Ig. and Met. Pet. Exam 1 Study Guide

Define the term petrography.  How does petrography differ from petrology?

Petrology is the study of rocks and the processes that produce them. Petrology is the study of rocks where you make detailed descriptions of them, using things like thin sections under microscopes. They differ because petrology is a broad study of rocks, while petrography is a detailed study and falls under the broad category of petrology.

Give one example of how classical controlled experimental work is used/useful in igneous petrology

Earth is thought to have a concentrically layered structure with a silicate crust and mantle and a metallic core.   Earth layers can be defined by differences in composition and also by differences in physical behavior.  The boundaries that separate the different portions of Earth’s interior are seismically defined, but our model for Earth’s interior structure are informed by many other data sources including gravity considerations, studies of Earth’s magnetic field, experimental petrologic data and our understanding of igneous and metamorphic petrogenesis.

What are the 7 most abundant elements in the Earth system.  About how much of Earth do these 7 elements comprise?

The 7 most abundant elements in Earth is:

• Iron (30%)

• oxygen (30 %)

• silicon (15%)

• magnesium (15%)

• 10 percent of 88 other naturally occurring elements (lectures).

Sulfur,

calcium,

aluminum are the other left.

These elements compose almost 100 percent of the Earth with only one percent being other elements.

Draw a cross-sectional view (cut-away view) of the Earths interior.  Label all major layers (both compositional layers and layers based on physical properties) within the Earth including depth ranges

crust (lithosphere)

crust (lithosphere): 0 to 100km silicates

Asthenosphere

Asthenosphere: 100km to 700 km

mantle

mantle: 100 to 2900 km iron and magnesium silicates

Outer core:

Outer core: 2900km to 5000km iron and nickel

Inner core

- Inner core: 5000km to 6300km and metals

How do oceanic and continental lithosphere differ from one another? (think structure/comp, thickness and density)

Continental crust is more thick than oceanic

oceanic is more dense hence subduction zone stuff.

Oceanic crust is basaltic (Si, O, Ca, Mg, Fe) and continental is more granitic (Si, O, Al, K, Na)

Draw a graph showing a qualitative profile of seismic velocity vs depth in the Earth.  Mark prominent seismic discontinuities (areas of rapidly changing seismic velocities, noting the depth of the discontinuity and its geologic significance.

What are the sources of heat that leads to increase in temperature with depth that is “observed” on Earth and its interior?

1.     Heat from the early accretion and differentiation of the Earth (secular cooling with time)

2.     Heat released by the radioactive breakdown of unstable nuclides (K, U, Th, PbL mostly in crust)

3.     Heat released due to crystallization of core

4. Increase in T due to compression

Dominant Heat Transfer Mechanisms:

Earth cools to space by radiative (electromagnetic) processes

Heat conducts outward from the core into the lower mantle

Heat then conducts through the lithosphere to the atmosphere and radiates to space

The mantle then convects transferring heat (and matter)

What is a typical geothermal gradient in the continental crust

In continental crust, the geothermal gradient is about 25-30/km

What is a typical geothermal gradient in the oceanic crust

oceanic crust the gradient is about 60/km

Is this geothermal gradient consistent through the entire thickness of Earth? Draw a general profile as above indicating change in temperature with depth in the mantle

The gradient is not consistent throughout the entire thickness. It has a general curve like \. This curve reflects more efficient adiabatic (constant heat content) convection of heat in the mobile asthenosphere and less efficient conductive heat transfer through the more rigid lithosphere.

How to calculate the pressure with increasing depth in Earths crust?  How does the mantle compare?

P=Pgh, p is density, g is gravity, and h is depth. Mantle experiences higher pressures.

Cite at least two sources of information that constrain the temperature of Earth at depth (i.e. the pressure-temperature regime).

Mineral composition, certain minerals crystallize at higher temps than others,

Are all places on Earth are magmatically active?

