through a series of collision in the southern hemisphere.  By the Cambrian Period shown here, Laurentia had moved northward and was straddling the equator. Note that western, southern, and eastern North America are yet to be formed.  During this time sediments eroding from the barren continent are being deposited into the surrounding oceans along passive margins. Lifeforms at this stage are limited to primitive marine and plants and animals.   Note: The ancient cratons of Africa and South America are combined in a supercontinent called Gondwana. Pink toothed lines are subduction zones.
	Figure 5. Laurentia is still near the equator, however it appears smaller because sea level has risen.  Thick deposits of sediments are accumulating on in the shallow seas and passive margins surrounding the exposed craton. Microcontinents--such as Baltica and Avalonia, approach Laurentia from the southeast while intervening ocean basins are consumed along oceanic subduction zones. These microcontinents will collide within the next 100 million years to form the Northern Appalachians. West of Laurentia, Island arcs are forming outboard of the coast along  subduction zones.  These volcanic arcs will collide much later in a series of  events that will cause uplift and extend the craton further west.
	Figure 6. The Permian Period marks the final assembly of Pangea, the latest of the large supercontinents.  Baltica and Avalonia have collided, followed by Gondwana from the south. This sequence of orogenies--or mountain building events, created the Central Pangean Mountain belt of which the North American Appalachians are part.  Island arcs are building and impacting western North America, where marine conditions still dominate.  Most of the western US lies along a marginal sea east of the arc. Mountains forming to the west, and a slight shift to more northerly latitudes brings arid conditions to the southwest that prevail off and on to this day.
	Figure  7.  Pangea rifts apart. A passive margin develops along the east coast. During the Jurassic Period the supercontinent tears along a series of continental rifts. By the Cretaceous Period (shown here) sea-floor spreading is progressing in full force as North America drifts towards higher latitudes.   Because newly created oceanic crust is warm and fairly buoyant, the rapid sea-floor spreading increases global sea level. Rising sea level combined with crustal loading of the western craton produces an interior seaway that extends from Alaska to the Gulf of Mexico.  The evolving passive margin along the East Coast is blanketed by a thick wedge of sediment eroded from the Appalachians. Subduction accretion continues along the entire West Coast, which is approaching its present-day configuration.  However, portions of Washington, Oregon, and California are yet to be added.
	Figure 8.  The continents have reached their current location as the Atlantic Ocean basin widens.  The West Coast is still active.  Subduction continues beneath Washington and Oregon, however a transform boundary (San Andreas Fault) has evolved along the California coast. The northern hemisphere is covered by continental glaciers (white), which have advanced and retreated numerous times over the last 2 million years. The East Coast is a stable passive margin and continues to receive sediments shed from the continent. (See figure 7.) Barrier islands, deltas, and sandy beaches extend along the unglaciated regions of the coast.
Image source: Courtesy of the Paleontology Portal.  To view additional paleomaps go to Exploring Time and Space, scroll to the bottom and choose the place (North America) and time.

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