Coasts are one of the most active environments found on Earth. The landscape of coasts is constantly changing, reflecting sensitive balances between sediment influx and outflow. Today, as through all of recorded history, most people live on or near the coast. Their interactions with the coast often have profound, frequently unforeseen, influences-most of them bad.
Besides being important, active modern environments, the world's oceans and coasts are very well represented in the rock record. Sedimentary rocks cover most of the land surface and most were deposited in marine or coastal depositional environments. To understand the rock record, we must therefore understand the modern processes active within the oceans on at the coasts.
The next one and a half lectures are about oceans and coasts. We will cover the chemistry of ocean water; the different forms of flow in the oceans (currents, tides and waves); the nature and effects of El Nino; shoreline morphology, including beaches, deltas and dunes; and the role of plate tectonics in shaping coastlines. We begin with a physical description of the world's oceans.
Most ocean basins are formed by the creation of new oceanic crust at mid-ocean spreading centers. This process imprints a general morphology on ocean basins which is well demonstrated in the Atlantic. In cross-section across the Atlantic, we see much topography. Throughout the basin there are scattered bathymetric lows (topographic highs) on the ocean floor. These underwater mountains are called seamounts (at last a name that makes sense!). They are volcanic in origin, but are usually long since dead, having been formed very near the mid-ocean ridge. In the Atlantic we also find oceanic islands sitting on large, gently sloping plateaus, of which Bermuda is a prime example. These islands are produced by hotspot volcanism, where plumes of hot mantle material rise from the core- mantle boundary at nearly 2900 km depth, partially melt and erupt at the surface. In the heart of the Atlantic lies the mid-Atlantic ridge, a through going ridge where new oceanic plate is created as the North America plate (to the west) and the African and Eurasian plates (to the east) spread apart. The ridge sits high because it is hot and less dense than the colder ocean lithosphere further from the ridge, which is denser and sinks down lower. Although it is subtle, one can see a general increase in ocean depth with distance from the ridge extending all the way to the continent rise, slope and shelf where the east coast of the US and western coast of Africa (or Europe) emerge and ocean gives way to continent. We will look at ocean basin morphology in much more detail in subsequent lectures.
Basin geometry has varied through time with the opening and closing of oceans due to plate tectonics. Today's oceans are just that, today's. Oceans 100, 500 or 1000 Ma in the past looked very different in map view. Within them, however, we would find the same features: seamounts, hotspot islands, mid-ocean ridges, etc. We also believe we would have found the similarly salty water.
These different elements originate from different sources. The cations (Na, Mg, Ca and K) are largely leached (chemically weathered) from rocks and transported to the oceans by streams. The anions are the product of mantle degassing through volcanoes, which emit CO2, Cl(1-) and SO4(2-). Hydrothermal metamorphism also releases these anions into ocean water.
Stream supply and mantle outgassing are ongoing processes. Looking at the rock record, we have reason to believe that ocean salinity (and the elemental mix responsible for the salinity) has been relatively constant. This evidence comes from evaporites (chemical sediments precipitated from water), which follow the same sequence of precipitation as modern sea water implying the chemistry of the water is similar. This presents a problem. If streams and volcanoes are constantly injected Cl, Na, etc. into the water, why hasn't the salinity of sea water increased through time? To avoid increasing salinity, these must be an equal output of Cl, Na, etc. to match the stream and volcanic gas input. In other words, what is added by streams and outgassing must be removed by some other processes to maintain the balance. What are the processes that remove Cl, Na, etc.?
Many cations are used by organisms to build shells, bone and soft parts. This includes Si, Ca, P, and HCO3 as used in the production of CaCO3. When the organism dies, it sinks to the sea floor, thus these elements are converted from dissolved content to sediment and are removed from the water. Because these processes work efficiently and are always going, Ca, and carbon have short residence times in sea water.
K and some Na is absorbed by clays and deposited as sediment; Cl and Na form evaporites in arid, isolated regions; and some Cl is taken up by rocks during hydrothermal alteration. Because these processes are slower or episodic, K, Na and Cl have long residence times in the oceans.
