Deformation and Metamorphism

Deformation and Metamorphism

  • When a rock is exposed to tectonic forces, it can be deformed and metamorphosed due to great temperatures and stresses.

  • Deformed structures are described using strike and dip.

Deformation

  • Geologists use measurements to describe the orientation of a rock layer using strike and dip.

  • Strike: Direction one would walk along a bed without changing elevation.

  • Dip direction: Direction water would flow down the bed (perpendicular to strike).

  • Dip angle: Angle of the bed from horizontal.

  • Strike and dip measurements describe the orientation of any plane.

Geologic Maps and Cross Sections

  • Representations of local geologic units and features at the surface (map) and below the surface (cross section).

Rock Deformation

  • Basic tectonic forces that deform rocks:

    • Tensional forces: Pull and stretch.

    • Compressive forces: Squeeze and shorten.

    • Shearing forces: Push two sides in opposite directions.

  • Brittle deformation occurs in the shallow crust, resulting in fractures.

  • Ductile deformation occurs in the deeper crust, resulting in bulging.

Brittle Rock Deformation: Faults

  • Faults

  • Hanging Wall

  • Foot Wall

Normal Faults

  • Extensional deformation.

  • Dip-slip motion.

  • Hanging wall moves down relative to the footwall

Reverse Faults

  • Compressional deformation.

  • Dip-slip motion.

  • Hanging wall moves up relative to the footwall.

  • Fault plane at a steep angle.

Thrust Faults

  • Compressional Deformation

  • Dip-slip motion

  • Hanging wall moves up relative to footwall

  • Fault plane at shallow angle: largely horizontal movement.

Strike-Slip Faults

  • Shear Deformation

  • Steep fault plane

  • Horizontal movement.

  • Left-lateral strike-slip fault

  • Right-lateral strike-slip fault

Oblique-Slip Faults

  • Combination of Forces

  • Strike-slip and dip-slip motion

  • Combination of shear forces with either tensional or compressive forces.

Ductile Rock Deformation: Folds

  • Compressive and Sometimes Shear Stresses

  • Anticlines fold upward; synclines fold downward.

  • A horizontal fold axis is horizontal; a plunging fold axis is at an angle to the horizontal.

  • Anticline: Limbs dip away from the fold axis; older rock units are found closer to the fold axis.

  • Syncline: Limbs dip toward the fold axis; younger rock units are found closer to the fold axis.

Fold Symmetry

  • Symmetrical folds have limbs that dip symmetrically from the axial plane.

  • Asymmetrical folds have one limb that dips more steeply than the other.

  • Overturned folds have limbs that dip in the same direction, but one limb has been tilted beyond the vertical.

Joints

  • Fractures with No Movement

Continental Tectonics: Tensional

  • Extension of continental crust produces normal faults with high dip angles in the upper crust that flatten with depth, forming curved fault surfaces.
    *Brittle upper crust
    *Ductile lower crust

  • Rift Valleys

Continental Tectonics: Compressive

  • Compression of continental crust occurs on thrust faults with low dip angles.

  • Fold and thrust belt

  • Thrust Faults

Continental Tectonics: Shearing

  • Shearing of continental crust occurs on a nearly vertical strike-slip fault. The case shown here is for a right-lateral fault.

  • A left bend in the fault results in local compression.

  • A right bend in the fault results in local extension.

Metamorphic Rocks

  • Rocks formed by the transformation of preexisting solid rock under the influence of high pressures and temperature.

  • Occurs at various locations:

    • Regional metamorphism at convergent plate boundaries at moderate to deep levels under moderate to ultra-high pressures and high temperatures.

    • High-pressure metamorphism along linear belts of volcanic arcs, produced by continent-continent collision, occurs at high pressures.

    • Contact metamorphism affects a thin zone of country rock around an igneous intrusion.

    • Burial metamorphism transforms sedimentary rocks at progressively increasing temperature and pressure.

