Map ocean-floor features and oceanic crust, contrast passive and active continental margins, read transgressing and regressing sea levels, and explain how streams build channels, floodplains, cross-beds, alluvial fans and deltas

Learning goal: Describe the features of the ocean floor (continental shelves and slopes, the abyssal zone, mid-ocean ridges, trenches, island systems, island arcs and reefs), the rock of the oceanic crust and the sediments that cover it; contrast passive and active continental margins; explain transgressing and regressing sea levels and how the rock record shows them; and explain how streams shape channels, streambeds and floodplains and build cross-bedding, alluvial fans and deltas, including the engineering that holds the Mississippi River in its course.

Where this lesson sits. Ohio's PG.IMS.4 (Ocean) lists eight topics, and this site teaches them in two lessons meant to be studied together. This lesson is part 2 of 2: ocean features (ridges, trenches, island systems, the abyssal zone, shelves, slopes, reefs and island arcs), passive and active continental margins, transgressing and regressing sea levels, and streams. Part 1, the lesson on tides, currents, waves and water density, teaches tides, deep and shallow currents, thermal energy and water density, and waves, with El Niño and energy from the ocean. Neither part alone covers PG.IMS.4.

Seeing the ocean floor. The deep ocean floor is dark and kilometers down, so it is mapped with sound. A ship's echo sounder sends a pulse down and times the echo; sound travels through seawater at about 1,500 m/s, and the pulse goes down and back, so depth = 1,500 × time ÷ 2. Here is a model worked through: an echo returning after 5.4 s means a depth of 1,500 × 5.4 ÷ 2 = 4,050 m, typical of the abyssal plain. Modern ships sweep a fan of beams to map a strip at once, and satellites measure tiny bumps in the sea surface caused by the gravity of seamounts and ridges below, mapping the whole ocean at coarser detail.

Features from the shore outward. The continental shelf is the gently sloping, submerged edge of a continent, from a few kilometers to hundreds of kilometers wide, ending at the shelf break, usually no more than a few hundred feet (on the order of 100 m) deep. The continental slope is steeper, a few degrees, and descends to the deep ocean; it is cut by submarine canyons down which dense, sediment-laden currents flow. On many margins a continental rise, an apron of sediment shed down the slope, grades into the abyssal plain, the flattest surface on Earth, where sediment has buried the rough volcanic floor at depths of about 3,000 to 6,000 m. The abyssal zone is the environment of this deep floor: dark, near-freezing and under enormous pressure. Mid-ocean ridges form a connected mountain chain about 65,000 km long through all the oceans, with a crest where plates separate and new crust forms (a rift valley along the crest at slow-spreading ridges such as the Mid-Atlantic Ridge, a smoother rise at fast ones such as the East Pacific Rise), and hot springs where seawater circulates through hot rock. Trenches are the deepest places, long narrow troughs where one plate sinks beneath another; the Mariana Trench reaches nearly 11 km. Island systems take two main forms. Island arcs are curved chains of volcanoes that rise beside trenches, above the sinking plate: the Aleutians, Japan and the Mariana Islands. Hot-spot chains form where a plate moves over a mantle plume, so the islands get older with distance from the active volcano, as in the Hawaiian chain. Old volcanic islands that erode flat and subside become flat-topped seamounts. Reefs of the kind that build islands and atolls are built by corals and other organisms that need warm, clear, shallow, sunlit water (a few corals grow slowly in cold, deep water, but they do not build these reefs); fringing reefs hug a coast, barrier reefs are separated from it by a lagoon, and an atoll is a ring of reef around a lagoon where a volcanic island has subsided out of sight. Because reef-builders need shallow water, a fossil reef is strong evidence of a shallow, warm sea at that place and time.

Oceanic crust and ocean sediments. Oceanic crust is about 7 km thick: pillow-shaped basalt lava on top, formed where lava erupts into cold water, then sheets of basaltic dikes, then gabbro below. It forms at ridges and is destroyed at trenches, so almost none of it in the ocean basins is older than about 200 million years (a remnant beneath the eastern Mediterranean, interpreted as about 340 million years old, is a rare exception), while continental rocks reach about 4 billion. Sediment settles on it slowly. Terrigenous sediment comes from land (sand and mud, thickest near continents and delivered to the deep sea by turbidity currents). Biogenous sediment is the shells of plankton: calcareous ooze of calcite shells accumulates on ridges and shallower floors, but below roughly 4 to 4.5 km (the calcite compensation depth) calcite dissolves in the cold, high-pressure water, leaving siliceous ooze of silica shells or fine red clay instead. Hydrogenous sediment precipitates from seawater, such as manganese nodules. Sediment is thin near ridges and thicker away from them, because older crust has had longer to collect it.

Passive and active margins. A passive continental margin is one where the edge of the continent is not a plate boundary; continent and ocean floor ride on the same plate. It has a wide shelf, a gentle slope and a thick rise, and few earthquakes or volcanoes. The Atlantic and Gulf coasts of the United States are passive margins. An active continental margin coincides with a plate boundary. Where an ocean plate subducts, as along the west coast of South America, the margin has a narrow shelf, a steep slope, a trench, frequent earthquakes and a chain of volcanoes on land. The misconception to avoid is that passive margins never had any activity: they began as rifts when a continent split apart, and have since been carried away from the boundary.

Transgressing and regressing sea levels. Sea level relative to the land changes for two kinds of reasons. Global sea level changes when water is locked up in or released from ice sheets, or when mid-ocean ridges swell or shrink and displace water; about 20,000 years ago, at the height of the last glaciation, sea level was roughly 120 to 125 m lower than today and much of today's continental shelf was dry land. Local sea level changes when land subsides or rises. When the sea advances over the land, a transgression, the shoreline moves inland and offshore sediments (mud, then limestone) come to rest on top of nearshore sand: the record deepens upward. When the sea retreats, a regression, nearshore sand is laid on top of offshore mud: the record shallows upward. Ohio's Paleozoic rocks record many such advances and retreats of shallow seas.