When sea water is evaporated, the first compound to precipitate is CaCO3. Because CaCO3 is nearly saturated in average sea water, it is easily secreted by organisms, but not easily dissolved in sea water, probably explaining why so many organisms use it to form hard parts. Below a depth of 4 to 5 km, CaCO3 is undersaturated and will dissolve. Undersaturation here is the result of the greater solubility of CO2 in water that is cold and at high pressure (this is why soda releases CO2 when you warm it or uncap it, lowering the pressure). Below this depth, called the CCD (short for carbonate compensation depth) shells made of CaCO3 dissolve. For this reason, we do not find deep-water calcareous sediments, and can rule out deep-sea deposition as the source of carbonate rocks.
Looking at a parcel of sea water, salinity is a function of evaporation, the formation or melting of glacial ice (ice is pure water), and freshwater influx from streams, rain and groundwater. As these vary, the salinity of sea water varies, ranging from 33 to 41. The salinity and temperature of sea water affect its density. Sea water density increases with increasing salinity and decreases with increasing temperature. Thus cold, saline waters are dense, whereas warm, fresh waters are not. Differences of density result in stratification of ocean water as dense water sinks to the bottom and warmer, less saline water rises to the top of the ocean. This stratification produces a form of ocean circulation that we will discuss later.
The origin of water: All water is ultimately due to outgassing of the planet through volcanism at mid-ocean ridges, subduction zones and hotspot islands. Each year about 15 cubic kilometers of new rock is erupted. This is derived from the mantle by partial melting and carries about 0.5% water by weight. If we assume that this water escapes the rock and finds its way to the oceans, then over the course of the Earth's 4.55 Ga history, outgassing would produce about 95% of the current oceanic volume of water. This assumes, however, that all the water in newly erupted rock ends up in the ocean and remains there. A more realistic set of assumptions that allows water to be recycled into the mantle reduces this number to about 25%, implying that outgassing must have been more vigorous in the past. This is acceptable since we know that the young Earth was warmer and produced much more volcanism. Today it appears that the rate at which water is outgassed is about equal to the rate at which water is recycled into the mantle by subduction of water-rich sediments and oceanic crust. In other words, the oceans don't appear to be growing or shrinking today (although sea level still rises and drops due to the volume of water held in glacial ice and changes in ocean basin bathymetry).
Coastal upwelling in California: California climate is strongly influenced by upwelling offshore. Upwelling implies the upward movement of cold deep waters towards the surface. Cold water at the surface cools the air and causes it to loose moisture (warm air carries more water than cold air), producing coastal California's cool, dry climate. It also allows cold-water fish species to survive far south of their usual habitat. Upwelling is the result of surface currents produced by winds and Ekman currents, the latter produced through the interaction of Coriolis forces and friction. To understand Ekman currents, let's think about what happens when wind blows over water. Wind generates surface currents by frictional drag as it blows along the water. These surface currents are deflected by Coriolis forces. By frictional drag with deeper water, they create deeper currents which are also deflected by Coriolis forces. As we move down through the water column, we find progressively slower currents (since frictional drag is not great) that are increasingly deflected from the wind direction. The deflection is great enough that the average current direction is perpendicular to the winds that produced it. At the surface they are parallel, at great depth the water actually moves in the exact opposite direction of the wind. The average, however, is perpendicular to the wind. Okay, so how do Ekman currents create upwelling in California? Well, in California the dominant wind direction is from the northwest. This wind creates a southwest Ekman current because Ekman currents are perpendicular to the wind. This southwest current moves water away from the coast. As near-surface water flows away from the coast, deep water rises to take its place, and we have upwelling.
Thermohaline circulation: Cold, saline water is denser than warm, less saline water and will sink below it. When it sinks, it displaces water which must flow out laterally to make room. As it sinks, water is sucked in to replace it, creating a net flow system. This system is driven by variations in water temperature and salinity, hence the name: thermohaline circulation (haline for halite, the prime dissolved constituent of sea water). The production of cold, saline water is related to surface currents, so thermohaline circulation is tied to surface currents and the winds.
