Alluvial fans

Alluvial fans

Fan- or cone-shaped deposits of fluvial gravel, sand, and other material radiating away from a single point source on a mountainside. They represent erosionaldepositional systems in which rock material is eroded from mountains and carried by rivers to the foot of the mountains, where it is deposited in the alluvial fans. The apex of an alluvia fan is the point source from which the river system emerges from the mountains and typically breaks into several smaller distributaries forming a braided stream network that frequently shifts in position on the fan, evenly distributing alluvial gravels across the fan with time. The shape of alluvial fans depends on many factors, including tectonic uplift and subsidence, and climatic influences that change the relative river load-discharge balance. If the discharge decreases with time, the river may downcut through part of the fan and emerge partway through the fan surface as a point-source for a new cone. This type of morphology also develops in places where the basin is being uplifted relative to the mountains. In places where the mountains are being uplifted relative to the basin containing the fan, the alluvial fan typically displays several, progressively steeper surfaces toward the fan apex. In many places, several alluvial fans merge together at the foot of a mountain and form a continuous depositional surface known as a bajada, alluvial apron, or alluvial slope. The surface slope of alluvial fans may be as steep as 10° near the fan apex and typically decreases in the down-fan direction toward the toe of the fan. Most fans have a concave
upward profile. The slope of the fan at the apex is typically the same as that of the river emerging from the mountains, showing that deposition on the fans is not controlled by a sudden decrease in gradient along the river profile. Alluvial fans that form at the outlets of large drainage basins are larger than alluvial fans that form at the outlets of smaller drainage basins. The exact relationships between fan size and drainage basin size is dependent on time, climate, type of rocks in the source terrain in the drainage basin, structure, slope, tectonic setting, and the space available for the fan to grow into. Alluvial fans are common sights along mountain fronts in arid environments but also form in all other types of climatic conditions. Flow on the fans is typically confined to a single or a few active channels on one part of the fan, and shifts to other parts of the fan in flood events in humid environments or in response to the rare flow events in arid environments. Deposition on the fans is initiated when the flow leaves the confines of the channel, and the flow velocity and
depth decrease dramatically. Deposition on the fans may also be induced by water seeping into the porous gravel and sand on the fan surface, which has the effect of decreasing the flow discharge, initiating deposition. In arid environments it is common for the entire flow to seep into the porous fan before it reaches the toe of the fan. The sedimentary deposits on alluvial fans include fluvial gravels, sands, and overbank muds, as well as debris flow and mudflow deposits on many fans. The debris flows are characterized by large boulders embedded in a fine-grained, typically mud-dominated matrix. These deposits shift laterally
across the fan, although the debris and mudflow deposits tend to be confined to channels. The fan surface may exhibit a microtopography related to the different sedimentary facies and deposit types. The development of fan morphology, the slope, relative aggradation versus downcutting of channels, and the growth or retreat of the toe and apex of the fan are complex phenomenadependent on a number of variables. Foremost among these are the climate, the relative uplift and subsidence of the mountains and valleys, base level in the valleys, and the sediment supply.

Aleutian Islands and Trench

Aleutian Islands and Trench

Stretching 1,243 miles (2,000 km) west from the western tip of the Alaskan Peninsula, the Aleutian Islands form a rugged chain of volcanic islands that stretch to the Komandorski Islands near the Kamchatka Peninsula of Russia. The islands form an island arc system above the Pacific plate, which is subducted in the Aleutian trench, a 5-mile (8-km) deep trough ocean-ward of the Aleutian Islands. They are one of the most volcanically active island chains in the world, typically hosting several eruptions per year. The Aleutians consist of several main island groups, including the Fox Islands closest to the Alaskan mainland, then moving out toward the Bering Sea and Kamchatka to the Andreanof Islands, the Rat Islands, and the Near Islands. The climate of the Aleutians is characterized by nearly constant fog and heavy rains, but generally moderate temperatures. Snow may fall in heavy quantities in the winter months. The islands are almost treeless but have thick grasses, bushes, and sedges, and are inhabited by deer and sheep. The local Inuit population subsists on fishing and hunting.
The first westerner to discover the Aleutians was the Danish explorer Vitus Bering, when employed by Russia in 1741. Russian trappers and traders established settlements on the islands and employed local Inuit to hunt otters, seals, and fox. The Aleutians were purchased by the United States along with the rest of Alaska from Russia in 1867. The only good harbor in the Aleutian is at Dutch Harbor, used as a transshipping port, a gold boomtown, and as a World War II naval base.