    • Shock metamorphism results from the heat and shock waves of a meteorite impact, transforming rock at the impact site.

Metamorphic Textures

  • Foliated texture: Platy minerals become aligned due to directed pressure.

  • Non-foliated (Granoblastic) texture: Composed mainly of crystals that grow in equant shapes and therefore cannot become aligned as with foliated rocks.

Foliation under Directed Pressure

  • Directed pressure causes sedimentary rocks, such as shale, to form cleavage planes that may differ from their bedding planes.

  • Regionally metamorphosed rock show foliation caused by compressive forces.

  • Rocks develop foliation when they contain platy minerals that align along a preferred orientation.
    *Depending on their preferred orientation, compressive forces cause the mineral crystals in the rock to grow or to align perpendicular to the compressive forces.

Non-foliated (Granoblastic) Rocks

  • Quartzite

  • Marble

Foliation: Indicative of Metamorphic Grade

  • As intensity of metamorphism increases, so does crystal size and coarseness of foliation.

  • Low grade: Slate (Slaty cleavage)

  • Intermediate grade: Phyllite (Slaty cleavage), Schist (Schistosity) (abundant platy minerals)

  • High grade: Gneiss (Banding) (fewer platy minerals), Migmatite (Banding)

Metamorphic grade and texture

  • As a parent rock is metamorphosed, it progresses from low-grade rock to high-grade rock.

Metamorphic Grade and Metamorphic Facies

  • Metamorphic facies and associated index minerals of metamorphic rock with shale as the protolith.

Regional Metamorphism and Metamorphic Grade

  • Isograds based on index minerals can be used to plot metamorphic grades over a regional metamorphic belt.

  • Not metamorphosed, Chlorite zone (Low grade), Biotite zone (Intermediate grade), Garnet zone (Intermediate grade), Staurolite zone (High grade), Sillimanite zone (High grade)

Interpreting Metamorphic Facies

  • Overlapping boundaries of metamorphic facies.
    Contact metamorphism, Conditions beneath
    mountain belts
    Zeolite, Hornfels, Greenschist, Amphibolite, Granulite, Eclogite. Subduction zone
    Blueschist

Interpreting P-T Paths: Plate Tectonics

  • Ocean-continent convergence: Low temperature-high pressure metamorphism at subduction zones

  • Continent-continent convergence: High temperature-high pressure metamorphism within mountain belts

Module 8 Geologic Time

  • Many geologic processes require enormous amounts of time to occur.

  • Earth is 4.564.56 Billion years old.

*Geologic materials (rocks, fossils, etc.) preserve a rich archive of the history of our planet.

Geologic Time

  • Relative dating: the process of determining the chronologic order of geologic features or events (old – young)

  • Absolute dating: the process of determining the numerical age of a given geologic feature or event (expressed as the number of years before present)

Relative Dating

  • Geologists use principles of rock relationships to determine the order of events in Earth’s history.

  • These principles are used in the science of stratigraphy, which is the study of layers (strata) within sedimentary rocks.

Principle of Original Horizontality

  • Sediments are deposited under the influence of gravity and therefore form nearly horizontal beds (or strata).

Principle of Superposition

  • Sedimentary beds in an undisturbed sequence are arranged with the oldest on the bottom and progressively youngers beds above.

Principles of Original Horizontality and Superposition

  • Ramifications: If sedimentary beds are not horizontal, then tectonic forces have impacted the region after the deposition of the deformed beds.

Principle of Faunal Succession

  • Fossils are preserved in sedimentary rocks.

  • Fossils occur in a definite sequence within the geologic record as a result of biological evolution over time.

Principle of Faunal Succession

  • Allows geologists to correlate sedimentary strata from different locations leading to the ability to construct composite (and more complete) geologic sections

Principle of Cross-Cutting Relationships

  • Intrusive rocks or deformation structures must be younger than the strata that are cut.