Note all places on Earth are magmatically active.  Magmatism only occurs in specific geologic environments where the normally solid mantle and/or crust are induced to melt via geologic processes.

What are the relation between mantle convection and plate tectonics?

Mantle convection causes plate tectonics. The density difference that causes rises and falls within the mantle cause the plates to shift around.

Convection currents drive the movement of Earth's rigid tectonic plates in the planet's fluid molten mantle. In places where convection currents rise up towards the crust's surface, tectonic plates move away from each other in a process known as seafloor spreading

What are the 7 principle environments where magmatism and volcanic activity occur on Earth?

1. Mid-Ocean Ridges
2. Intracontinental Rifts
3. Island Arcs
4. Active Continental Margins
5. Back-Arc Basins

6. Ocean Island/Hotspot
7. Miscellaneous Intra-Continental Activity: kimberlites, carbonatites, anorthosite

Define the terms aphanitic and phaneritic and relate each to textures observed in plutonic and/or volcanic rocks

Aphanitic means that crystals are too small to see with your eyes. Phaneritic is when you can see the constituent minerals.

Phaneritic

Phaneritic: Majority of the crystals in the rock can be seen with the naked eye.

Fine grained <1 mm diameter

Medium grained <1-5mm diameter

Course grained 5-50 mm diameter

Very course grained > 50 mm diameter

This texture typically means that the rock crystallized slowly beneath the surface of the Earth (plutonic/intrusive)

Aphanitic

Aphanitic: This texture typically means that the rock crystallized rapidly at the Earth’s surface (volcanic/extrusive)

Porphyritic:

Porphyritic:

Rocks can have two dominant grain sizes. The larger crystals are phenocrysts, and the finer crystals are called groundmass. In volcanic porphyritic rocks the groundmass is aphanitic.

Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning all types of igneous rocks can display some degree of porphyritic texture.

Glassy/Holohyaline

Glassy rocks are considered extrusive igneous rocks, meaning they form when magma cools rapidly at the Earth's surface, resulting in a glassy texture with minimal crystal formation

Rocks can be classified based on their crystallinity.

Rocks can be classified based on their crystallinity.
• Holocrystalline: a rock consisting entirely of crystals.
• Hypocrystalline: a rock consisting both of crystals and glass.
• Holohyaline: a rock consisting entirely of glass

What is modal mineralogy?  What is normative mineralogy?  Why isn’t modal mineralogy the official method to compositionally classify glassy/aphanitic rocks?

Modal Mineralogy:

volume percent of observable mineral phases, Point counted by area.

Normative Mineralogy:

Composition of a rock recast into “normal” or idealized (predicted) mineral phases w.t%

NOT REAL mineralogy-simplified

Limited mineral set

Assumes anhydrous mineralogy, end-member formulas

Assumes low pressure

 

Modal mineralogy isn’t used to classify glassy/aphanitic rocks because the grains are often too small to estimate properly, and the mineral elements needed for modal mineralogy are not present in the rocks because they never has the chance to develop any or many minerals

What is the significance of a rock having normative nepheline?  Quartz?

Rocks with “normative nepheline” indicates a relatively low silica content and an alkaline magma composition, while a rock with “normative quartz” suggest a higher silica content and a more typical granitic magma source

What is a porphyritic texture?  In which kinds of igneous rocks (plutonic or volcanic) are porphyritic textures most commonly observed?

Porphyritic- bimodal grain size distribution.

Rock can have two dominant grain sizes. The larger crystals are called phenocrysts, and the finer crystals are called groundmass. In volcanic porphyritic rocks the ground mass is aphanitic.

They indicate a two-stage cooling process where larger crystals formed first followed by the rapid cooling of the remaining magma, making it most commonly observed in volcanic igneous rocks as they often experience this rapid cooling at the earth’s surface after a period of slower crystallization deep within the crust.

What are the minerals used in the IUGS classification scheme for felsic and intermediate plutonic rocks?

