Intro to Marine Biology | Florida International University | Chapter 2.2
Difficulty: Introductory | Prerequisites: Chapter 2.1 (Tectonic Processes) notes recommended, since the geological features discussed there are the starting material that weathering and erosion act on.
Once tectonic processes create rock and landforms, weathering, erosion and sedimentation reshape them. These three processes form a sequence: rock is broken down (weathering), the fragments are transported (erosion), and they settle in a new location (sedimentation). This cycle determines the character of every coastline, from rocky cliffs to sandy beaches to mudflats. Understanding it is essential for later topics on marine habitats, because the type of shore directly controls which organisms can live there.
Rocks break down through chemical, physical and biological weathering. The fragments are moved by ice, gravity, wind or water (erosion) and eventually settle somewhere new (sedimentation). The type of sediment that accumulates, and how exposed the shore is to wave action, determines whether a coastline becomes rocky, sandy, muddy, an estuary or a delta.
Weathering
The chemical and physical processes that break down rocks in place, without transporting them. Think of it as rock slowly falling apart where it sits.
Chemical weathering
The breakdown of rock through chemical reactions, typically involving exposure to water and/or oxygen. Rainwater is a common agent. In simple terms, rock dissolves or reacts with water and air.
Physical weathering (mechanical weathering)
The breakdown of rock into smaller particles by physical forces such as temperature changes, freezing water or wave action, without altering the rock's chemical composition. Think of it as rock cracking apart from pressure, temperature swings or pounding waves.
Biological weathering
The breakdown of rock caused by living organisms: plant roots growing into cracks, burrowing animals, or microorganisms secreting acids onto rock surfaces. In simple terms, living things slowly break rock apart, either physically or chemically.
Erosion
The movement of weathered material from one location to another by natural forces. Think of it as the transport step: weathering breaks rock down, erosion carries the pieces away.
Ice erosion (glacial erosion)
Erosion caused by glaciers moving over land, physically breaking down underlying rock and depositing trapped sediments wherever the glacier travels. In simple terms, a glacier grinds rock beneath it and dumps the debris as it moves.
Gravity erosion
The movement of weathered rock from one location to another by the force of gravity (e.g. rockslides, landslides). Think of it as loose material simply falling or sliding downhill.
Wind erosion
The movement of weathered particles by wind. Beach sand is a common example. In simple terms, wind picks up loose grains and carries them elsewhere.
Water erosion
The transport of rock particles or sediments by water action, including run-off, river flow and wave action. Water can also hold particles in suspension and deposit them elsewhere. Responsible for cutting canyons over geological time. Think of it as water carrying, rolling and dropping sediment.
Sedimentation
The final stage of the weathering-erosion sequence: particles are deposited and settle as sediment in a new location, becoming part of the landmass. The rate of sedimentation varies with particle size (larger particles settle faster). In simple terms, the transported material lands somewhere and stays.
Littoral zone (intertidal zone)
The benthic (bottom) zone between the highest and lowest spring tide shorelines, where land meets sea. Classified by shore shape, wave exposure, substrate and resident organisms. Think of it as the strip of coast that is alternately underwater and exposed as tides rise and fall.
Morphology (of shores)
The study of the form and shape of things. Shore morphology focuses on the slope of the shore and the size of sediment present. In simple terms, what the shore looks like and why.
Rocky shore
A shoreline characterised by a rocky substrate, ranging from vertical cliff faces to wide, flat rock platforms. Rocks exist in a gradient: larger rocks further from the waterline (less eroded), smaller rocks closest to it (more eroded by waves). Resistant to erosion, very open and exposed.
Sandy shore
A shoreline composed of loose sand, small gravel and shells, formed by erosion of sandstone and deposition by waves. Constantly in motion, with a gradual slope toward the ocean.
Muddy shore (mud flat)
A flat, low-energy shoreline with little to no slope and minimal wave action. Silt particles and organic materials accumulate in layers. Least exposed to erosion. Examples in the US include Cape Cod Bay (Massachusetts) and Padilla Bay (Washington).