Conformable vs. Unconformable

  • Strata that were deposited continuously and that represent a complete record of time are termed conformable strata.

  • Unconformable strata are those that do not represent continuous sedimentation.

  • The boundary between unconformable strata is called an unconformity.

  • Unconformities represent gaps in time.

  • There are three types of unconformities:

Disconformity

  • Upper set of strata are deposited upon the erosional surface of underlying, undeformed strata.

  • Disconformities represent a gap in time normally associated with uplift and erosion followed by subsidence and renewed sedimentation.

Angular Unconformity

  • Upper set of strata are deposited upon lower folded strata that have been eroded to a flat plane.

  • Two sets of bedding are not parallel.

  • Angular unconformities represent a gap in time associated with uplift, deformation, and erosion followed by subsidence and renewed sedimentation.

Nonconformity

  • Upper set of strata are deposited directly on metamorphic or igneous rocks.

  • Nonconformities represent a gap in time between the metamorphic/igneous process and the initiation of sedimentation upon this surface.

Relative Dating: Construction of the Geologic Time Scale

  • Utilizing stratigraphic principles, and especially the principle of faunal succession, early geologists reconstructed the worldwide history of geologic events.

  • Earth history is divided into time slices known as eons, eras, periods, and epochs (in order of longest to shortest duration).

  • Subdivisions of time were defined by unique faunal assemblages observed within the fossil record.

Geologic Time Scale

  • Relative Dating: Construction of the
    Time Scale
    *EPOCH, PERIOD, ERA
    Cenozoic Era: Holocene, Pleistocene, Neogene, Paleogene
    Mesozoic Era: Cretaceous, Jurassic, Triassic
    Paleozoic Era: Permian, Carboniferous, Devonian, Silurian, Ordovician, Cambrian

Regional Correlations of the Colorado Plateau

  • Relative Dating:

Absolute Dating

  • Determination of the number of years before present that a geologic event occurred

  • Absolute dating is important so that geologists can understand the rates of geologic processes.

  • The most common tool used for absolute dating is the utilization of radioactive isotopes that are present in geologic materials.

Isotope Basics

  • Atomic number: number of protons (defines element)

  • Atomic mass: number of protons plus number of neutrons

  • Isotopes of a given element have the same atomic number, but different atomic masses (different number of neutrons).

  • Most isotopes are stable.

  • Radioactive (unstable) isotopes’ nuclei will spontaneously disintegrate.

Radioactive Decay

  • A radioactive parent decays to a daughter.

  • The rate of this decay is predictable.

  • The predictability of decay can be used as a “clock” within the rock: Parent → Daughter

Radioactive Decay

  • Half-life: time required for one-half of the original number of parent atoms to decay to daughter atoms

  • Half-life is predictable and reproducible.

  • By knowing the half-life of a given isotope, and the proportion of that parent that remains in a sample, an absolute date can be calculated for any sample.

Key isotopic systems

  • Parent isotope, Daughter isotope, Half-life, Effective dating range, Examples of Minerals and Materials That Can Be Dated

    • Uranium-238, Lead-206, 4.44.4 billion years, 1010 million-$4.6$ billion, Zircon, apatite

    • Uranium-235, Lead-207, 0.70.7 billion years, 1010 million-$4.6billion,Zircon,apatite</p></li><li><p>Potassium−40,Argon−40,billion, Zircon, apatite</p></li><li><p>Potassium-40, Argon-40,1.3billionyears,billion years,50,000-4.6billion,Muscovite,biotite,hornblende</p></li><li><p>Rubidium−87,Strontium−87,billion, Muscovite, biotite, hornblende</p></li><li><p>Rubidium-87, Strontium-87,47billionyears,billion years,10 million-$4.6 billion, Muscovite, biotite, potassium feldspar

    • Carbon-14, Nitrogen-14, 57305730 years, 100−70,000100-70,000, Wood, charcoal, peat, bone and tissue, shells and other calcium carbonates

Putting It All Together:

  • Eons: Hadean, Archean, Proterozoic, Phanerozoic
    *Eras: Precambrian, Mesozoic, Cenozoic, Tertiary Quaternary

  • Periods: Cambrian, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, Holocene
    *Epoch: Paleocene

Module 9 History of Earth

  • Earth is 4.564.56 billion years old.