QAPF: Quartz, alkali Feldspar, Plagioclase, and Feldspathoids (foids)

 

NOTE that quartz and feldspathoids cannot be in the same rock together and thus are the two extremes of the ternary rhombus shaped diagram.

Why is this IUGS classification scheme not used to classify different kinds of mafic and ultramafic rocks

They don’t have quarts as a primary minerals

What are the minerals use to classify mafic plutonic rocks (anhydrous)

Mafic: Plagioclase feldspar, Orthopyroxene, and Clinopyroxene are used to distinguish mafic rocks because they are some of the next forming after Olivine and thus are not ultramafic but definitely not intermediate or felsic

What are the minerals use to classify ultra mafic plutonic rocks

Olivine, clinopyroxene, and orthopyroxene are used to classify ultramafic rocks because these are the mineral present to distinguish ultramafic rocks (they form first on Bowen’s reaction series.)

What is the main chemical index used to subdivide volcanic rocks into the compositional categories felsic, intermediate, mafic and ultramafic. Be specific, citing the abundance range of that index for each category (in Wt%)

Felsic: From feldspar + silica; a rock that is >65% silica

Intermediate: between felsic and mafic; 55%-65% silica

Mafic: from magnesium + ferric iron; 45%-55% silica

Ultramafic: a rock that consists of over 90% mafic minerals

What chemical parameters are used to classify volcanic rocks as alkaline or subalkaline? Within subalkaline, what is the key difference between calc-alkaline and tholeiitic? Why is this parameter used to subcategorize rocks into these different groups (Hint: consider the next question)?

3 categories of magmas/igneous rocks based in alkali content (Na, K) relative to other chemistry.

Sub-alkaline: Lowest alkalis, tend to have quartz in their norm

Alkaline: Silica deficient compared to otherwise broadly similar subalkaline magmas (e.g mafic, felsic, ect.) and lack quartz in their norm.

Peralkaline: highest alkalis

-relatively rare

-alkalis so high you get sodic pyroxene and amphibole

·       What is a rock series (or lineage)? 

Chemistry of groups of rocks and relations

·       Which series tends to be highly silica undersaturated? Which one(s) tend to be silica undersaturated?

Ultramafic are highly silica undersaturated (olivine)

Mafic/intermediate are silica undersaturated to mildly silica undersaturated.

What is the primary classification criteria used to distinguish different kinds of pyroclastic rocks?

Based on type of material:

Vitric tuff/ vitric ash (glass)

Lithic tuff/ lithic ash (rock)

Crystal ash/ crystal tuff (crystal)

What is a tuff?  How does a lithic tuff differ from a vitric tuff?

A tuff is a volcanic rock formed from compacted volcanic ash. Rocks that contain greater than 75 percent of ash is considered tuff. 

A lithic tuff is composed primarily of rock fragments, vitric tuff is made up of mostly volcanic glass shards. Crystalline tuff is made of mostly crystal.

How is a lapilli-stone different from a tuff in terms of the pyroclastic classification scheme?

Lapilli stone is a pyroclastic rock with lapilli >75%,

lapilli tuff is a rock which bombs and blocks 25% and lapilli and ash 75%

Classification of cooling rates

Igneous textures are broadly categorized as either primary (occurring during crystallization due to crystal growth and interactions between minerals and melt) and secondary (occurring after complete solidification. aka below the solidus temperature).  Primary textures are strongly controlled by (and reflect) nucleation and growth rates, chemical equilibrium or disequilibrium, and cooling rate.

How does the rate of crystal nucleation change with differing degrees of undercooling?  How does growth rate relate to with different degree of undercooling?

Undercooling is the degree to which you suck heat out of magma (rate of heat loss). As you increase the rate of cooling, you create more individual mineral grain nucleation sites that limits the growth of each individual mineral meaning the mineral grains are smaller (aphanitic has many more individual mineral grains).

