Why is it important to understand the earth system?
Earth and Atmospheric Sciences
In simple terms, sediment is the raw material (the loose “stuff”), and sedimentary rocks are the finished product (the “stuff” glued together).1
Think of it like a beach vs. a sidewalk: the loose sand on the beach is the sediment, and if you were to cement that sand into a solid block, it would be a sedimentary rock.2
1. What are Sediments?
Sediments are loose, solid particles that have been moved and deposited in a new location.3 They aren’t “grown” like crystals; they are created when existing rocks are broken down.4
- How they form: Through weathering (breaking rocks into smaller pieces via wind, rain, or ice) and erosion (carrying those pieces away).5
- What they’re made of:
- Rock fragments: Pebbles, sand, silt, and clay.6
- Organic matter: Shells, bones, or decayed plant material.7
- Chemical precipitates: Minerals like salt that “fall out” of water when it evaporates.8
2. What are Sedimentary Rocks?
Sedimentary rocks are formed when layers of sediment are buried and hardened into solid rock over millions of years.9 This transformation is called lithification.10
The Two-Step Recipe:
- Compaction: As more layers of sediment pile up, the weight of the top layers squashes the bottom layers, squeezing out water and air.11
- Cementation: Dissolved minerals (like silica or calcite) act like “nature’s glue,” filling the tiny gaps between the grains and sticking them together.12
3. Types of Sedimentary Rocks
Geologists usually group these rocks into three main categories based on how they started:13
| Type | How it Forms | Examples |
| Clastic | From broken pieces (clasts) of other rocks. | Sandstone, Shale, Conglomerate |
| Chemical | When minerals dissolved in water crystallize. | Rock Salt (Halite), Gypsum |
| Organic | From the remains of plants or animals. | Coal, Limestone (often made of shells) |
Why do they matter?
Sedimentary rocks are like a history book for Earth. Because they form in layers (called strata), they often trap fossils and preserve clues about what the environment was like millions of years ago—whether it was a desert, a deep ocean, or a swamp.
How do particles and dissolved substances create the raw materials of sediment?
The raw materials for sediment are created through two distinct paths of weathering: physical (mechanical) and chemical.1 Think of this process as nature’s way of recycling old mountain ranges into the building blocks of new landscapes.
1. Physical Weathering: Creating Particles2
Physical weathering is like a hammer. It breaks solid rock into smaller fragments—called clasts—without changing the rock’s chemistry.
- Frost Wedging: Water seeps into cracks, freezes, and expands (ice takes up about 10% more space than liquid water), prying the rock apart.3
- Abrasion: Rocks tumbling in a river or blown by wind act like sandpaper, grinding each other down into pebbles, sand, and silt.4
- Thermal Expansion: In deserts, rocks expand when heated by the sun and contract at night. Over time, this stress causes the outer layers to peel off like an onion (exfoliation).5
2. Chemical Weathering: Creating Dissolved Substances
Chemical weathering is like acid.6 It uses water, oxygen, and natural acids to rot or dissolve minerals, turning them into ions (atoms with an electric charge) that float invisibly in water.7
- Dissolution: Rainwater picks up carbon dioxide from the air to become a weak carbonic acid.8 This acid eats away at minerals like calcite (found in limestone).9 The rock literally disappears into the water as dissolved calcium and bicarbonate ions.10
- Hydrolysis: Water reacts with common minerals like feldspar, turning them into soft clay minerals while releasing extra potassium or sodium into the water.11
- Oxidation: Oxygen reacts with iron-bearing minerals, creating “rust” (iron oxides).12 This weakens the rock, making it easier for it to crumble.
3. How they become “Raw Materials”
Once these processes occur, the materials are ready to be moved by erosion:
- The Particles (Clastic Sediment): Gravity, wind, or water carry the solid chunks (sand, mud, gravel).13 They eventually settle when the water or wind slows down.14
- The Dissolved Substances (Chemical Sediment): These ions travel in solution—meaning you can’t see them.15 They stay in the water until it evaporates (like salt forming in a drying puddle) or until a biological organism, like a coral or a clam, pulls the ions out of the water to build its shell.
| Material Type | Primary Process | Resulting Sediment |
| Solid Particles | Physical Weathering | Sand, Silt, Clay, Gravel |
| Dissolved Ions | Chemical Weathering | Salt, Gypsum, Calcium (for shells) |
Summary of the Rock Cycle
Weathering and Erosion
This video provides a visual breakdown of how physical and chemical forces work together to dismantle solid rock and prepare it for transport.