*Earth has experienced a rich and diverse history.

Lecture Outline

  • Origin of the Solar System

  • Early Earth: Formation of a Layered Planet

  • Diversity of the Planets

  • Evolution of the Continents

  • Biological Events in Earth’s History

Formation of the Solar System

  • Nebular hypothesis

  • Matter within a given orbit accretes together to eventually form planets.

  • Leads to formation of planets that rotate within orbits around the sun

The Solar System

  • Inner rocky planets were formed from the accretion of dense materials.

  • Outer gaseous planets were formed from the accretion of less dense materials.
    *Note: Pluto is no longer a planet as of 2006.

Early Earth

  • Earth was very hot at its origin.

  • Violent impacts from planetesimals and other bodies were common.

  • Elements within Earth differentiated to form layered internal structure.

Early Earth: A Giant Impact!

  • During the middle to late stages of Earth's accretion, a Mars-sized body collided with Earth.

  • The giant impact quickly propelled a shower of debris from both the impacting body and Earth into space.

  • The impact sped up Earth's rotation and tilted Earth's orbital plane 2323.

  • Earth re-formed as a largely molten body.

*and the Moon aggregated from the debris.

Early Earth: Differentiation

  • During gravitational differentiation, iron sank to the center and lighter material floated upward to give us Earth as a layered planet.
    *Lighter Crust

  • Mantle
    *Liquid iron outer core
    *Solid iron inner core

Early Earth: Atmosphere and Oceans

  • Light elements that make up our atmosphere and oceans were aggregated to Earth from planetesimals.

  • Volcanic degassing fed the molecules to the early atmosphere and oceans.

Early History of Inner Planets

  • Earth had largely differentiated by 4.44.4 Billion years ago.

  • Other inner planets experienced similar differentiation during this time.

  • Significant differences are evident in the relative sizes of these planets’ cores.

*Earth’s moon preserves a history of early solar system events since its crust has not been constantly recycled through plate tectonic processes.

Early History of Inner Planets

  • The Moon's surface provides a record of impact-bombardment history.

Moon History Relevant to Inner Planets

  • Planetary accretion
    *Moon forms
    *Oldest Moon rocks
    *Core-mantle differentiation complete
    *Oceans form on Earth, liquid water on Mars
    *Late Heavy Bombardment
    *Oceans form on Earth, liquid water on Mars
    *Late Heavy Bombardment
    *Oldest Earth minerals
    *End of abundant liquid water on Mars; youngest sedimentary rocks?
    *Hadean, Archean, Proterozoic, Phanerozoic.
    Age of lunar highlands
    Age of basalts in lunar maris
    Age of Vallis Marineris on Mars
    Age of Mars's ancient cratered terrains.
    Age of youngest lavas on Olympus Mons
    olcano, Mars.
    Age of oldest
    surfaces on Venus

The Inner Planets: Mercury

  • Mercury’s surface is heavily cratered, implying a tectonically dormant planet with no recycling of crust.

  • Prominent scarps suggest horizontal compression resulting from early cooling.

The Inner Planets: Venus

  • Venus’ surface features suggest a geologically active planet with convecting mantle.
    *Maat Mons: A volcanic mountain

The Inner Planets: Mars

  • Mars’ surface consists of a combination of ancient terrain and younger lavas and sediments.

  • Topography is more extreme than either Earth or Venus.

Mars Rovers

  • As of 20222022, there have been six successful deployments of rovers on Mars.

Mars and Water

  • Sedimentary strata provide evidence of former standing water on Mars.