 

Cool too rapidly kill nucleation entirely and get a glass

 

Less nucleation sites increases the ability for each mineral to grow individually leading to large phaneritic rocks

What is a holohyaline rock?  What does this imply about the cooling rate?

Holohyaline: a rock consisting entirely of glass, fast cooling

• What is a holocrystalline rock? What does this imply about the cooling rate?

A rock made of entirely crystals, slower than other two cooling rates

• Name and describe (drawings may be used) at least 2 mineral textures indicative rapid cooling/quenching.

Swallowtail plag, skeletal structures

What is chemical zoning in a mineral? What process does chemical zoning in a mineral imply?

There are rings of different chemical compositions in mineral grain.

It implied the depletion of an element, where one zone is rich in an element, and as the grain is crystalized the surrounds are depleted of that element, so they begin to crystalize with another element, creating another zone layer.

Define the terms euhedral, subhedral, and anhedral and interstitial.  How/why can observations of crystal morphology be useful in considering crystallization sequence of a plutonic rock?

Euhedral: well-formed crystal

subhedral: sub-formed crystal

anhedral: amorphous shaped crystal.

Minerals that began to crystallize first are euhedral, as they have more time and space to form good crystal faces

What is an ophitic texture? In what composition of rock would one look to find an ophitic texture?

In an ophitic texture, Plagioclase and pyroxenes crystallize simultaneously- Cpx nucleated less, grew faster, and enclosed the growing plagioclase.

What is a trachytic texture and how/why does it form?

Trachytic texture is when the plagioclase (microlites) is aligned by flow. They are parallel forming flow lines along the directions of lava flow and around inclusions.

• What is a phenocryst?  How does it differ in origin from the groundmass (in terms of the conditions of formation/solidification

Phenocrysts are the larger crystals with in porphyritic rocks. phenocrysts indicate slow cooling before eruption. The ground mass cooled fast due to small crystals, and likely cooled after eruption

How is a microlite different from a crystallite?  Would these be phenocryst phases or groundmass phases

Microlites show birefringence, and crystallites do not and are smaller than microlites. These are micro phenocrysts, or ground mass because they are small

What are sieve textures indicative of? 

Sieves are crystals filled with channel ways due to dissolution

What are embayed textures?

Commonly quartz embayments are interpreted as the result of magmatic corrosion (resorption, dissolution) resulting from a change of conditions that cause a previously stable crystal to become unstable with respect to the liquid, and so to begin dissolving, for example, a change in pressure or a change in chemical.

Secondary textures

Secondary textures form after solidification of a magma and because they do not involve melt, are really metamorphic in nature (often referred to as autometamorphic).  Secondary textures can include Oswald ripening, polymorphic transformation, secondary twinning (from polymorphic change or deformation), exolution, and a wide range or reaction/replacement textures to interaction with chemically reactive fluids (commonly deuteric alterations).  Diagenetic and weathering processes are generally not included in the categorization of autometamorphic (generally too low of a temp to be considered such)

What is a deformation twin

A deformation twin has an atomic arrangement that is the mirror image of the parent crystal. Due to the mirror symmetry, the crystallographic structure is conserved, but the crystal has a different shape.

• What is a transformation twin?  Site one example of transformation twinning.

A transformation twin is a crystal that developed twinning feature when it undergoes a phase change fue to a change in temperature or pressure. Means that a pre-existing crystal transforms into a new crystal with a slightly different orientation creating a twin structure.

Describe in common terms the phenomena of exolution.  Site 2 common kinds of minerals that are known to show, in some instances, exsolution.

The process through which an initially homogeneous solid solution separates into at least two different crystalline minerals without this addition or removal of any materials. In most cases, it occurs upon colling below the temperature of mutual solubility or stability of the solution.

Pyroxenes will exsolve, olivine will as well

• What is seritization?  To what mineral does it generally occur?

A process of mineral alteration cause by hydrothermal fluids invading permeable country rock

Feldspars to white micas

• What is Saussuritization?  To what mineral does it generally occur?