How does sediment get transported?
Sediment transport is the process of moving “raw materials” from the site of weathering to a new location where they can settle.1 Geologists categorize this movement based on the agent (the force moving it) and the mode (how the particle actually travels).
1. The Four Agents of Transport
Nature uses four primary forces to move sediment across the globe:2
- Water (Fluvial/Marine): The most powerful agent. Rivers carry everything from tiny clay particles to massive boulders during floods.3 In the ocean, currents and tides move sediment along coastlines and into the deep sea.4
- Wind (Aeolian):5 Highly effective in dry environments. Wind is excellent at “sorting” sediment because it can only pick up lighter grains, like sand and dust, leaving heavier rocks behind.
- Ice (Glacial):6 Glaciers act like giant bulldozers.7 Because ice is a solid, it can carry any size of sediment—from “glacial flour” (fine dust) to house-sized boulders—without dropping them until the ice melts.
- Gravity: This is the primary driver of “mass wasting” events like landslides, rockfalls, and mudflows. It moves sediment directly downslope without needing a fluid like water or air.
2. How Particles Move (Modes of Transport)
Depending on the size of the particle and the energy of the water or wind, sediment travels in one of four ways:
| Mode | Description | Particle Type |
| Solution | Dissolved minerals travel invisibly in the water. | Ions (Salt, Calcium) |
| Suspension | Tiny particles float in the flow without touching the bottom. | Clay, Silt |
| Saltation | Medium-sized grains “hop” or bounce along the floor. | Sand |
| Traction | Large, heavy rocks roll or slide along the bottom. | Pebbles, Boulders |
3. Physical Changes During Transport
As sediment travels, it changes physically.8 The further a particle is moved, the more “mature” it becomes:
- Rounding: Sharp edges are knocked off as particles bump into each other (abrasion), turning jagged rocks into smooth pebbles.9
- Sorting: Currents separate particles by size.10 A “well-sorted” sediment (like beach sand) has grains that are all roughly the same size.
- Size Reduction: Constant bumping and grinding break large rocks into smaller and smaller pieces.11
[Image showing the evolution of sediment rounding and sorting over distance]
See it in action
Transportation of Sediment – Wind, Ice, Water and Gravity
This video provides a visual breakdown of how different natural forces move debris and how gravity affects sediment at the continental shelf.
How is sedimentation the end of the line?
“Sedimentation” (also called deposition) is the “end of the line” because it represents the exact moment when the energy of the wind, water, or ice is no longer strong enough to keep a particle moving.1
Once gravity overcomes the force of the transporting medium, the journey ends, and the sediment settles into a permanent layer.
1. The “Energy Drop”
The main reason transport stops is a loss of kinetic energy.2 Imagine a fast-moving mountain stream carrying heavy pebbles; when it reaches a flat valley or a calm lake, the water slows down.3
- Heaviest first: The biggest particles (boulders and gravel) are the first to drop out because they require the most energy to stay in motion.4
- Lightest last: The tiniest particles (clay and silt) can stay suspended in even very slow-moving water, only settling when the water is almost completely still, such as in the deep ocean or a quiet pond.
2. Common “End of the Line” Environments
Where sediment stops depends on where the energy runs out. Geologists call these depositional environments:
| Environment | Why Transport Stops | Typical Sediment |
| River Delta | The river hits the standing water of a lake or ocean and stops flowing. | Sand and Silt |
| Alluvial Fan | A mountain stream suddenly hits a flat plain and spreads out. | Boulders and Gravel |
| Desert Dunes | The wind hits an obstacle or simply dies down. | Well-sorted Sand |
| Deep Ocean | There are no strong currents to keep fine particles floating. | Microscopic Clay/Mud |
3. Preservation: From Sediment to History
Not every “stop” is permanent. If sediment is deposited on a beach, a storm might pick it up and move it again the next day. For sedimentation to truly be the “end of the line,” the particles must be buried.
