Sedimentary and Metamorphic Rocks

• Lecture 4/5: Earth Materials II & III review earth materials, with a focus on sedimentary and metamorphic rocks.

Sedimentary Rocks

• Sedimentary rocks are formed from rock fragments that settle and accumulate in layers after being transported or precipitated.
• The term "sediment" comes from the Latin word "sedimentum," which means "to settle."
• Sedimentary rocks originate from mechanical or chemical breakdown.
• They make up only 5% of Earth's outer 15 km by volume, but cover 75% of exposed rocks.
• These contain evidence of past environments, including tectonics, climate, and life (fossils).
• Rock types are based on the source of the material
• Detrital or clastic rocks are transported sediment as solid particles.
• Chemical rocks are sediment that precipitates from a solution or extracted from water by organisms (converted to shells/skeletons).

Sedimentary processes

• Physical features in the rock reveal how sediments were deposited and the environmental conditions of deposition.
• Begins, with weathering that breaks down rock.
• Follows, erosion that leads to mass wasting.
• Next, transport leading to relocation.
• Afterward, deposition of sediments.
• Ends, with lithification via compaction & cementation to create rock.

Weathering

• Weathering involves physical and chemical changes that weaken and break down rocks when exposed to the atmosphere and biosphere.
• Factors controlling weathering include the parent rock (e.g., granite vs. salt), climate (temperature and rain), soil and vegetation cover, and time.

Types of Sedimentary Rocks

• Physical and chemical weathering provide the particles and dissolved ions needed for all sedimentary rocks
• Geologists classify them into four classes:
• Clastic: Loose rock fragments (clasts) held together by cement.
• Biochemical: Cemented shells of organisms.
• Organic: Carbon-rich remains of once-living organisms.
• Chemical: Minerals that crystallize directly from the water.

Clastic Sedimentary Rocks

• These rocks are made of quartz, feldspars, micas, and clay minerals, which are stable on Earth's surface.
• Texture reflects process of deposition.
• Grain size is related to the parent rock properties and is proportional to the energy level of the transport medium (hi-E = smaller grains).
• Sorting is determined by distribution of sizes, shapes, and densities from a transport medium that selects particles of different sizes, shapes, and sorting..
• Grain Shape: ranging from angular to rounded shapes that are proportional to energy level or distance traveled (more collisions = rounded)

Mudstone

• Consists of clay to silt-sized particles in thin layers.
• Commonly forms in calm waters (lakes, lagoons, and deep ocean).
• Sedimentary rocks are more than 50% mudstone
• Mudstone color varies: red (Fe & O rich), green (Fe & O poor), black (not enough O to decompose organic matter).

Sandstone

• Mostly composed of sand-sized particles; primarily quartz
• Found in rivers, beaches, glaciers, mountains, and deserts.

Conglomerate and Breccia

• Particles are over 2 mm in diameter within a finer-grained matrix.
• Conglomerate contains rounded gravels.
• Breccia contains angular particles.

Chemical Sedimentary Rocks

• Chemical sedimentary rocks consists of precipitated material that was once in solution.
• Precipitation occurs through:
• Inorganic processes where minerals in water precipitate or
• Organic processes (biogenic origin).

Limestone

• Limestone is a common chemical sedimentary rock.
• It forms is 10-15% of sedimentary rocks.
• It is composed chiefly of calcite (CaCO3).
• This rock forms in warm, tropical, shallow, clear, and O2-rich marine water.
• Marine biochemical limestones form from coral reefs.
• They are made of sediments derived from the shells of once-living organisms.
• Hard mineral skeletons accumulate after the death of the organisms
• Inorganic limestones include travertine, speleothems and tufa.
• Travertine is calcium carbonate (CaCO3) that is precipitated from groundwater.
• Dissolved calcium (Ca2+) reacts with bicarbonate (HCO3-).
• When CO2 is expelled into the air, it causes CaCO3 to precipitate, and this process occurs in thermal (hot) springs and caves.
• CaCO3 precipitation is controlled by the solubility of CaCO3, which is proportional to CO2 in water and a water's level of agitation.

Chert

• Chert is a chemical sedimentary rock that contains microcrystalline quartz.
• It may contain silica, skeletons of small marine plankton, or varieties.

Evaporites

• Trigger deposition of inorganic chemical precipitates.
• Form is restricted basins with warm temperatures
• Rock salt and rock gypsum are some examples of this sedimentary rock.

Coal

• Coal is a chemical sedimentary rock that is made from the decay and compression of land plants rich in resins, waxes, and lignins.
• Organic matter accumulates in anoxic environment swamps.
• Peat converts into lignite, bituminous, and anthracite coal after burial.
• Largely distributed and readily available supply.