Process by which calcium-bearing plagioclase feldspar is altered to a characteristic assemblage of minerals called saussurite

Common in chlorite, amphiboles, and carbonates

• What is a myrmekite?  In what kind of igneous rock does it occur?

Quartz intergrowth with feldspar

Igneous rocks such as granite.

Magma composition

Magma composition is an important control on viscosity and eruptive behavior.  The kind of volcanic structure that forms depends on the eruptive behavior which is in large part controlled by magma composition.  Magma composition is controlled by magmatic process which is, in turn, affected by tectonic/magmatic environment.   Different kinds of volcanic structures are classified based on their morphology and structure which relates to their manner of formation.

What are the primary variables that affect magma viscosity?  What is the basic relationship between each factor and viscosity?

Viscosity increases with:

Decreased Temperature

Increased SIO2- Greater polymerization

Exsolution of volatiles- gas bubbles

 

Viscosity decreases with:

Increased Temperature

Decreased SIO2- Less polymerization

Dissolved volatiles and alkalis- lower polymerization

Shield volcanos (Concave down):

Shield volcanos (Concave down):

• Broad, slightly domed-shapes

• Constructed by lateral flow of viscosity basaltic lava

• Have a slope and cover large geographic area

• Muana Loa on Hawaii is a perfect example.

Stratovolcano

Stratovolcano (composite volcanoes, slope concave up)

Large, cone-shaped volcanoes with steeper slopes

Made of alternating layers of lava, tephra, and debris

Often symmetric; can be odd shapes from landslides

Basaltic-andesite

Cinder cone

Cinder cone: Conical piles of tephra

The smallest type of volcano

Built of ejected lapilli-sized fragments piled up at a vent

Slope are at the angle of repose

Often symmetrical with a deep summit crater.

Volcano category sizess

• Shield volcanoes—largest

• Stratovolcanoes—intermediate in size

• Dome - small to moderate
• Cinder cones—smallest.

Crater

Crater: a bowl-shaped or steep-sided depression atop a volcano

Craters are up to 500 m across, 200 m deep

Form as erupted lava piles up around the vent

-summit eruptions- locate within the summit crater

-Flank eruption- located along the side of a volcano

Caldera

Caldera:

A caldera is a gigantic volcanic depression

-Much larger than a crater

-Usually steep sidewalls and flat floors

As the magma chamber empties, a major collapse of the magma chamber forms a caldera

Sometimes it forms a lava lake that directly connects to the magma chamber

Domes

Domes:

Domes form when largely degassed, viscous, silicic magma, such as dacite or rhyolite (less commonly andesite) moves slowly and relatively quietly to the surface. Domes range in size from less than 100 m to several kilometers in diameter.

Flank cones

Flank cones are parasitic cones that form on the sides of volcanoes, or flanks, during eruptions. They are made of loose fragments of volcanic ash, cinders, and other pyroclastic material. 

Explanation

• Flank: The side of a volcano 

• Parasitic cone: A cone that forms on the side of a volcano 

• Pyroclastic material: Fragments of volcanic ash, cinders, and other materials created by eruptions 

Basaltic Lava Flows

• Mafic lava—very hot, low silica, and low viscosity.
• Basalt flows are often thin and fluid.
• They can flow rapidly (up to 30 km/hr or even more).
• They can flow for long distances (up to several hundred km).
• Most flows measure less than 10 km.
• Long distance flow facilitated by lava tubes.

Lava tubes

Lava tubes:

conduits for basaltic lava.
• A cooled crust forms on top of a basalt flow.
• A conduit—a lava tube—develops in the flow.
• Tubes prevent cooling, facilitating flow for miles.
• Lava tubes become caves that can transmit water

Pahoehoe (pa-hoy-hoy)

Pahoehoe (pa-hoy-hoy)

• a Hawaiian word describing basalt with a glassy, ropy texture

• Lava advances by the propagation of individual toes or lobes.