Once a layer is buried by new sediment, it is protected from the wind and water above. Over millions of years, the weight of the layers above will begin the process of turning that “settled stuff” into solid sedimentary rock.
See it in action
Sediment Transport
This video is a great resource because it visualizes the specific physics of how different types of sediment loads (bed load vs. suspended load) eventually reach their final destination and settle.
What is diagenesis and what is lithification?
While they are closely related, think of diagenesis as the “entire journey” and lithification as the “final destination” where sediment officially becomes rock.1
1. What is Diagenesis?
Diagenesis is the umbrella term for every physical, chemical, and biological change that happens to sediment after it is deposited, but before it turns into metamorphic rock.2 It happens at relatively low temperatures (under 200°C) and pressures.3
Key diagenetic processes include:
- Bioturbation: Critters like worms and crabs tunneling through the sediment, mixing it up.
- Recrystallization: Minerals changing their crystal structure into a more stable form (e.g., aragonite in shells turning into calcite).4
- Replacement: One mineral dissolving and being replaced by another (this is how petrified wood or pyrite-covered fossils form).5
2. What is Lithification?
Lithification is a specific part of diagenesis.6 It is the actual process of turning loose, unconsolidated sediment into solid, “lithified” rock.7
It relies on two main stages:8
A. Compaction (The Squeeze)9
As more layers of sediment pile up, the weight of the “overburden” becomes immense.10 This pressure:
- Squashes the grains together.11
- Reduces porosity (the empty space between grains).12
- Squeezes out trapped water.13
Note: Fine-grained sediments like clay can lose up to 80% of their volume during compaction!
B. Cementation (The Glue)
As water is squeezed out, it carries dissolved minerals.14 These minerals precipitate (solidify) in the remaining tiny gaps, acting as a natural glue.15
- Common Cements: Calcite (calcium carbonate), Silica (quartz), and Iron Oxide (which gives rocks like the Grand Canyon their red color).
Diagenesis vs. Lithification: At a Glance
| Feature | Diagenesis | Lithification |
| Scope | Includes all changes (chemical, biological, physical). | Focuses specifically on solidifying the rock. |
| Result | Can result in rock, but also altered minerals or fossils. | Always results in a solid sedimentary rock. |
| Key Actions | Dissolving, replacing, burrowing, hardening. | Compaction and Cementation.16 |
Lithification and Diagenesis
This video provides a more in-depth academic look at how these processes determine the texture and mineralogy of the rocks we see today.
How are sediments classified in sedimentary rocks?
To classify sedimentary rocks, geologists first determine how the sediment was created.1 They then use specific physical properties, like the size of the grains or the minerals present, to give the rock a precise name.2
1. Classification by Origin (Three Major Groups)
The broadest way to classify sedimentary rocks is by their “birth certificate”—how they were originally formed.
- Clastic (Detrital): Made of solid fragments (clasts) of pre-existing rocks.3 These are classified primarily by grain size.4
- Chemical: Formed when dissolved minerals precipitate directly out of water (like salt forming when a lake dries up).5 These are classified by mineral composition.6
- Biochemical / Organic: Formed from the remains of living things, such as shells, coral, or plant matter.7 These are also classified by their composition.8
2. Classifying Clastic Rocks (Grain Size)
For rocks made of pieces (sandstone, shale, etc.), geologists use the Udden-Wentworth Scale to measure the average diameter of the particles.9 This is the most common classification tool in geology.
| Particle Name | Size Range | Rock Name |
| Boulder | > 256 mm | Conglomerate (rounded) or Breccia (angular) |
| Cobble / Pebble | 2 mm – 256 mm | Conglomerate or Breccia |
| Sand | 1/16 mm – 2 mm | Sandstone |
| Silt | 1/256 mm – 1/16 mm | Siltstone |
| Clay10 | < 1/256 mm11 | Shale or Mudstone12 |
3. Classifying Chemical and Organic Rocks (Composition)
Because these rocks aren’t usually made of “pieces,” grain size doesn’t matter as much. Instead, we look at what they are made of.