  • The former presence of water suggests the possibility of former life on Mars.

Internal Structure of the Rocky Planets

*Rocky Crust Low Density..
*Mantle Medium Density
*Metal Core
*Highest Density
Rigid Lithosphere
Crust and part of Mantle

Earth: Evolution of the Continents

  • Continental crust contains the oldest crustal material on Earth.

  • Continents are divided into tectonic provinces, which are large-scale regions formed by a particular set of tectonic processes.

*North America provides a well-known example of tectonic provinces.

North American Provinces

  • Stable Craton: Precambrian shield, Platform cover and basins

  • Folded and Faulted Belts: Paleozoic, Mesozoic-Cenozoic

*Passive Margins: coastal plain, continetal shelf

History of Earth

  • Stable craton formation in the precambrian

Three Stages of Craton Evolution

  • Initial Stage (>3.5 billion years ago)
    *Middle Stage (3.5-2.9 billion years ago)
    *Final Stage (2.8-2.6 billion years ago)

Canadian Shield

  • Oldest part of the North American crust

  • Precambrian granitic and metamorphic rocks

Interior Platform

  • Precambrian basal rocks (same as on Shield) are overlain by Paleozoic sedimentary rocks

  • Basin and dome structures
    *Basins accumulate relatively thick sedimentary packages.

History of Earth

  • Appalachian orogeny at ~400400 Ma
    *Cordillera developed in Mesozoic and Cenozoic Precambrian Cambrian Meso and Cenozoic.

Appalachian Fold Belt

*Record the history of the assembly of Pangaea.

North American Cordillera

*Records the history of the Farallon Plate beneath North American continent

Continental Growth

  • Magmatic addition: In subduction zones, differentiation of silica-rich rock produces new continental crustal material. – Vertical process

  • Accretion: Fragment of buoyant, continental crust becomes attached to the edge of a craton through tectonic processes. – Horizontal process

Biological Events in Earth’s History

  • The earliest evidence of life on Earth dates back approximately 3.83.8 billion years.

  • The oldest fossils discovered are around 3.53.5 billion years old.

    • These fossils are remnants of single-celled bacteria.

Origin of Life

  • Life is believed to have emerged through simpler elements organizing into complex molecules. These complex molecules are capable of self-replication and metabolism (consuming other molecules).

  • Amino acids, fundamental building blocks of life, have been found in meteorites.

Origin of Eukaryotes

  • Eukaryotic cells are thought to have evolved from prokaryotic cells roughly 2 billion years ago.

  • Eukaryotic cells are more complex and larger than their prokaryotic counterparts.

Origin of Multi-Cellular Life

  • The first signs of multi-cellular life appeared approximately 1 billion years ago.

  • The earliest multi-cellular organisms were algae that inhabited the oceans.

Cambrian Explosion

  • A significant diversification of multi-cellular life occurred at the beginning of the Paleozoic Era.

  • The Cambrian Explosion marks this sudden increase in the diversity and abundance of multi-cellular life.

  • This period also saw the appearance of skeletons and shells in organisms. This development allowed for better preservation in the fossil record, giving us a clearer picture of the life forms present at that time.

Colonization of Land

  • Plants began to colonize land about 475475 million years ago, transforming terrestrial environments and paving the way for other organisms.

  • Arthropods and tetrapods followed, colonizing land around 400400 million years ago. This transition marked a major step in the evolution of life, as organisms adapted to survive and thrive in terrestrial habitats. The interplay between plants and animals on land led to the development of complex ecosystems that continue to evolve to this day.

Major Extinction Events

  • Throughout Earth's history, there have been several major extinction events that dramatically reduced the planet's biodiversity.

    • The Permian-Triassic extinction event, for example, is the largest known extinction event in Earth's history, wiping out approximately 96% of marine species and 70% of terrestrial vertebrate species.

    • These extinction events have had a profound impact on the course of evolution, opening up ecological niches for new species to evolve and diversify.