• Pahoehoe develops when the rate of effusion is low

• Usually fed by a well- insulated underground tube system

• More spherical vesicles, less degassing

A’a’ (ah-ah)

A’a’ (ah-ah) is a Hawaiian word describing basalt that
solidifies with a jagged, sharp, angular texture

 

A’a lava advances as a thick,
single unit steadily.
• A’a’ forms when the rate of effusion
is fast- a lot of lava is discharged
once.
• Usually develops when lava is
being transported through open
channels on the ground surface.
• Irregular vesicles due to the
continued stretching motion of the
lava right up- degassing as it flows
• More crystalline

Andesitic Lava Flows:

Andesitic Lava Flows:
• Higher SiO2 content makes andesitic lavas viscous.
• Unlike basalt, they flow slowly and remain close to the vent
• The crust fractures into rubble, called blocky lava

Rhyolitic Lava Flows:

Rhyolitic Lava Flows:
• Rhyolite has the highest SiO2 and the most viscous lava.
• Rhyolitic lava rarely flows.
• Rather, lava plugs the vent as a lava dome.
• Sometimes, lava domes are blown off

·       What are the different size categories of pyroclastic materials and the names of rocks made from each of them?

 Basaltic eruptions.
• Released gases eject clots and drops
of molten magma.
• Sometimes, basaltic eruptions form
dramatic fountains.
• Pele’s hair—thin glass strands.
• Ash - < 2mm
• Lapilli—pea-sized fragments.
• Pele’s tears—frozen round droplets.
• Blocks, bombs—large fragments

Andesitic or rhyolitic eruptions.
• Explosive eruptions generate huge volumes of debris.
• Pumice lapilli—angular pumice fragments.
• Accretion lapilli—clumps formed by falling through moist air.
• Pumice—frothy volcanic glass （rhyolitic).
• Ash

Effusive eruptions

Effusive eruptions:

produce a vast outpouring of lava.

-Lava flows stream away from vents.
-Lava lakes can form near, or inside, the vent
-Can produce huge lava fountains 500 m high.
-Common with mafic magma (basalt)

Explosive eruptions

Explosive eruptions

• release pressure catastrophically.

• High gas pressure is from more viscous SiO2-
  rich magma.

• Create pyroclastic flows and cover the land
  with tephra

• Mostly andesitic and rhyolitic compositions.
  

pyroclastic flow

A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) a fast-moving current of hot gas and tephra

Tephra

Tephra is a term used to describe any rock fragment that
is ejected from a volcano during an eruption

Strombolian eruptions

Strombolian eruptions:

• regular “burps” of magma.

• Basaltic, basaltic andesites

• Every 10 to 20 minutes it shoots out lapilli and blocks.

• Trace arcs in the sky at night.

• Stromboli is an example of a basaltic pyroclastic eruption

Vulcanian eruptions

Vulcanian eruptions:

• moderately sized and explosive.

• Emit pyroclastic eruptions.

• Named for the island of Vulcano

Surtseyan eruptions

Surtseyan eruptions:

• vent erupts in shallow seawater/lake.

• Basaltic in composition

• Creates a large amount of steam. Ash is carried out of the water by the steam

Eruptive style is controlled by:

Eruptive style is controlled by:
magma viscosity (resistance to flow)
the amount of gas that has exsolved out of the magma
The ease at which that gas can escape

Magma viscosity and gas content are controlled by:

magma composition.
Magma composition is controlled by:

igneous processes, and ultimately can be related to P-T
environment.

   What is an ignimbrite?

·       What is an ignimbrite?

·       Ignimbrite—tuff deposited while hot that welds together.

     What are larhars?

Lahar—water-rich debris flow of ash and blocks; materials move
after deposit.
• Can move very fast (~50 km per hour) and very far (~tens of km per
second).
• Extremely destructive, sometimes deadly.
• A very real hazard for people living near active volcanoes.