| Rock Name | Primary Composition | Origin Type |
| Limestone | Calcite ($CaCO_3$) | Often Biochemical (shells/coral) |
| Chert | Silica ($SiO_2$) | Chemical or Biochemical |
| Rock Salt | Halite ($NaCl$) | Chemical (Evaporite) |
| Coal | Carbon (Plant remains) | Organic |
| Gypsum | Calcium Sulfate | Chemical (Evaporite) |
4. Textural Maturity
Geologists also classify sediment based on how “mature” it is.13 This tells us how far the sediment traveled before it stopped.
- Immature: Grains are different sizes (poorly sorted) and have sharp, jagged edges (angular). This suggests they haven’t traveled far from their source.
- Mature: Grains are all roughly the same size (well-sorted) and are smooth and round. This suggests a long journey through rivers or across beaches.
[Image comparing poorly sorted angular grains vs well sorted rounded grains]
Composition, Texture, & Classification of Sedimentary rocks
This video explains the detailed differences between clastic and chemical rock textures and how geologists use “maturity” to reconstruct Earth’s past.
What are bedding and what are sedimentary structures?
In geology, bedding is the general way sediment is stacked, while sedimentary structures are the specific patterns or “clues” found within those stacks that tell us what was happening at the time.
1. What is Bedding?
Bedding (or stratification) is the most obvious feature of sedimentary rocks.1 It refers to the arrangement of sediment into distinct layers, called beds or strata.2
- How it forms: Each bed represents a single “event” of deposition.3 A new bed starts when something in the environment changes—like a flood bringing in new sand, a change in water depth, or a different type of sediment being washed in.
- Bedding Planes: These are the flat surfaces that separate one bed from another.4 They represent a brief “pause” in deposition or a sharp change in conditions.5
- Law of Original Horizontality: Most beds are deposited in flat, horizontal layers due to gravity.6 If you see tilted layers today, it means tectonic forces moved them after they turned into rock.7
2. What are Sedimentary Structures?
Sedimentary structures are specific patterns found within or on the surface of beds.8 They are “fossilized” snapshots of physical processes like wind, waves, or drying mud.
Common Primary Structures (Formed during deposition):
| Structure | What it looks like | What it tells geologists |
| Cross-Bedding | Angled layers inside a horizontal bed. | Formed by migrating sand dunes or ripples; shows the direction of wind or water flow. |
| Graded Bedding | Coarse grains at the bottom that get finer toward the top. | Formed when a fast current slows down (like a submarine landslide); shows which way is up. |
| Ripple Marks | Small “waves” on the surface of a bed. | Symmetrical ripples suggest a beach (waves); asymmetrical ones suggest a river or desert (current). |
| Mud Cracks | Polygonal cracks in fine-grained sediment. | Tells us the area was once wet and then dried up in the sun. |
Common Secondary Structures (Formed after deposition):
- Bioturbation: Burrows or tracks left by animals (like worms or crabs) that churn up the sediment before it hardens.9
- Sole Markings: Grooves or scours made on the bottom of a bed when an object (like a stick) was dragged across the mud by a current.10
Why are they important?
Geologists call these structures “way-up indicators.” Over millions of years, Earth’s crust can flip or tilt rock layers completely upside down.11 By looking at things like mud cracks (which always widen toward the top) or graded bedding, geologists can figure out the original orientation of the rock and reconstruct ancient environments.12
What are clastic sediments in sedimentary rocks?
Clastic sediments are the solid fragments—ranging from microscopic clay to house-sized boulders—that are broken off from pre-existing rocks through physical weathering.1
In the world of sedimentary rocks, clastic materials are the “building blocks.” While chemical rocks form from dissolved minerals, clastic rocks are essentially “recycled” pieces of older igneous, metamorphic, or even other sedimentary rocks.2
1. Defining “Clasts”
The word “clastic” comes from the Greek word klastos, meaning “broken.” A clast is an individual grain or fragment.3
- Composition: Most clastic sediments are made of quartz and feldspar because these minerals are very hard and resistant to chemical rot.4
- Matrix: When these rocks form, the larger clasts are often surrounded by a “matrix”—a filler of much smaller particles (like silt or clay) that occupies the spaces between the big pieces.5
2. Classification by Grain Size
Geologists name clastic sedimentary rocks based almost entirely on the size of the sediments they contain.6 This is the most important classification system in sedimentary geology.