Thermohaline circulation patterns are complex and very slow (0.0001 m/s). We know they
exist by measurements of the salinity and temperature of ocean waters and by tracking
distinctive features within the water column. Unlike surface currents, thermohaline
circulation involves the entire ocean. A major component of the circulation is the
movement of water in the Atlantic. Here warm, salty water flows north in the Gulf Stream.
Its high salinity is due to evaporation; its temperature due to time spent at the surface near
the equator where solar heating is most efficient. As this water flows north it cools. By
the time it reaches Greenland it is cold and very dense and sinks. This water then flows
along the ocean floor south toward Antarctica, follows Antarctica around to the southern
Pacific and flows north. It eventually upwells off Alaska which blocks its northerly path.
This water then flows south through Indonesia, around Africa and back to the southern
Atlantic, warming and mixing with fresher water, to start the cycle again.
(For a more extensive survey of the global ocean conveyor belt see the following links:
Warnings from the Ice (Time, 1997)
The Conveyor Belt -- A Key Global Phenomenon? (1997)
1998: International Year Of The Ocean
The JHU/APL Ocean Currents Web Site
EPA's Global Warming Site
Rapid Climate Change (American Scientist, july 1999) by Kendrick Taylor, the original article
Climate rides on ocean conveyor belt (1999)
Study hints at extreme climate change (1999)
Global Warming FAQ (New Sientist, 2000)
Research projects:
NSERC World Ocean Circulation Experiment
Collaborative Special Project (2000)
Halldór Björnsson (2000)
Uwe Mikolajewicz (2000)
Vicki S. McKenna (2000)
Ike, Oct 2000).
El Nino: El Nino is the name given to a series of climatic effects associated with changes in oceans currents. The name is derived from Spanish for Christ since the effects of El Nino are felt most strongly during the Christmas season. El Nino results from a reversal in the easterly Trade Winds which usually produce upwelling off Peru. When these winds reverse, they send a warm water front across the Pacific toward Peru which inhibits upwelling, raising water and air temperature. The warm water jet spreads out along the western margin of South and Central America, affecting water and air temperatures as far away as Alaska. The presence of warm waters in regions of upwelling kills cold water species. That effect is felt most strongly in South America where the anchovy harvest fails during El Nino years. It can be felt locally in depleted squid stocks.
Low pressures zones formed in Indonesia follow the warm water front across the Pacific, bringing warm, water-laden air and storms to the eastern Pacific. These usually hit Santa Cruz in late winter and early spring, resulting in floods, debris and mud flows and significant coastal erosion as storm surges reach high levels.
Lastly, El Nino appears to be semi-periodic, recurring every 4 to 7 years on average. Specific El Nino years vary in intensity. The last really strong one was 1982. This year's promises to be very severe.
The following two links contain movies showing the change in sea surface temperatures during
the last year. The tongue of warm water spreading eastward across the Pacific is
very obvious. Near the end of the movie (July of this year), warm water starts to spread both north
and south away from the Peruvian coast, bringing warm water up the California coast and
depressing deep water upwelling (resulting in some very warm late summer days in Santa Cruz).
El Nino Movie (MPEG) El Nino Movie
(QuickTime) (images copyright by NASA).
It is the small differences between centrifugal force and the force due to the Moon's gravity that raises tides. The Sun raises much smaller tides despite having a greater force due to gravity. The reason Sun tides are smaller is that the difference between centrifugal force and gravity is smaller. This is because the Sun is much further away from the Earth and its gravitational attraction is nearly constant over the surface of the Earth (recall that centrifugal force is constant), such that the difference between gravitational force and centrifugal force is small.