Air Pressure

Air Pressure

The weight of the air above a given level. Thisweight produces a force in all directions caused by constantly moving air molecules bumping into each other and objects in the atmosphere. The air molecules in the atmosphere are constantly moving and bumping into each other with each air molecule averaging a remarkable 10 billion collisions per second with other air molecules near the Earth’s surface. The density of air molecules is highest near the surface, decreases rapidly upward in the lower 62 miles (100 km) of the atmosphere, then decreases slowly upward to above 310 miles (500 km). Air molecules are pulled toward the Earth by gravity and are therefore more abundant closer to the surface. Pressure, including air pressure, is measured as the force divided by the area over which it acts. The air pressure is greatest near the
Earth’s surface and decreases with height, because there is a greater number of air molecules near the Earth’s surface (the air pressure represents the sum of the total mass of air above a certain point). A one-square-inch column of air extending from sea level to the top of the atmosphere weighs about 14.7
pounds. The typical air pressure at sea level is therefore 14.7 pounds per square inch. It is commonly measured in units of millibars (mb) or hectopascals (hPa), and also in inches of mercury. Standard air pressure in these units equals 1,013.25 mb, 1,013.25 hPa, and 29.92 in of mercury. Air pressure is equal in all directions, unlike some pressures (such as a weight on one’s head) that act in one direction. This explains why objects and people are not crushed or deformed by the pressure of the overlying atmosphere. Air pressure also changes in response to temperature and density, as expressed by the gas law: Pressure = temperature × density × constant (gas constant, equal to 2.87 × 106 erg/g K). From this gas law, it is apparent that at the same temperature, air at a higher pressure is denser than air at a lower pressure. Therefore, high-pressure regions of the atmosphere are characterized by denser air, with more molecules of air than areas of low pressure. These pressure changes are caused by wind that moves air molecules into and out of a region. When more air molecules move into an area than move out, the area is called an area of net convergence. Conversely, in areas of low pressure, more air molecules are moving out than in, and the area is one of divergence. If the air density is constant and the temperature changes, the gas law states that at a given atmospheric level, as the temperature increases, the air pressure decreases. Using these relationships, if either the temperature or pressure is known, the other can be calculated. If the air above a location is heated, it will expand and rise; if air is cooled, it will contract, become denser, and sink closer to the surface. Therefore, the air pressure decreases rapidly with height in the cold column of air because the molecules are packed closely to the surface. In the warm column of air, the air pressure will be higher at any height than
in the cold column of air, because the air has expanded and more of the original air molecules are above the specific height than in the cold column. Therefore, warm air masses at height are generally associated with high-pressure systems, whereas cold air aloft is generally associated with low pressure. Heating and cooling of air above a location causes the air pressure to change in that location, causing lateral variation in air pressure across a region. Air will flow from highpressure areas to low-pressure areas, forming winds. The daily heating and cooling of air masses by the Sun can in some situations cause the opposite effect, if not overwhelmed by effects of the heating and cooling of the upper atmosphere. Over large continental areas, such as the southwestern United States, the daily heating and cooling cycle is associated with air pressure fall and rise, as expected from the gas law. As the temperature rises in these locations the pressure decreases, then increases again in the night when the temperature falls. Air must flow in and out of a given vertical
column on a diurnal basis for these pressure changes to occur, as opposed to having the column rise and fall in response to the temperature changes.