| Sediment Name | Size Range | Resulting Rock | Typical Environment |
| Gravel/Boulders | > 2 mm | Conglomerate (rounded) or Breccia (angular) | Mountain streams, landslides |
| Sand | 1/16 mm to 2 mm | Sandstone | Beaches, deserts, river channels |
| Silt | 1/256 mm to 1/16 mm | Siltstone | Floodplains, lake bottoms |
| Clay | < 1/256 mm | Shale or Mudstone | Deep ocean, quiet lagoons |
3. What Clastic Sediments Tell Us
The shape and arrangement of these sediments act like a “fingerprint” for the Earth’s past:
- Angularity: If the sediments are jagged and sharp (angular), they didn’t travel far.7 If they are smooth and rounded, they were likely tumbled in a river or ocean for a long time.
- Sorting: If the rock contains a mix of many sizes (poorly sorted), it was likely deposited quickly by a “chaotic” event like a landslide.8 If the grains are all the same size (well-sorted), they were likely moved by a steady force like wind or waves.9
- Maturity: A “mature” clastic rock consists of well-rounded, well-sorted quartz grains, indicating a very long journey from its source.10
[Image comparing angular poorly sorted grains vs rounded well sorted grains]
Key Clastic Rock Examples:
- Sandstone: The most common clastic rock; often shows “cross-bedding” from ancient dunes.
- Shale: A fine-grained rock that splits into thin layers (fissility); often contains well-preserved fossils because the sediment is so gentle.11
- Breccia: Contains large, sharp fragments; usually found near the base of cliffs or fault zones.12
What are chemical and biochemical sediments in sedimentary rocks?
While clastic rocks are made of “broken pieces,” chemical and biochemical sedimentary rocks are made from minerals that were once dissolved in water as invisible ions (like calcium, sodium, or silica).1
The main difference between the two is who (or what) does the work of pulling those minerals out of the water.
1. Chemical Sedimentary Rocks (Inorganic)2
These form strictly through physical or chemical processes without any help from living things.3 They typically form in environments where water is evaporating or where the chemistry of the water changes suddenly.4
- Evaporites: When seawater or lake water evaporates, the minerals left behind become too concentrated to stay dissolved.5 They “fall out” of the solution and crystallize.6
- Examples: Rock Salt (Halite) and Rock Gypsum.7
- Travertine: This is a type of limestone that forms in caves (as stalactites) or near hot springs.8 It happens when $CO_2$ escapes from the water, causing calcium carbonate to precipitate instantly.
- Oolitic Limestone: Small, round grains called “ooids” form in shallow, warm tropical waters when calcite layers grow around a tiny grain of sand as it rolls in the current.9
2. Biochemical Sedimentary Rocks (Organic)
These form when living organisms—from giant corals to microscopic algae—extract ions from the water to build their shells, skeletons, or tissues.10 When these organisms die, their remains pile up and become rock.11
- Biochemical Limestone: By far the most common type.12 It is made of the calcite remains of shells, coral reefs, or tiny plankton.
- Chalk: A very soft, white limestone made entirely of the microscopic shells of ancient floating organisms.13
- Coquina: A rock that looks like a “granola bar” of sea shells, loosely cemented together.14
- Biochemical Chert: Formed from the silica-rich shells of tiny marine organisms like diatoms or radiolarians.15 When they die, their glass-like shells carpet the ocean floor.
- Coal: A unique organic rock formed from the remains of ancient swamp plants that were buried before they could fully decay.16 Over millions of years, heat and pressure turned that “peat” into solid carbon.