Estuary
A partially enclosed, tidal, coastal body of water where fresh river water meets ocean saltwater. The water is described as "brackish" (a mix of fresh and salt). Sheltered from wave action, with a bottom of fine sand and silt. Also referred to as bays, lagoons, sounds or sloughs. In simple terms, where a river flows into the sea in a sheltered, semi-enclosed area.
Brackish water
Water that is a mixture of fresh water and saltwater, typically found in estuaries. Think of it as "in-between" water: saltier than a river, less salty than the open ocean.
Delta
A low-lying, triangular landform at the mouth of a river, built up by the deposition of sediments as the river slows on meeting the sea. Over time, deposits accumulate into sandbars, land masses and fan-shaped structures. Well-known examples: the Mississippi River Delta and the Nile River Delta. In simple terms, a fan of new land created where a river dumps its sediment into the ocean.
Weathering is the breakdown of rock in place. Three types:
Chemical weathering: Rocks break down through exposure to water and/or oxygen. Rainwater is a common cause. The rock's chemical composition changes.
Physical weathering (mechanical): Temperature changes, freezing water expanding in cracks, and wave action physically shatter rock into smaller particles. The composition stays the same.
Biological weathering: Living organisms break rock apart. Tree roots grow into cracks and force them wider. Some organisms secrete acids that chemically dissolve rock (this overlaps with chemical weathering).
Erosion is the transport of weathered material from one place to another. Four types:
Ice erosion: Glaciers move over land, grinding underlying rock and depositing trapped sediments wherever they travel.
Gravity erosion: Gravity pulls weathered material downhill (rockslides, landslides).
Wind erosion: Wind carries loose particles. Beach sand is a familiar example.
Water erosion: Water transports particles through run-off, river flow and wave action. Water can hold sediment in suspension and deposit it elsewhere. Over geological time, water erosion cuts canyons. Water can also act as both a weathering and an erosion agent.
Sedimentation is the final step in the sequence. Weathered and eroded particles are deposited and settle in a new location, becoming part of the landmass. Larger particles settle faster than smaller ones.
The littoral zone is the benthic (bottom) zone between the highest and lowest spring tide shorelines. It is where land meets sea. It is also called the intertidal zone.
The littoral zone is classified by:
The shape of the shore
Wave action and erosion
The substrate (material making up the shore)
The organisms that live there
The shape of the seashore depends primarily on (1) the land lying closest to the shore, and (2) the level of exposure to erosion. Shore morphology studies focus on the slope of the shore and the size of sediment found there.
Rocky shores
Characterised by a rocky substrate: vertical cliff faces, wide flat rock platforms, or anything in between.
Rock size varies from large boulders to gravel and pebbles.
A gradient exists: larger rocks sit further from the waterline (less eroded); smaller rocks sit closest (more eroded by waves).
Rocky shores are resistant to erosion (the rock itself is hard-wearing) and tend to be very open and exposed.
The rocks are resistant to weathering and take a long time to break down.
Sandy shores
Made of loose sand, small gravel and shells.
Formed by erosion of sandstone and deposition of sediments by waves.
Constantly in motion: waves move sand up and down the beach.
Gradual slope toward the ocean.
Muddy shores (mud flats)
Least exposed to erosion.
Usually flat, with no slope.
Little to no wave action.
Silt particles and organic materials accumulate in layers over time.
US examples: Cape Cod Bay (Massachusetts), Padilla Bay (Washington).
Estuaries
Partially enclosed, tidal, coastal bodies of water where river fresh water meets ocean saltwater.
The water is brackish (a mixture of fresh and salt).
Sheltered from wave action.
Bottom made of fine sand and silt that settles out of suspension.
Often referred to as bays, lagoons, sounds or sloughs.
Deltas
Low-lying, triangular areas at the mouth of a river.
Formed by sediment deposition: the river carries sediments, slows as it meets the sea, and the sediment settles out.