Twice each lunar month (28 days) the Sun, Moon and Earth align along a common axis, creating the highest high tides, which are called Spring tides, not for the season but rather for the height of the tide. Also twice a month, Neap tides occur when the Moon and Sun pull perpendicularly to one another, creating the lowest high tides. Even during high high tides, tidal amplitude is only about 0.5 m in open water. Near coasts and in bays, however, constricted flow of water can amplify the tides, creating tides that rise or fall as much as 12 m! The height of the tide in estuaries and bays can create tidal currents as the tide rushes in and out of constricted openings. Tidal currents are an important erosive agent in tidal flats and some bays.
V = L / T
In words, wave velocity is wavelength divided by wave period. Why? A propagating wave's crests take T seconds to move one wavelength, the distance between crests. Velocity is distance divided by time, or wavelength divided by period. Wave velocity varies between 3 and 30 m/s. This relation will be used again shortly.
With increasing depth, wave motion decreases, vanishing at a depth of L/2 (one half wavelength), a depth known as the wave base. The limited depth of wave motion explains why surfers can dive beneath a wave and not get worked. Within the upper L/2 (above the wave base), water moves in circles. If you could tag a small parcel of water, you would see that it moves in a circular orbit, similar to a point on the rim of a drum rolling in the direction of wave propagation. Within the orbit, water moves up, then forward, then down, then back, explaining why surfers move forward then back as a wave passes.
When the bottom shallows to less than wave base (L/2 depth), friction slows the wave down and compresses the circular orbits. Although wave velocity V drops, the period T remains the same, requiring the wavelength L to decrease. This causes wave crests to be packed closer together. Because wave height doesn't decrease, the water becomes more steeply crested. Eventually the wave breaks when it becomes too steep. Breaking waves create surf.
Surf zone: Surf is turbulent white water. It is high energy and can do work to the beach and backing cliff. Surf picks up sand; the turbulent motion within surf uses sand to scour the water bottom, eroding it down. Surf intrudes into cracks and hydraulically wedges them, physically weathering rock. It can also apply tremendous pressure to cliff faces capable of shattering rock (the strain rate is high and the rock fails in brittle fashion). Most of the work accomplished by waves happens within the surf zone.
Wave refraction: Waves impeding on a shoreline are refracted. Refraction means that the wave changes direction, turning to face the shoreline at a more nearly perpendicular angle. This is the result of bottom friction and the reduction of wave velocity in shallow water, causing the portion of a wave closest to the beach to travel slower than the portion further out. Because the portion of the wave further from shore travels faster, it catches up to the near-shore portion, resulting in a wavefront at nearly right angles to the shore. Wave refraction also causes waves to focus on headlands with shallow water projections since the waves bend to hit them square on. It also directs waves away from deep-water bays, resulting in sheltered dockage, but also resulting in a lower energy environment and sediment deposition that fills bays. Either way, headland or bay, wave refraction works to produce a uniform, linear coastline.
Longshore current: Wave refraction isn't complete, meaning that waves don't break at perfect right angles to shorelines. When waves strike at an acute angle they produce longshore currents. To understand longshore currents, let's imagine following a parcel of water near the shore. As a wave runs up the beach (swash) it moves along the same direction as the waves, that is, it moves at an angle up the beach. When the water runs back down the beach (backwash) it does so under the force of gravity only, causing it to flow straight down the beach (perpendicular to the shoreline). The result of repeated run up and run down is a zig-zag trajectory that moves water down the shoreline. This is the longshore current. Sand is moved with the longshore current, resulting in longshore or beach drift. The longshore current results in the net transport of water along shore. Longshore drift results in net transport of sand along shore. Although the distance traversed in a single swash/backwash cycle is small, longshore currents and drift are rapid. For instance, in Santa Cruz, the longshore drift of sand measured at the Yacht Harbor jetty is around 250,000 cubic meters a year, meaning 0.25 million cubic meters of sand flows along the beaches of Santa Cruz each year!