Afar Depression, Ethiopia

Afar Depression, Ethiopia

One of the world’s largest, deepest regions below sea level that is subaerially exposed on the continents, home to some of the earliest known hominid fossils. It is a hot, arid region, where the Awash River drains
northward out of the East African rift system, and is evaporated in Lake Abhe before it reaches the sea. It is located in eastern Africa in Ethiopia and Eritrea, between Sudan and Somalia, and across the Red Sea and Gulf of Aden from Yemen. The reason the region is so topographically low is that it is located at a tectonic triple junction, where three main plates are spreading apart, causing regional subsidence. The Arabian plate is moving northeast away from the African plate, and the Somali plate is moving, at a much slower rate, to the southeast away from Africa. The southern Red Sea and north-central Afar Depression form two parallel north-northwest-trending rift basins, separated by the Danakil Horst, related to the separation of
Arabia from Africa. Of the two rifts, the Afar depression is exposed at the surface, whereas the Red Sea rift floor is submerged below the sea. The north-central Afar rift is complex, consisting of many grabens and horsts. The Afar Depression merges southward with the northeast-striking Main Ethiopian Rift, and eastward with the east-northeast-striking Gulf of Aden. The Ethiopian Plateau bounds it on the west. Pliocene volcanic rocks of the Afar stratoid series and the Pleistocene to Recent volcanics of the Axial Ranges occupy the floor of the Afar Depression. Miocene to recent detrital and chemical sediments are intercalated with the volcanics in the basins. The Main Ethiopian and North-Central Afar rifts are part of the continental East African Rift System. These two kinematically distinct rift systems, typical of intracontinental
rifting, are at different stages of evolution. In the north and east, the continental rifts meet the oceanic rifts of the Red Sea and the Gulf of Aden, respectively, both of which have propagated into the continent. Seismic refraction and gravity studies indicate that the thickness of the crust in the Main Ethiopian Rift is less than or equal to 18.5 miles (30 km). In Afar the thickness varies from 14 to 16 miles (23–26 km) in the south
to 8.5 miles (14 km) in the north. The plateau on both sides of the rift has a crustal thickness of 21.5–27 miles (35–44 km). Rates of separation obtained from geologic and geodetic studies indicate 0.1–0.2 inches (3–6 mm) per year across the northern sector of the Main Ethiopian Rift between the African and Somali plates. The rate of spreading between Africa and Arabia across the North-Central Afar rift is relatively faster, about 0.8 inches (20 mm) per year. Paleomagnetic directions from Cenozoic basalts on the Arabian side of the Gulf of Aden indicate 7 degrees of counterclockwise rotation of the Arabian plate relative to Africa, and clockwise rotations of up to 11 degrees for blocks in eastern Afar. The initiation of extension on both sides of the southernmost Red Sea Rift, Ethiopia, and Yemen appear coeval, with extension starting between 22 million and 29 million years ago.

Accretionary Wedge

Accretionary Wedge

Structurally complex parts of subduction zone systems, accretionary wedges are formed on the landward side of the trench by material scraped off from the subducting plate as well as trench fill sediments. They typically have wedge-shaped cross sections and have one of the most complex internal structures of any tectonic element known on Earth. Parts of accretionary wedges are characterized by numerous thin units of rock layers that are repeated by numerous thrust faults, whereas other parts or other wedges are characterized by relatively large semi-coherent or folded packages of rocks. They also host rocks known as tectonic mélanges that are complex mixtures of blocks and thrust slices of many rock types (such as graywacke, basalt, chert, and limestone) typically encased in a matrix of a different rock type (such as shale or serpentinite). Some accretionary wedges contain small blocks or layers of high-pressure lowtemperature
metamorphic rocks (known as blueschists) that have formed deep within the wedge where pressures are high and temperatures are low because of the insulating effect of the cold subducting plate. These high-pressure rocks were brought to the surface by structural processes. Accretionary wedges grow by the progressive offscraping of material from the trench and subducting plate, which constantly pushes new material in front of and under the wedge as plate tectonics drives plate convergence. The type and style
of material that is offscraped and incorporated into the wedge depends on the type of material near the surface on the subducting plate. Subducting plates with thin veneers of sediment on their surface yield packages in the accretionary wedge dominated by basalt and chert rock types, whereas subducting plates with thick sequences of graywacke sediments yield packages in the accretionary wedge dominated by
graywacke. They may also grow by a process known as underplating, where packages (thrust slices of rock from the subducting plate) are added to the base of the accretionary wedge, a process that typically causes folding of the overlying parts of the wedge. The fronts or toes of accretionary wedges are also characterized by material slumping off of the steep slope of the wedge into the trench. This material may then be recycled back into the accretionary wedge, forming even more complex structures. Together, the processes of offscraping and underplating tend to steepen structures and rock layers from an orientation that is near horizontal at the toe of the wedge to near vertical at the back of the wedge. The accretionary wedges are thought to behave mechanically somewhat as if they were piles of sand bulldozed in front of a plow. They grow a triangular wedge shape that increases its slope until it becomes oversteepened and mechanically unstable, which will then cause the toe of the wedge to advance by thrusting, or the top of the wedge to collapse by normal faulting. Either of these two processes can reduce the slope of the wedge and lead it to become more stable. In addition to finding the evidence for thrust faulting in accretionary wedges, structural geologists have documented many examples of normal faults where the tops of the wedges have collapsed, supporting models of extensional collapse of oversteepened wedges. Accretionary wedges are forming above nearly every subduction zone on the planet. However, these accretionary wedges presently border open oceans that have not yet closed by plate tectonic processes. Eventually, the movements of the plates and continents will cause the accretionary wedges to become involved in plate collisions that will dramatically
change the character of the accretionary wedges. They are typically overprinted by additional shortening, faulting, folding, and high-temperature metamorphism, and intruded by magmas related to arcs and collisions. These later events, coupled with the initial complexity and variety, make identification of accretionary wedges in ancient mountain belts difficult, and prone to uncertainty.

