Sedimentary Rocks: Formation and Classification

Detrital Sedimentary Rocks

• Solid particles are products of physical weathering
• Examples include sandstones, shales, and conglomerates.

Chemical Sedimentary Rocks

• Examples include limestone and chert.

Sedimentary Rocks and the Rock Cycle

• Weathering breaks down rocks.
• Transportation moves eroded products.
• Deposition involves particles settling due to decreased transportation energy.
• Burial is the accumulation of sediment, burying older material.
• Diagenesis/lithification is when sediments turn into solid rocks.

Transportation Mechanisms

• Water is the most effective transportation mechanism.
• Wind transports finer grained materials like dust.
• Ice glaciers can transport material of any size.
• Gravity has limited range of transportation.

Energy, Particle Size, and Transportation

• Consistent grain size indicates consistent energy.
• Inconsistent grain size indicates inconsistent energy.
• Particles become smaller and more rounded with longer transport.
• High -> Low Energy Transportation.

Sorting

• Sorting refers to particles that are all the same size.

Rounding

• Rounding represents a long travel distance or time.

Rounding Occurrence

• Rounding may occur from a long travel distance.

Diagenesis and Lithification

• Compaction and cementation bind grains together.
• Volume and water decrease.
• Heat and pressure cause precipitation of a solid, such as calcite.

Stratification (Bedding)

• Layers in beds indicate changes in deposition over time.

Cross Bedding

• Wave-like formations in rocks between horizontal layers indicate deposition by a moving current.
• The dip points in the direction the current was moving.
• Environments include beaches and deserts.

Graded Bedding

• Layers transition from coarse and old at the bottom to young and fine-grained at the top.
• Found in ocean environments and formed by turbulent currents causing avalanches.

Fossils

• Fossils can define certain periods of time if they are easily identifiable and existed for a limited period.

Local Rock Type: Chuckanut Formation

• Thick sequence of clastic sedimentary rocks, mostly sandstone.
• 40-50 million years old with plant fossils, like palm tree trunks.
• Contains lots of coal.

Sources of Heat for Metamorphic Rocks

• Contact metamorphism occurs near a magma melt.
• Regional metamorphism is due to the geothermal gradient from the mantle/core.
• Mylonites generate heat from friction across a fault plane.
• Heat originates from radioactive decay, remnant heat from Earth's formation, and pressure.

Pressure Types for Metamorphic Rocks

• Confining pressure is the same in all directions.
• Differential pressure involves squeezing horizontally.

Parent Rock (Protolith)

• The protolith limits the minerals that can form during metamorphism.
• A single parent rock can turn into many metamorphic rocks with heat and pressure.
• A single metamorphic rock can derive from many parent rocks
• Marble forms from limestone and quartzite from sandstone.

Foliation

• Foliation involves the aligning of minerals.

Regional Metamorphism

• High pressure occurs under mountain ranges at convergent boundaries.
• Forms a variety of foliated rocks.
• Examples include slate, schist, phyllite, and gneiss.

Contact Metamorphism

• Occurs when magma comes into contact with rocks without melting them.
• Heat transforms the rocks with low pressure.
• Rocks are not foliated due to high temperature and low pressure.
• Creates localized zones.
• Marble forms from limestone.

Burial Metamorphism

• Occurs at the bottom of sediment piles.
• Non-foliated rocks.

Metamorphic Grade

• High grade indicates high temperature and pressure.
• Low grade indicates low temperature and pressure.
• Increasing grade: Slate-phyllite-schist-gneiss.

Location of Metamorphism

• Regional metamorphism takes place at convergent boundaries.
• Burial metamorphism occurs at convergent boundaries.
• Other locations include subduction zones, divergent areas, and deep within the crust.

Principles of Relative Dating

• Comparing two different entities to determine which is older and younger.

Law of Original Horizontality

• Layers of sediment are originally deposited horizontally under the action of gravity.

Law of Superposition

• Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top.

Cross-Cutting Relations

• The geologic feature which cuts another is the younger of the two features.

Unconformities

• Represent a gap in the geologic record, interrupting sedimentation.
• Commonly accompanied by erosion of rocks below the break.

Absolute Dating

• Determines age in years using radioactivity and radiometric dating.

Isotope

• Variations of an atom with different numbers of neutrons.
• Carbon-14 is an example.

Parent

• The isotope that is decaying.

Daughter

• The decay product.

Half-Life

• The decay rate of an isotope.

Viscosity

• Resistance to flow.
• High viscosity = does not flow.
• Low viscosity = flows easily.

Pahoehoe

• A type of basaltic lava with a ropey, braided surface texture.

Aa

• A type of basaltic lava with a blocky flow and higher viscosity.

Common Gases Emitted from a Volcano

• Sulfur dioxide, carbon dioxide, and water vapor.

Pyroclastic Material

• Includes bombs and ash, classified by grain size.

Shield Volcanoes

• Thick stacks of individual basalt lava flows with shallow slopes.
• Example: Mauna Lao.

Cinder Cones

• Basaltic, made of pyroclastic material, but also erupt lava.
• Formed from one event.
• Usually small and associated with other volcanoes.

Composite Volcanoes

• Subduction-related, generally intermediate (andesite).
• Viscous, with alternating layers of lava and pyroclastic material.
• Eruptive events create these layers.
• Associated with series eruptions.
• Example: Mt. Rainer.

Volcanic Domes

• Felsic to intermediate.
• Steep-sided with little lava spreading.
• Can trap gas, become over-steep, and landslide.
• Relatively small.
• Example: St. Helens 1980 and 2004 eruptions with newer lava domes.

Craters and Calderas

• Formed from a composite cone that blows its top and collapses.
• Results from serious eruptions of composite volcanoes.
• Example: Mt. Mazama forms Crater Lake.

Fissure Eruptions/Flood Basalts

• Basaltic and can occur at divergent plate boundaries.
• Earth opens, creating flood basalt plateaus in horizontal layers.
• Non-viscous.
• Example: Siberian Traps in Russia.

Columbia River Basalts

• Each layer was an individual lava flow.
• Occurred 12-15 million years ago under a continental plate.
• Not a linear pattern.

Pyroclastic Flows

• A mixture of hot ash and lava, denser than air.
• Burns anything in its path.

Lahars

• Mud flow, due to pyroclastic material mixing with glacier ice or snow.
• Most significant and harmful effects of eruptions.
• Dangerous with little warning.

Clastic Sedimentary Rocks

• Formed from pieces of weathered rocks and mineral grains.

Chemical Sedimentary Rocks

• Precipitated from solution.

Depositional Environment for Limestone

• Shallow Marine.

Depositional Environment for Chert

• Deep Marine.

Carbon-14 Dating

• Carbon-14 is created in the atmosphere and combines with oxygen to form carbon dioxide.
• Living things take it up, maintaining equilibrium.
• Upon death, Carbon-14 decays into nitrogen, starting the depletion clock.
• Each half-life is 5,700 years.
• Measurable until about 8 half-lives (50,000 years).
• Can date anything that was once alive.

Index Fossils

• Pinpoint a period of geologic time.
• Preserved in sedimentary rocks.
• Easily identifiable and existed for a limited period of time.

Columnar Joints

• Form when lava cools and contracts.
• Hexagonal, six-sided structures that run up and down.

Gases and Eruptions

• The ease of gas escape determines eruption size.
• Viscous lava doesn't allow gas to escape, leading to bigger explosions in felsic/intermediate eruptions.