Beach shorelines have a sand budget that determines their health. Inputs that grow the beach consist of sediment eroded from backshore cliffs, sediment brought in by longshore currents and beach drift, and sediments brought in by rivers. Outputs that shrink beaches consist of sediments blown into backshore dunes, sediment removed by longshore current and beach drift, and sediments transported to deep water by currents and waves. Beach size varies with these inputs and outputs on a variety of time scales ranging from days to many years. Beaches that have persisted for many years must be in near steady state balance. Thus if sand is lost during a large storm it must be replaced, for instance by stream input. Disrupting this delicate balance are man made perturbations, such as jetties and groins. These stop longshore current and beach drift, building beaches on the up current side, but starving beaches on the down current side. A less obvious but often more important perturbation is the damming of rivers. River dams trap sediment, reducing the riverine influx and causing beaches to shrink. As the extent of damming increases (for energy and agricultural purposes) so will the effect on sandy beaches. And as we saw with rivers, a beach robbed of sediment influx has greater capacity to erode.
Rocky shorelines: The elements of rocky shorelines are: a wave-cut bench, wave-cut notch and the wave-cut cliff. The bench is a gently dipping bedrock flat created by surf erosion. It grows in the landward direction by the action of surf cutting a notch at the base of backing cliffs. When the notch grows deep enough, or when storm surges raise wave energy levels sufficiently, the wave-cut cliff will fail (an example of mass wasting). The rock debris is quickly broken down, becoming grist used to scour the wave-cut bench. Beaches on rocky shorelines are usually limited to pockets between headlands. Here wave refraction results in a lower-energy environment where sediment is deposited.
Sometimes in the cutting of the bench, portions of the backing cliff will be left standing. As the cliff face retreats, these "stacks" become isolated columns in the foreshore or offshore. Santa Cruz has many examples of these. Santa Cruz also had examples of wave-cut arches, which are produced when notches in cliff faces reconnect to the surface, creating a hole through the rock. Unfortunately, the arches at Natural Bridges state park have all been broken down to stacks.
Santa Cruz's terraces: The shoreline near Santa Cruz is being slowly uplifted, the result of compression across the San Andreas fault. The uplift has produced many marine terraces above sea level. Each terrace is a former wave-cut bench that is now uplifted far above sea-level. The topography of Santa Cruz is due to these terraces. For instance, as you travel down Western Ave or Bay Ave, you will notice alternation between steep downslopes and relatively flat stretches. The flats are wave-cut terraces, the steep slopes are partially eroded backing cliffs, made less steep by mass wasting. It is this succession of bench, cliff, bench, cliff, ..., that has produced the "theater seating" exploited by home builders, each of whom wants to offer buyers a ocean view.
Barrier islands are long sandbars offshore that form a barricade between open ocean waves and the main shoreline. They are common along low lying coasts where sediment is abundant. They are thought to have formed from the submergence of berms during sea level rise or by the progradation of spits. They are maintained by influx of sand eroded by breakers. Like sandy beaches, barrier islands are subject to a sand budget and can grow and shrink on very human time scales. Padre and Mustang islands offshore of eastern Texas are prominent examples of barrier islands.
We have already talked about deltas in "Rivers and Wind", but it's worth mentioning that the existence of a delta is evidence that sediment influx exceeds the ability of waves and tidal currents to erode sediment.
Atolls are coral reefs arranged in a circular form, sometimes bounding a volcanic island. They represent the last stages in the evolution of an oceanic island. In the first stage, the island is created by volcanic eruptions creating an edifice that extends above sea level, forming a shoreline. Corals grow in the warm, shallow waters surrounding the island. The island subsides as it cools and as the plate it sits on cools. The corals continue to grow, matching the rate of subsidence, resulting in a circular coral reef growing up from deeply submerged sea floor. This process was first proposed by Charles Darwin during the cruise of the HMS Beagle. Darwin failed to convince people, largely because he had no mechanism for subsidence. We now recognize subsidence as a natural product of plate tectonics, a theory developed long after Darwin's death, which explains the creation of oceanic plates at mid-ocean ridges, their cooling and subsidence and eventual subduction back into the mantle.