WEEK 3 - Sedimentary Rocks Flashcards
Sedimentary Rocks
Formed from accumulated & hardened sediment over time
SEDIMENT: solid material that is moved and deposited in a new location
Three Types of Sedimentary Rocks
1️⃣ Clastic – Rock fragments (e.g., sand)
2️⃣ Chemical – Minerals from dissolved solutions (e.g., salt)
3️⃣ Biochemical – Organic materials (e.g., shells, wood)
Sources of Clastic Sediment: Weathering of Rocks
Rocks break down at Earth’s surface by weathering:
- Mechanical (Physical) Weathering - Breaks rocks into smaller pieces without changing composition
- Caused by: Water, wind, ice, & biological activity (e.g., tree roots).
- Chemical Weathering - Breaks down minerals through chemical reactions with water or air
- Caused by: Acid rain, oxidation, & dissolution
Mechanical vs. Chemical Weathering
🛠️ Mechanical Weathering:
- Breaks rocks into smaller pieces without changing composition
- Common in cold, dry climates (e.g., Arctic)
🧪 Chemical Weathering:
- Breaks down minerals through chemical reactions (e.g., oxidation, dissolution)
- Follows Bowen’s Reaction Series – High-temperature minerals weather faster than low-temperature ones
- Most intense in warm, humid climates (e.g., rainforests)
Why Do Some Minerals Weather Faster?
🔥 High-Temperature Minerals (e.g., Olivine, Pyroxene):
- Less stable at Earth’s surface → Weather easily
❄️ Low-Temperature Minerals (e.g., Quartz):
- More stable at Earth’s surface → Resistant to weathering
EXAMPLE: Quartz is abundant in sedimentary rocks because it weathers slowly
Bowen’s Reaction Series & Weathering
🌡️ Minerals that crystallize at high temps (e.g., olivine) break down faster than those forming at low temps (e.g., quartz)
- It is for this reason that quartz is very abundant in sedimentary rocks, whereas olivine is very rare
🌿 Chemical weathering is strongest in hot, wet climates (e.g., tropical rainforests)
What’s Left After Chemical Weathering?
- Quartz → Quartz grains (resistant to weathering)
- Feldspar, Amphibole, Olivine → Clay minerals, Limonite, Hematite
- Iron oxides (Hematite) remain as residue
💧 Dissolved ions (e.g., silica, Mg²⁺, Ca²⁺) are carried away in water → Transported to the sea
How Are Sediments Transported?
🌍 Gravity – Mass wasting (landslides, rockfalls)
🌊 Water – Rivers carry sediment downstream
💨 Wind – Moves small particles (e.g., dust storms)
❄️ Ice – Glaciers transport and deposit sediments
How Do Sediments Get Deposited?
Transport stops when the medium (water, wind, ice) can’t carry them anymore
Lighter/smaller particles → settle in calm environments (e.g., lakes, deep ocean)
Heavier/larger particles → settle in high-energy environments (e.g., rivers, debris flows)
- 🌊 Beach Sand → Well-sorted & rounded (constant wave action)
- 🌪️ Mudflows/Debris Flow → Poorly sorted & angular (sudden transport)
Where Do Sediments Accumulate?
Beaches – Well-sorted sand, rounded grains
Swamps – Fine organic-rich mud, plant material
Offshore Marine – Fossil-rich, fine sediments
Rivers, Lakes, Deltas – Mix of sediment sizes, influenced by flow strength
🌎 Sedimentary rocks help geologists identify past environments!
How Does Sediment Become Rock?
🪨 Lithification → Turns loose sediment into rock
1️⃣ Compaction – Sediment layers get buried, squeezing particles closer together
2️⃣ Cementation – Minerals from water fill spaces between grains, bonding them together
🔹 Result: More solid, rock-like material
Clastic Sedimentary Rocks
🪨 Clastic sedimentary rocks are classified by the grain size of their sediments:
🔹 Clay (Sediment Type) → Shale/Mudstone (Clastic Rock)
- Shale = fine-grained, splits into layers
- Mudstone = similar but does not split easily
🔹 Silt (Sediment Type) → Siltstone (Clastic Rock)
- Slightly larger than clay, still fine-grained
🔹 Sand (Sediment Type) → Sandstone (Clastic Rock)
- Visible sand-sized grains
- Common in deserts, beaches, and riverbeds
🔹 Gravel (Sediment Type) → Conglomerate (Clastic Rock)
- Conglomerate = rounded pebbles/cobbles
- Breccia (a variant) = angular fragments
Chemical Sediment & Seawater Composition
🌊 Water dissolves minerals → forms chemical sediments
🔹 Common dissolved ions in seawater (% by weight):
Positive Ions:
Sodium (Na+): 30.61%
Magnesium (Mg2+): 3.69%
Calcium (Ca2+): 1.16%
Potassium (K+): 1.10%
Negative Ions:
Chloride (Cl-): 55.04%
Sulfate (SO4 2-): 7.68%
Others (0.72%): Trace metals (gold, zinc, copper, etc.)
Chemical Sedimentary Rocks: Evaporites
Evaporite rocks form when dissolved ions in water become too concentrated and precipitate as minerals
This occurs in isolated basins where water can’t escape back to the ocean
More evaporation → More concentrated dissolved ions → Minerals precipitate & deposit
Two Common Minerals in Evaporate Deposits
1️⃣ Gypsum (Calcium Sulphate)
2️⃣ Halite (Sodium Chloride)
These minerals form as water evaporates and ions bond together
Rock Types from Evaporites
Rock Salt (from halite) → Used as table salt
Alabaster (from gypsum) → Used in sculptures
What is Travertine?
Travertine is a sedimentary rock that’s a type of limestone
Chemical Sedimentary Rocks: Chemical Limestones
Limestone is a sedimentary rock made of calcite (CaCO₃)
Forms in caves & hot springs where calcium & carbonate ions precipitate
Travertine = A banded variety of limestone
Some travertine has pores from gas bubbles released by bacteria
Travertine Structures
Travertine forms in different environments:
🏔️ Cave Stalactites (e.g., Lehman Caves, Nevada)
🌋 Geothermal Terraces (e.g., Yellowstone, Wyoming)
🏠 Polished travertine used in tabletops & floor tiles
Biochemical Sediment
Formed by biological activity (organic materials processed into rock)
Includes: Shells, bones, teeth, plant remains
Three Types of Biogenic Sedimentary Rocks
1️⃣ Fossiliferous Limestone (made of calcite shells/skeletons)
2️⃣ Chert (formed from silica-rich organisms)
3️⃣ Coal (compacted plant material)
Fossiliferous Limestone
Fossiliferous limestone contains visible skeletal fragments (e.g., crinoids)
Chalk
Chalk is a variety made of microscopic algae skeletons
Chert: A Biochemical Rock
Formed from silica skeletons of planktonic organisms & sponges
Silica dissolves into a gel on the seafloor, then solidifies into fine-grained quartz rock
Very hard due to its quartz composition