Streams. A stream flows in a channel, whose floor is the streambed. Faster water carries larger particles, so gravel and sand collect on the streambed where the current is strong and mud settles where it is slow. In a meandering stream, water flows fastest on the outside of each bend, eroding a steep cut bank, and slowest on the inside, depositing a point bar, so bends migrate across the valley. The floodplain is the flat land beside the channel that floods when discharge exceeds the channel's capacity; as floodwater spills over, it slows at once, dropping coarser sediment as natural levees along the banks and fine mud farther away. Sand on the streambed moves as ripples and dunes; sand avalanches down each one's downstream face, building cross-bedding, sets of inclined layers that dip downstream and preserve the current direction in rock. An alluvial fan forms where a steep mountain stream emerges onto a flat valley floor: the stream spreads out, slows and drops its load in a fan-shaped deposit, common along dry mountain fronts. A delta forms where a stream enters a lake or the sea: the current slows to nearly nothing, sediment drops, and the river splits into distributaries that build land outward.

Engineering rivers. The Mississippi River has shifted its lower course and built new delta lobes many times over thousands of years, as its channel grew long and a shorter, steeper route to the Gulf became available. In the twentieth century the Atchafalaya River threatened to capture its flow, and in 1963 the U.S. Army Corps of Engineers completed the Old River Control Structure to hold the flow split. Levees protect towns and shipping, but they also keep flood sediment from reaching the delta, which contributes to the loss of coastal wetlands in Louisiana. Farther west, dams and diversions on the Colorado River now use so much of its water that since the 1960s it has rarely reached the Gulf of California (the Sea of Cortez), apart from some wet years in the 1980s and a planned release in 2014, and its delta wetlands have shrunk greatly.

Worked example, echo sounding a profile: Along a hypothetical line of soundings the echo times are 0.12 s, 0.20 s, 2.4 s, 5.6 s and 10.4 s. Depths = 1,500 × t ÷ 2: 90 m, 150 m, 1,800 m, 4,200 m and 7,800 m. The first two lie on the continental shelf, the third on the continental slope, the fourth on the abyssal plain, and the last, far deeper than the plain, fits a trench. A trench beside the continent means an active margin.

Worked example, reading sediment thickness: Hypothetical drill cores show sediment 20 m thick on crust 2 million years old and 400 m thick on crust 60 million years old. Average rates: 20 m ÷ 2 million years = 10 m per million years, and 400 ÷ 60 ≈ 6.7 m per million years. The older site has much more sediment in total, because the crust has had longer to collect it; its lower average rate is consistent with the crust having moved away from the productive ridge crest and down below the depth where calcite shells dissolve. Thickness increasing away from the ridge is evidence that the crust gets older away from the ridge.

Worked example, stream discharge and sediment: A hypothetical stream channel 20 m wide and 2 m deep carries water at 1.5 m/s. Discharge = width × depth × velocity = 20 × 2 × 1.5 = 60 m³/s. In a flood the depth rises to 4 m and the velocity to 2.5 m/s: 20 × 4 × 2.5 = 200 m³/s, more than three times as much, and the faster current can move gravel that stayed still before. When the flood spills onto the floodplain and slows to 0.2 m/s, only mud stays suspended, so sand drops beside the channel as levees.

Worked example, transgression from a column: A hypothetical outcrop shows, bottom to top: cross-bedded sandstone with plant fragments, gray shale with clams, limestone with corals. Nearshore sand overlain by offshore mud and then clear-water carbonate: the environments deepened upward, so, provided the layers follow one another without an erosion surface or fault between them, the sea transgressed across the site. A column showing the same three rocks in the opposite order would record a regression.

Investigation, a stream table: Materials: a long tray tilted on a block about 5 cm high, sand mixed with a little fine gravel, a bucket with a small hole or a hose clamp to control flow, a catch basin at the low end that holds standing water, a ruler and a phone camera. Run water down the sand for 10 minutes at a low flow and photograph every 2 minutes; then repeat with a higher block (steeper slope) and with a higher flow. Record channel width, the number of bends, and the length of the deposit that grows into the basin. Result that supports the claim: the stream sorts sediment, cuts the outside of bends and builds point bars, and drops sediment as a delta where it meets the standing water; a steeper slope and higher flow carry larger grains farther. What the model cannot show: vegetation that holds real banks together, the long time scales of real rivers, and the effect of floods of different sizes.

Representation task: Draw a labeled cross-section from a passive margin across an ocean to an active margin: continental shelf, slope, rise, abyssal plain, mid-ocean ridge, trench and volcanic arc. Mark where each kind of ocean sediment accumulates and draw arrows for plate motion. Then, on a world map, mark and name at least one ridge, one trench and one island system in each of the Atlantic, Pacific and Indian Oceans (for example the Mid-Atlantic Ridge, Puerto Rico Trench and Lesser Antilles; the East Pacific Rise, Mariana Trench and Hawaiian chain; the Southeast Indian Ridge, Java Trench and Indonesian arc). In Google Earth, find one meandering river and label its channel, a cut bank, a point bar, the floodplain and any delta or alluvial fan.

Independent performance, a river decision: Research both the lower Mississippi River's control structures and the Colorado River delta using maps and aerial photographs from several decades, then write your recommendation on one of them. Success criteria: the report explains, with the stream vocabulary of this lesson, how the river would behave without engineering; describes what the engineering does; gives at least two benefits and two costs, one of them to the delta or wetlands; and recommends one change, supported by evidence from the photographs or maps.