Define the following: Maar, tuff ring, tuff cone?

·       Maar:

A wide, flat crater, usually filled with water, formed by a phreatomagmatic eruption where magma meets groundwater below the surface. 

·       Tuff ring:

A low-lying, ring-shaped volcanic feature with a wide crater, created by explosive eruptions where magma interacts with surface water, forming a ring of ejected material around the crater. 

·       Tuff cone:

A smaller, steeper volcanic cone with a more defined crater, similar to a cinder cone, resulting from the accumulation of volcanic debris from an eruption involving magma and water. 

·       Pyroclastic deposits. Fall, flow, surge deposits,  welded tuff, ignimbrite,

Fall deposits:

·        Formed when volcanic ash and debris settle out of the air after an eruption, creating layers of varying thickness depending on the distance from the vent. 

·        Can include fine ash, lapilli (small pebbles), and larger volcanic bombs. 

Flow deposits:

·        Consist of a rapidly moving, dense mixture of hot ash, gas, and volcanic debris that travels down slopes. 

·        When solidified, these deposits are often called "ignimbrites". 

·        Can be very destructive due to their high temperature and velocity. 

Surge deposits:

·        Low-density, fast-moving clouds of ash and gas that can travel long distances, often depositing thin, widespread layers. 

·        Typically have a more sorted texture than flow deposits. 

Welded tuff:

·        A type of pyroclastic flow deposit where the ash particles were hot enough to partially melt and fuse together, creating a dense, welded rock. 

Ignimbrite:

·        The geological term for a large, widespread pyroclastic flow deposit, usually characterized by a massive, poorly sorted texture and often containing welded tuff layers. 

Key points to remember:

·        The type of pyroclastic deposit depends on the eruption style and the characteristics of the volcanic material. 

·        Pyroclastic deposits can be studied to understand past volcanic eruptions and their potential hazards. 

What are flood basalts/LIPS .  What kind of volcanic landform do they form (how different from central vent constructs).

Key points about flood basalts/LIPs:

·        Eruption style:

Eruptions occur through large fissures, releasing vast amounts of low-viscosity basaltic lava that spreads out over wide areas, creating flat plateaus. 

·        Landform:

Unlike a typical volcano with a central cone, flood basalts create a relatively flat, layered plateau with multiple lava flows stacked on top of each other. 

·        Examples:

The Columbia River Basalt Province in the Pacific Northwest, the Deccan Traps in India, and the Siberian Traps are well-known examples of flood basalt provinces. 

How they differ from central vent constructs:

·        Eruption source:

Central vent volcanoes erupt from a single vent, whereas flood basalts erupt from long fissures, causing lava to spread out over a much larger area.

·        Volcanic shape:

Central vent volcanoes tend to build up a cone-shaped structure due to the

 

·       What are columnar jointing and how do they form?

Columnar jointing:

When thick basalt lava flows cool, they tend to contract with vertical fractures and are hexagonal in cross-section.

The lava contracts as it cools, forming cracks. Once the crack develops it continues to grow. The growth is perpendicular to the surface of the flow.

·       What is the difference between a concordant and discordant pluton?

• What term is a generic term for a non-tabular intrusive rock body?

Plonic rocks classified by:
Tabular - injected along planes of weakness (fractures)
sheet-like dikes (discordant) and sills (concordant)

Discordant: Cross cutting the country rock body
Concordant: parallel to the country rock structure

 

 

·       What term is a generic term for a non-tabular intrusive rock body?

Pluton

·       What is a laccolith and how is it different from a lopolith?

Laccolith

• Sufficiently viscous (silicic) to limit magma flow along the horizon plane

• Shallow enough to lift the roof rock

• Much smaller than lopolith

 

Lopoliths

• Down warped (saucer-shaped) concordant mafic intrusion.

