Week 8 Flashcards
What is a stream?
a body of water that carries rock particles and dissolved ions and flows down slope along a clearly defined path, called a channel
Why are streams important?
1) They deliver much of the water from land to the sea
2) They erode lots of sediment particles and transport then to lower elevations
3) They carry dissolved ions
4) They are major sources of water and serve as transportation routes for humans
Origin of streams
Water that falls to the ground as precipitation can:
1. Evaporate back into the atmosphere 2. Soak into the ground
3. Remain at the surface as runoff
Runoff initially moves over the surface in a thin, continuous sheet (sheetflow), but eventually becomes confined to tiny channels called rills.
Ability to downcut
Streams tend to erode downward in an effort to reach base level (sea level). The greater the elevation above sea level, the greater the rate of downcutting. Thus, erosion tends to dominate at the headwaters (highest elevation above base level).
Gradient
The gradient (slope) tends to be higher at the upper reaches of a stream (headwaters) than its lower reaches (close to its mouth).
Bedload
large and/or dense particles that remain on the bed of a stream most of the time, but can move by tumbling and saltation
Suspended load
small and/or light particles that are suspended in water most of the time
Types of flow
1) Turbulent (non-linear) flow
2) Laminar (linear) flow
Turbulent Flow
dominates near the bottom and sides of a stream channel where swirling eddies are generated around elements of surface roughness.
Laminar Flow
dominates water that is out of reach of turbulent eddies.
Valleys
River channels are bounded on either side by valleys. Valleys reflect the erosive action of streams, which undercut and destabilize their banks. Destabilized material tumbles down into the stream channel, providing sediment particles for the stream to transport as bedload or suspended load.
Oxbow Lakes
Meander loops become more pronounced through time, resulting in cutoffs. When a meander is isolated due to a cutoff, it can form an arc-shaped lake
Floodplain Deposition
Floods are important events that deposit sediments beyond a river’s channel.
This is particularly important in lower river courses where the gradient is very low
During a flood, water spills over the river’s banks.
The sudden reduction in the speed of the water allows sediment to be deposited on the floodplain.
Levees
The coarsest sediment (sand and silt) is deposited closest to the channel, forming ridges called levees (these are exposed when the river returns to its normal water level).
Coastal System:
A beach is part of a coastal system, which includes several zones defined by their proximity to shore and the dominant processes that occur within them.
Offshore to Shoreface
Offshore and shoreface portions of the coastal profile are permanently submerged in water, below the low tide mark.
An important feature that separates the offshore from shoreface zones is fairweather wave base – the depth at which water is affected by wave movement under normal weather conditions.
Offshore Zone
The offshore zone lies below fairweather wave base and is therefore unaffected by normal waves.
Normally only receives fine sediment that settles from suspension .
Shoreface Zone
The shoreface zone lies above fairweather wave base and is constantly affected by normal waves.
The shoreface has a gentle, concave-upward shape.
The sediment is mostly sand
Surf Zone
The zone of breaking waves. Near the top of the shoreface zone, the base of a wave is slowed down due to friction with the seabed. The wave is oversteepened, and breaks, losing much of its energy.
Foreshore
Once a wave breaks, its water moves as a sheet upslope as swash, and falls back toward the sea as backwash.
Backshore
The upper, dry part of the “beach”
Sediment from Coastal Erosion
Minerals of beach sediment derived mostly from eroded rocks along the coast match those of the source rocks.
Sediment from Rivers
most sediment supplied to beaches along continental coastlines is delivered to the coast by rivers. When a river enters a large body of water (e.g. ocean), its flow rapidly decreases, resulting in the deposition of sediment at the river mouth. The resulting sediment deposit is a delta.
Longshore Drift
Water in the shoreface zone flows toward the shore at an angle, and flows back at a right angle to the shore. The net result is a current that flows parallel to the shore. This is called a longshore current. The movement of shoreface sediment by a longshore current is called longshore drift.
Rip Currents
Breaking waves approaching the beach carry water toward the beach. The water can’t just pile up there: it has to escape back out to sea somehow. Various “paths of least resistence” (e.g low areas along sandbars) provide areas for water to flow back to the sea.
Undertow
Results when water cannot escape as a rip current.
Reefs
natural structures of rock formed by marine animals
Atoll Reef
a special kind of reef that is ring-shaped and has a central lagoon
Precious Metals
Metals of high economic value (often used to make coins or jewellery)
EX. Silver, gold, platinum
Many of these metals (with the exception of silver) are desirable for the manufacture of electrical components due to their resistance to corrosion and oxidation.
Base Metals
Metals of low intrinsic value. Most base metals oxidize (tarnish) in air.
EX. iron, lead, copper, lead, zinc, and tin.
Magmatic Cooling
Some metals can be concentrated by simple gravitational settling of heavy minerals from cooling magma within an intrusive body. These heavy minerals are formed during the earliest stages of cooling and sink to the bottom of the magma chamber.
Immiscibility
the separation of metal rich liquid from silicate rich liquid
Disseminated Deposits
Pressure from the intrusion of an igneous body can generated lots of microscopic cracks into which residual fluid can penetrate and cool
Vein (lode) deposits
Fluids super-concentrated in metals can penetrate larger fractures, forming mineral-filled veins
Pegmatite
Generally felsic in composition and has enormous crystals. Commonly contain high concentrations of rare metals such as beryllium and rare earth elements