Abyssal Plains

Abyssal Plains

Flat, generally featureless plains that form large areas on the seafloor. In the Atlantic Ocean, abyssal
plains form large regions on either side of the Mid-Atlantic Ridge, covering the regions from about 435–620 miles (700–1,000 km), and they are broken occasionally by hills and volcanic islands such as the Bermuda platform, Cape Verde Islands, and the Azores. The deep abyssal areas in the Pacific Ocean are characterized by the presence of more abundant hills or seamounts, which rise up to 0.6 miles (1 km) above the seafloor. Therefore, the deep abyssal region of the Pacific is generally referred to as the abyssal hills instead of the abyssal plains. Approximately 80–85 percent of the Pacific Ocean floor lies close to areas with hills and seamounts, making the abyssal hills the most common landform on the surface of the Earth. Many of the sediments on the deep seafloor (the abyssal plain) are derived from erosion of the continents and are carried
to the deep sea by turbidity currents, wind (e.g., volcanicash), or released from floating ice. Other sediments, known as deep-sea oozes, include pelagic sediments derived from marine organic activity. When small organisms die, such as diatoms in the ocean, their shells sink to the bottom and over time can create significant accumulations. Calcareous ooze occurs at low to middle latitudes where warm water favors the growth of carbonate-secreting organisms. Calcareous oozes are not found in water that is more than 2.5–3 miles (4–5 km) deep, because this water is under such high pressure that it contains dissolved CO2 that dissolves carbonate shells. Siliceous ooze is produced by organisms that use silicon tomake their shell structure.

Aa Lava

Aa Lava

Basaltic lava flows with blocky broken surfaces.The term is of Hawaiian origin, its name originating from the
sound that a person typically makes when attempting to walkacross the lava flow in bare feet. Aa lava flows are typically10–33 feet (3–10 m) thick and move slowly downhill out ofthe volcanic vent or fissure, moving a few meters per hour.The rough, broken, blocky surface forms as the outer layer ofthe moving flow cools, and the interior of the flow remainshot and fluid and continues to move downhill. The movementof the interior of the flow breaks apart the cool, rigid surface, causing it to become a jumbled mass of blocks with angular steps between adjacent blocks. The flow front is typically verysteep and may advance into new areas by dropping a continuous  supply of recently formed hot, angular blocks in front of the flow, with the internal parts of the flow slowly overriding the mass of broken blocks. These aa lava fronts are rather noisy places, with steam and gas bubbles rising through the hot magma and a continuous clinking of cooled lava blocks rolling down the lava front. Gaps that open in the lava front, top, and sides may temporarily expose the molten lava within, showing the high temperatures inside the flow. Aa flows are therefore hazardous to property and may bulldoze buildings,
forests, or anything in their path, and then cause them to burst into flames as the hot magma comes into contact with combustible material. Since these flows move so slowly, they are not considered hazardous to humans.