Over time, deposits build sandbars, land masses and fan-shaped structures.
Well-known examples: Mississippi River Delta, Nile River Delta.
The Grand Canyon is a textbook example of water erosion over millions of years. Beach nourishment projects (pumping sand onto eroding beaches) are a direct response to wave-driven erosion threatening coastal property. Estuaries such as the Chesapeake Bay are among the most biologically productive ecosystems on Earth, serving as nursery grounds for commercially important fish and shellfish species. River deltas like the Mississippi's are critical agricultural and urban regions, but they are sinking and eroding as sediment supply decreases due to upstream damming.
Students often confuse weathering and erosion. Weathering breaks rock down in place; erosion transports the broken material. They are sequential, not the same process.
Students sometimes think biological weathering is always physical (e.g. roots cracking rock). It can also be chemical, as when organisms secrete acids that dissolve rock.
A common mistake is assuming sandy beaches are permanent. They are constantly reshaped by wave action and sediment transport; sand is deposited and removed continuously.
Students sometimes believe estuaries are stagnant. They are tidal and dynamic, with water salinity and levels changing regularly.
⚠️ Know the three types of weathering (chemical, physical, biological) and be able to give an example of each.
⚠️ Know the four types of erosion (ice, gravity, wind, water) and understand that water is both a weathering and an erosion agent.
⚠️ Understand the sequence: weathering → erosion → sedimentation. Exams often ask you to order these or explain how they relate.
⚠️ Be able to compare shore types (rocky, sandy, muddy) by substrate, slope and exposure to wave action.
⚠️ Know what an estuary is (where fresh and salt water meet) and the term "brackish."
⚠️ Know how deltas form (river slows at the sea, sediment drops out, builds up over time).
True or False: Erosion and weathering are the same process. Answer: False. Weathering breaks rock down in place; erosion transports the fragments.
Fill in the blank: The three types of weathering are chemical, physical and ______. Answer: Biological.
True or False: Muddy shores experience the most wave action of any shore type. Answer: False. Muddy shores have little to no wave action.
Fill in the blank: Water in an estuary is called ______ because it is a mix of fresh and salt water. Answer: Brackish.
True or False: A delta forms where a river meets the sea and deposits sediment. Answer: True.
Q: Name and briefly describe the three types of weathering.
A: Chemical (rock broken down by water/oxygen exposure), physical (rock broken into smaller particles by temperature changes, freezing or waves, without chemical change), biological (rock broken by organisms such as plant roots or acid-secreting microbes).
Q: What is the difference between erosion and sedimentation?
A: Erosion is the transport of weathered material from one place to another. Sedimentation is the final step, where transported particles are deposited and settle in a new location.
Q: Compare rocky shores and muddy shores in terms of slope, wave exposure and erosion resistance.
A: Rocky shores have varied slopes (cliff faces to flat platforms), are very exposed to wave action, and are resistant to erosion because the rock is hard-wearing. Muddy shores are usually flat (no slope), have little to no wave action, and are the least exposed to erosion.
Q: What is an estuary, and what does "brackish" mean?
A: An estuary is a partially enclosed, tidal, coastal body of water where river fresh water meets ocean saltwater. "Brackish" describes the resulting mixed water, which is saltier than fresh water but less salty than the open ocean.
Q: Describe how a river delta forms.
A: A river carries sediment downstream. When it meets the sea, the current slows and the sediment settles out of the water. Over time, these deposits accumulate on the river bottom and at its mouth, building up sandbars, land masses and fan-shaped structures.
This material follows directly from Chapter 2.1 (Tectonic Processes): tectonics creates the rock and landforms, and weathering, erosion and sedimentation reshape them. It also sets the stage for Chapter 2.3 (Tides and Ocean Currents), since tidal range and wave action determine how much erosion a shore experiences. Later in the course, understanding shore types will matter when studying intertidal ecology, because different substrates support very different communities of organisms.
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