Wave & Wind Erosion and Deposition: Sixth Grade Science
Coastal ocean waves and atmospheric wind currents are powerful geological forces that sculpt dynamic shoreline landforms and arid desert landscapes through mechanical erosion and deposition.
Coastal Wave Energy and Shoreline Dynamics
Ocean waves are energy traveling across the sea surface, generated primarily by wind blowing across open water. When a wave approaches shallow coastal waters:
- Friction with the rising seabed slows down the bottom of the wave.
- The wave height increases, its wavelength shortens, and the top of the wave curls forward and collapses as a breaking wave (surf).
- Breaking waves crash against coastal headlands with colossal hydraulic pressure—exceeding thousands of pounds per square foot—compressing air in rock cracks, shattering rock blocks, and hurling abrasive sand against cliffs.
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| COASTAL EROSIONAL EVOLUTION |
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| 1. Sea Cliff under attack by breaking waves. |
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| 2. Wave-Cut Notch hollowed out at cliff base -> Roof collapses! |
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| 3. Waves erode cracks in protruding headland to form a SEA CAVE. |
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| 4. Waves erode through the entire headland to carve a SEA ARCH. |
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| 5. Arch roof collapses, leaving an isolated pillar: a SEA STACK! |
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Longshore Drift and Coastal Deposition
Waves rarely hit a coastline at a perfect 90-degree angle; they typically strike diagonally. This angled approach generates longshore drift, an ocean current that runs parallel to the beach:
- Swash pushes sand diagonally up the beach face.
- Backwash pulls sand straight down the slope under gravity.
- Sand particles zigzag continuously down the coast like an invisible river of sand.
- Depositional Landforms: Where the coastline bends, the longshore current enters quiet water, dropping sand to build features such as sand spits (a finger of sand projecting into a bay), baymouth bars (a sand spit that closes off a bay), and barrier islands (long, narrow sandy islands parallel to the mainland that protect coasts from hurricane storm surges).
Wind Erosion in Arid Environments (Aeolian Processes)
Wind is an effective geological agent of erosion in arid and semiarid deserts where dry loose sediment lacks vegetation and moisture to hold particles together.
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| TWO MODES OF WIND EROSION |
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| 1. Deflation: |
| Wind lifts and blows away small clay, silt, and sand grains, |
| leaving behind a dense, protective armor of coarse gravel and |
| pebbles known as a DESERT PAVEMENT. |
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| 2. Abrasion: |
| Wind blows hard quartz sand grains within 1 meter of ground, |
| acting like natural sand-blasting to carve polished, faceted rocks |
| called VENTIFACTS and streamlined bedrock ridges called YARDANGS. |
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Wind Deposition: Sand Dunes and Loess
When wind velocity slows down behind obstacles or across flat plains, it unloads its airborne sediment, building two primary depositional formations:
- Sand Dunes: Mounds or ridges of wind-deposited sand that migrate slowly across deserts. Sand dunes typically have a gentle windward slope where sand grains roll up via saltation (bouncing), and a steep leeward slip face (at roughly 34 degrees, the angle of repose) where sand cascades down.
- Loess Deposits: Extensive, thick blankets of windblown silt and dust. During the Ice Ages, glacial grinding pulverized rock into fine rock flour, which dry winds carried hundreds of kilometers across continents, depositing deep, nutrient-rich loess soils in the American Midwest and central China that support major agricultural breadbaskets today.
Exercises and Solutions
Exercise 1: Reconstructing Coastal Landform Evolution. A coastal resort features a scenic rocky promontory with a natural rock bridge known as an arch where waves crash beneath a stone span. A marine geologist warns the town council that within a century, the arch will disappear and transform into an isolated stone tower out at sea. (a) What is the scientific name for the current rock bridge and the future isolated stone tower? (b) Describe the physical wave processes that will cause this transformation.
Solution 1: (a) The current natural rock bridge is a sea arch; the future isolated stone pillar is a sea stack. (b) Waves relentlessly pound the headland through hydraulic pressure and mechanical abrasion. As breaking waves hammer the base of the sea arch, water undercuts the rock pillars, widening the opening. Concurrently, subaerial weathering (rain, salt crystallizing in rock cracks, and gravity) weakens the overarching stone roof. Eventually, the structural strength of the rock span is exceeded, and the central arch collapses catastrophically into the ocean, leaving the disconnected outer pillar standing alone in the surf as a sea stack.
Exercise 2: Evaluating Artificial Groins and Longshore Drift. A beachfront community builds a series of concrete walls (groins) extending perpendicular from the beach into the ocean to trap sand and widen the beach in front of their luxury hotels. While the beach widens on the up-drift side of the groins, homeowners living 500 meters down-drift report that their sandy beaches have completely eroded away, threatening their home foundations. Explain the physics of longshore drift that caused this unintended coastal erosion.
Solution 2: Longshore currents carry a continuous river of sand parallel to the coastline through zigzag swash and backwash. When the concrete groins were constructed perpendicular to the beach, they acted as physical dams that blocked the natural longshore transport of sand, trapping it on the up-drift side and widening the hotel beach. However, longshore currents do not stop; as the water passes the groin, it is starved of sediment ("hungry water"). To restore its equilibrium sediment load, the wave current rapidly erodes sand from the unshielded beach immediately down-drift. By trapping sand in front of the hotels, the groins starved the downstream coast, accelerating erosion and washing away the beaches of the neighboring homeowners.