• Almost all Precambrian

• Much larger than laccolith

 

What is stoping

Stoping: Magma’s Way of Making Space

Stoping is a geological process in which magma intrudes into the crust by breaking off and engulfing fragments of the surrounding rock (host rock or country rock). These fragments, called xenoliths, may either sink into the magma, dissolve, or become incorporated into the final igneous body.

How Stoping Works

1.     Magma Rises – Buoyant magma moves upward through fractures and weaknesses in the crust.

2.     Rock Fractures & Breaks – The heat and pressure from the magma cause the surrounding rock to crack and detach.

3.     Xenolith Incorporation – These broken-off pieces fall into the magma chamber, where they may partially or fully melt.

4.     Magma Chamber Growth – This continuous breaking and assimilation allow the magma body to expand.

Geologic Importance of Stoping

·        Helps plutons intrude at shallow crustal levels without needing large-scale deformation.

·        Produces xenolith-rich igneous rocks, which provide clues about the composition of the crust.

·        Can contribute to metasomatism (chemical alteration of surrounding rock).

·       What is the difference between a dike and sill?

·       Dike vs. Sill: Key Differences

·       Both dikes and sills are intrusive igneous bodies, but they differ in their orientation relative to the host rock’s layering.

Feature	Dike	Sill
Orientation	Cuts across rock layers (discordant)	Forms parallel to rock layers (concordant)
Intrusion Type	Vertical or steeply inclined	Horizontal or gently inclined
Formation Process	Forms when magma forces its way through fractures and weaknesses in the rock	Forms when magma squeezes between pre-existing bedding planes
Thickness	Can vary but often narrow and elongated	Usually more uniform in thickness
Associated Features	Often related to dike swarms and feeder systems for volcanic eruptions	Can form extensive layered intrusions
Cooling Rate	May cool quickly, often showing chilled margins	Slower cooling, leading to more uniform grain size
Example Locations	Palisades Sill (USA), Great Dike (Zimbabwe)	Salisbury Crags (Scotland), Whin Sill (England)

·       What is a volcanic neck/plug?   Would this be considered epizonal or catazonal?

Volcanic Neck (Plug) Definition

A volcanic neck (or plug) is a solidified remnant of magma that once occupied the conduit of an extinct volcano. Over time, surrounding volcanic and sedimentary rocks erode away, leaving behind this resistant, steep-sided feature.

Formation Process

1.     Magma Hardens – During volcanic activity, magma ascends through the conduit but cools and solidifies before reaching the surface.

2.     Erosion – Softer surrounding materials (lava flows, ash deposits) erode over time, exposing the denser, erosion-resistant plug.

3.     Landform Development – The neck remains as a prominent structure, often appearing as an isolated peak or tower.

Epizonal or Catazonal?

Volcanic necks are considered epizonal because they form at shallow crustal levels (<10 km) and are associated with surface volcanic activity. Their textures are typically fine- to medium-grained (like basalt or andesite), and they often exhibit columnar jointing due to rapid cooling.

In contrast, catazonal plutons form at great depths (>20 km) and experience slow cooling, leading to coarse-grained, deformed textures (e.g., gneissic granites and migmatites). Since volcanic necks are remnants of shallow magma systems, they fit clearly within the epizonal category.

Ring Dikes

Ring Dikes

·        Definition: Ring dikes are circular or arcuate, near-vertical intrusive bodies that form when magma intrudes along concentric fractures caused by the collapse of a magma chamber.

·        Formation Mechanism:

o   A magma chamber undergoes roof collapse, often due to the withdrawal of magma.

o   This creates concentric tension fractures, allowing magma to rise and intrude in a ring shape.

·        Appearance:

o   Steeply dipping to vertical dikes forming circular patterns around the collapsed magma chamber.

·        Associated Settings:

o   Commonly found in calderas and large volcanic centers.

·        Example: The ring dikes of the Isle of Arran, Scotland and the Tuolumne Intrusive Suite in Yosemite.