Comparison at a Glance
| Feature | Chemical Sedimentary | Biochemical Sedimentary |
| Driver | Evaporation or Chemical Change | Biological Activity (Life) |
| Common Minerals | Halite, Gypsum, Calcite | Calcite, Silica, Carbon |
| Key Example | Rock Salt | Fossiliferous Limestone |
| Location | Salt flats, Caves, Hot springs | Tropical Oceans, Swamps |
Chemical Sedimentary Rocks
This video demonstrates the inorganic process of chemical precipitation using simple analogies and explores how environments like the Petrified Forest were created through these chemical shifts.
Solved Problems
Practice Problems: Sediments & Sedimentary Rocks
- Question: What is the primary difference between weathering and erosion? Solution: Weathering is the physical or chemical breaking down of rocks in place, while erosion is the removal and transport of that material by agents like water, wind, or ice.
- Question: Which mineral is most common in clastic sedimentary rocks due to its high resistance to chemical weathering? Solution: Quartz. It is chemically stable and physically hard, allowing it to survive long transport distances.
- Question: Describe the process of “Frost Wedging.” Solution: Water enters cracks in a rock, freezes, and expands by about 10%. This expansion exerts pressure that widens the crack, eventually breaking the rock apart.
- Question: A rock contains large, jagged, and angular fragments. Did this sediment travel a long distance or a short distance? Solution: Short distance. Angularity indicates the sediment has not been tumbled enough in water or wind to wear down its sharp edges.
- Question: What are the two main steps involved in lithification? Solution: Compaction (weight of overlying layers squeezing grains together) and Cementation (minerals precipitating in pore spaces to glue grains together).
- Question: How does Coal differ from most other sedimentary rocks? Solution: Most sedimentary rocks are made of minerals; Coal is organic, formed from the compressed remains of plant matter in oxygen-poor swamp environments.
- Question: What does “well-sorted” sediment indicate about the transport environment? Solution: It indicates a consistent energy level (like a beach or a desert dune) that was able to separate particles by size, leaving only one specific grain size behind.
- Question: What is the name of the rock formed from the lithification of clay? Solution: Shale (or mudstone).
- Question: If you find Ripple Marks in a rock, what can you infer about its formation? Solution: The sediment was deposited in moving water (a stream or a beach) or by wind, which created small wave-like structures on the surface.
- Question: What is an Evaporite, and give one example? Solution: A chemical sedimentary rock formed when mineral-rich water evaporates. Examples include Rock Salt (Halite) or Gypsum.
- Question: Which sedimentary structure can tell a geologist if a rock layer has been flipped upside down? Solution: Graded Bedding (where grains should be coarse at the bottom and fine at the top) or Mud Cracks (which v-shape downward).
- Question: What is the difference between a Conglomerate and a Breccia? Solution: Both have large grains (>2mm), but a Conglomerate has rounded fragments, while a Breccia has angular fragments.
- Question: What is Diagenesis? Solution: The sum of all chemical, physical, and biological changes that occur to sediment after deposition and during/after lithification (excluding metamorphism).
- Question: How does Limestone typically form biochemically? Solution: Marine organisms (like coral or plankton) extract calcium carbonate from seawater to build shells; when they die, these shells accumulate on the ocean floor and are cemented together.
- Question: What is “Cross-Bedding,” and what does it reveal? Solution: Layers deposited at an angle to the main horizontal bedding; it reveals the direction of the ancient wind or water current.
- Question: Why is Shale usually found in deep ocean or quiet lake environments? Solution: Shale is made of tiny clay particles that are so light they only settle out of the water when the environment is extremely calm (low energy).
- Question: What mineral acts as the “glue” in a rock that fizzes when touched with hydrochloric acid? Solution: Calcite (Calcium Carbonate).
- Question: What is the Udden-Wentworth scale used for? Solution: It is used to classify sediments based on their diameter/size (e.g., distinguishing sand from silt).
- Question: Define Bioturbation. Solution: The churning and stirring of sediment by living organisms (like worms or crustaceans), which often destroys original bedding structures.
- Question: Why are fossils almost exclusively found in sedimentary rocks rather than igneous or metamorphic? Solution: The heat and pressure required to form igneous and metamorphic rocks would destroy the organic remains. Sedimentary rocks form at low temperatures and pressures that preserve them.
Learn how we bridge these gaps: [The Starline Philosophy: The Modern Polymath]