Geo MID: Geologic Structures, Igneous Rocks & Volcanism

Intact Rock

• Solid, unbroken samples

• High strength

• Tested in labs

Rock Mass

The actual ground contains fractures, faults, and weak planes

Engineering Rule

the weakest plane governs the stability of the entire project

Stress

force applied to the rock (Compression, Tension, Shear)

Strain

the resulting deformation (change in shape/volume)

Brittle Deformation

• rocks break (faults, joints)

• occurs shallowly

Ductile Deformation

• Rocks bend/flow (folds)

• occurs deeply

Bedding Planes or Stratification

• distinct layers formed during sedimentary deposition

• create continuous, massive, planes of weakness

Bedding Planes or Stratification

Engineering Risk: Slopes cut parallel to dipping bedding planes will inevitably fall (landslides)

Unconformities

buried erosional surfaces representing missing geologic time

Unconformities

Engineering Risk: Drilling or tunneling can suddenly pass from hard, competent rock into highly weathered, soft material with zero warning

Angular Unconformity

Younger, horizontal sedimentary layers rest upon older, tilted, or folded sedimentary layers

Disconformity

An erosion surface separates two sets of parallel sedimentary rock layers, often representing a significant time gap

Nonconformity

Younger sedimentary rock layers are deposited on top of older, eroded igneous or metamorphic rock bodies

Paraconformity

a type of unconformity where the bedding planes above and below are parallel, and there is no obvious erosion surface, making it hard to identify without fossil evidence

Joints and Fracture Networks

• fractures where rock has not moved

• usually occur in repeating, parallel “joint sets”

• form due to tensile stress (which can be caused by tectonic forces, unloading cooling, and groundwater pressure.

Joints and Fracture Networks

Engineering Risk:

• Intersecting joint sets create loose rock blocks that fall into tunnels or rockfall on highways.

• Dictates blasting fragmentation

Faults

fractures where rock masses have moved relative to one another

Normal Fault

• tension (pulling apart)

• hanging wall drops

Reverse/Thrust Fault

• compression (pushing together)

• hanging wall pushed up

Strike-Slip Fault

• shear (sliding past)

• horizontal movement

Grabens

blocks that move down relative to the other blocks

Horsts

elevated blocks with graben on either side

down
normal
compressional

1.) The hanging wall went _____ relative to the footwall

2.) That makes this as a _______ fault, caused by _________ stress

Fault Gouge

fault movement grinds surrounding rock into powder called?

Fault Gouge

turns solid rock into a weak, clay-like, highly permeable zone

Fault Gouge

Engineering Risk: Floods tunnels, traps boring machines, ruins dam foundation integrity

Folds

warping of rock layers due to slow compression

Folds

Engineering Risk: The “hinge” (sharpest bend) is highly fractured. Synclines act as underground bowls that trap pressurized groundwater

Antiform

Arch shape “A”

Synform

Trough shape “U”

Foliation and Cleavage

alignment of minerals into thin sheets due to extreme pressure (e.g. slate, schist)

Foliation and Cleavage

Engineering Risk: Extremely strong when loaded perpendicular to the sheets; extremely weak when loaded parallel to them

Anisotropy

Rock strength becomes strictly directional

Dams

bedding strike and dip determine if water will bypass the dam through the rock walls

Tunnels

joint sets and faults dictate the exact placement of rock bolts, shotcrete, and steel

Slope Stability

the angle of structural weaknesses determines if a slope will suffer planar, wedge, or toppling failures

Rock Cycle

demonstrates that any rock type, under the right circumstances, can be transformed into any other type

Igneous Rocks

form as magma cools and crystallizes

Magma

molten rock generated by partial melting of rocks in Earth’s mantle and in the lower crust in smaller amounts

Magma

consists mainly of the elements found in silicate materials

Si and O
Al Fe Ca Na K Mg

In magma, the main constituents are ??? and ??? with lesser amounts of ???

Lava

• molten rock that reaches the surface

• emitted as fountains produced when escaping gases propel molten rock skyward

• most eruptions are not violent; rather volcanoes more often emit quiet outpourings of ???