Economic and Scientific Considerations

• Economic:
  • Coal
  • petroleum & natural gas
  • iron, aluminum, & manganese
  • phosphates
• Science:
  • Fossils
  • Geologic Histories

Important factors regarding Sedimentary Rocks

• Sedimentary rocks can be formed in glacial, desert, fluvial, and lake environments.

Metamorphism

• The term "metamorphism" comes from the Greek words meta (to change) and morphe (form).
• Metamorphism occurs when temperature (T), pressure (P), and chemical reactions alter mineral content and structure of pre-existing rock.
• This process only occurs in solid-state, without melting.
• Metamorphism happens when T & P break some atomic bonds.

Metamorphic rocks

• Produced from:
  • Igneous
  • Sedimentary
  • Other metamorphic rocks
• To understand all metamorphic rocks, it's important to consider three questions:
  • What is the rock now?
  • What was it before?
  • What caused the change tectonically?

Controls of metamorphism

1. Temperature
   • Is not absolute, but a change in temperature.
   • Rocks shift to equilibrium with new environmental conditions.
   • This produces recrystallization and stable minerals.
   • Geothermal gradient ranges from 10°C/km in stable continents to over 100°C/km in tectonically active areas.
2. Pressure (stress)
   • lithostatic/confining:
     • pushes on all directions equally.
     • rock gets more dense without change in shape.
     • This process increases at 0.3 kbar/km depth.
   • directed (differential):
     • Pressure is at its greatest in one direction.
       • Results to change in shape.
       • New growth minerals are aligned perpendicular to pressure to create foliation.

Differential Stress

• Normal stress operates perpendicular to surface, and includes push-together compression and pull-apart tension.
• Shear stress acts parallel to a surface, smearing it.

3. Fluid activity
   • Includes H2O and CO2, which enhances migration and potential for reaction.
   • Hydrothermal alteration is called metasomatism.

• The degree of metamorphism is controlled by the protolith

Example of Metamorphism

• At 200°C, sandy limestone consists of quartz and calcite.
• At 600°C, minerals are no longer stable together; each exists alone but together forms an aluminosilicate which are from the chemical formula: SiO2 + CaCO3 = CaSiO3 + CO2 is wollastonite.
• Elements do not change only restructure and regrow.
• During this process, CO2 is released.
• If CO2 leaves and rock cools reverts to quartz + calcite cannot occur unless CO2 is added.

Metamorphic Environments

• Metamorphic environments are most common at the plate boundaries

Regional Metamorphism

• Burial environments involved in the T & lithostatic P in sedimentary basins.
• Mississippi Delta, Bay of Bengal are the locations.
• Pressure increases as growing pile of overlying sediments increases T with increasing depth
• During Dynamothermal (Orogenic) conditions there is response to changes in pressure and temperature with large-scale tectonics and directed pressure that causes foliation in Himalayas, Appalacian regions.

Metamorphic Grade

• The metamorphic grade from low to high occurs between slate, phyllite, schist, gneiss.

Contact Metamorphism

• It is the response to change in temperature in cooler rocks, and is a local scale.
• An aureole has a zone of alteration that forms in the surrounding magma to the original rock.
• Hydrothermal Metamorphism*
• Includes chemical reactions caused by hot, ion-rich fluids in rock fissures and cracks and is common along mid-ocean ridges.

Classification

• The degree of metamorphism is reflected in texture and this is also reflected in mineralogy.

Granoblastic Rocks

• Includes non-foliated rocks that have crystal shapes that grow uniformly and are interlocking.
• Monomineralic rocks are granoblastic.

Hornfels

• Is developed through high-T contact metamorphism in uniform that undergoes minimal deformation.

Quartzite

• Is very hard and a nonfoliated white rock found in sandstone.

Marble

• Marble is a metamorphic products of T & P on limestone.

Foliated Rocks

• It is named after its "leaf-like" appearance.
• A common textural feature is the nature of foliation (flat or wavy parallel planes)

Slate

• It is a foliated rock made in low temperature and pressure environments.

Phyllite

• Forms at a higher level of temperature and pressure than slate.

Schist

• This is more intensely metamorphosed, the rock has flaky and larger crystals, easily seen by the naked eye.

Gneiss

• This textured rock forms under a high pressure and temperature environment.

Migmatite

• Forms when at a high temperature where beginning to melt a rock's precursor.

Exhumation

• Metamorphic rocks that return to earth's surface through a process called exhumation.
• This process is a result of mountains decaying through eduction, extensional thinning, and erosion.