Igneous Rocks

this form when molten rock solidifies at the surface are classified extrusive or volcanic

Extrusive or volcanic

igneous rocks formed at the surface

Intrusive or plutonic

igneous rocks formed at the depth

Crystallization Process

• cooling slows atomic movement → atoms arrange into orderly patterns

• crystals form and grow until edges meet, creating an interlocking solid mass

Effect of cooling rate on crystal size

• slow cooling (deep underground): large crystals (visible to the naked eye)

• rapid cooling (surface/near surface): small crystals (microscopic)

• instant quenching (eruptions): no crystals, forms volcanic glass/ash

Other influencing factors

• Magma composition

• Amount of dissolved gases

Igneous Rock Classification

• based on texture (crystal size/glass)

• mineral composition

Texture of Igneous Rocks

• overall appearance of igneous rock based on size and arrangement of crystals

• reveals rock’s cooling history and origin

Fine-grained

rapid cooling → small crystals

Coarse-grained

slow cooling → large crystals

Glassy texture

extremely rapid cooling

Fine-grained

small crystals, often with gas bubbles (vesicular texture)

Coarse-grained

Large, visible intergrown crystals (e.g. granite)

Porphyritic

large crystals (phenocrysts) embedded in a smaller crystal matrix

Glassy

atoms frozen in place, no crystals (e.g. Obsidian)

Vesicular

gas bubbles from holes/voids in lava rocks

Pumice

frothy, glassy rock with many air-filled voids can float in water

Igneous compositions

• composed mainly of silicate minerals

• Si and O most abundant → expressed as Silica (SiO2) content

• Other major elements: Al, Ca, Na, K. Mg, Fe

• Minor elements: Ti, Mn, trace amounts of Au, Ag, U

Dark Silicates

• also called ferromagnesian

• rich in Fe and Mg, low in silica

  • EX: Olivine, Pyroxene, Amphibole, Biotite Mica

Light Silicates

• rich in K, Na, Ca, high in Silica

  • EX: Quarts, Muscovite, Feldspars

• Feldspars are the most abundant mineral group (>= 40% of igneous rocks)

texture
mineral composition

The classification of igneous rocks are based on:

1. cooling history

2. parent magma chemistry

Granitic Rocks
Andesitic Rocks
Basaltic Rocks
Ultramafic Rocks

groups are divided by light vs dark mineral proportions

Granitic (Felsic) Rocks

• composed mainly of quartz + potassium feldspar

• about 70% silica, 10% dark minerals (biotite, amphibole)

• Major rock of continental crust

• Granite: coarse-grained, intrusive, widely exposed (e.g. Yosemite, Mount Rushmore)

• Rhyolite: fine-grained, extrusive, less common, found in Yellowstone

Andesitic (Intermediate) Rocks

• composition between felsic and mafic

• Mix of light and dark minerals (amphibole, plagioclase)

• Andesite: fine-grained, extrusive, common in continental margins

• Diorite: coarse-grained, intrusive equivalent

Basaltic (Mafic) Rocks

• rich in dark silicates and calcium plagioclase, NO QUARTZ

• darker, denser than felsic rocks

• Basalt: fine-grained, extrusive, forms oceanic crust and volcanic islands (e.g., Hawaii, Iceland)

• Gabbro: coarse-grained, intrusive, forms much of oceanic crust

Ultramafic Rocks

• composed almost entirely of olivine + pyroxene

• very low silica, very dark minerals

• Peridotite: Intrusive, main rock of Earth’s upper mantle

Lava

A single volcano can produce ??? of different compositions

Magma

It suggests that this “evolves” into different types of igneous rocks

Norman L. Bowen
20th century

Bowen’s reaction series was revolutionized by this person in this year

200 degrees Celsius

In Bowen’s reaction series, magma crystallizes over a range of ???, not at one fixed temperature

Specific order

In Bowen’s reaction series, Minerals crystallize in ??? as magma cools

Magmatic Differentiation

• different minerals crystallize from magma in a systematic order (Bowen’s series")

• as crystals form, they remove specific elements, changing magma composition

Crystal Settling

• early-formed minerals are denser and sink to the bottom of the magma chamber

• remaining liquid becomes chemically different from the parent magma

• when solidified, produces rocks of different composition

Formation of diverse magmas

• Magmatic differentiation = formation of new magmas from one parent magma

• Solid and liquid components can separate at various stage

• Results in chemically diverse magmas and a wide variety of igneous rocks

Assimilation

rising magma incorporates pieces of surrounding host rock, altering its composition

Magma Mixing

two magmas of different composition intrude and mix, creating a new composition

Convective Flow

this can stir magmas together

Mount St. Helens

this volcano is explosive, destructive, ash-rich eruption

Kilauea

this volcani is quiet, lava flows, less violent but still damaging

Magma composition
Temperature
Dissolved gases

Eruption style (violent or gentle) depends on ???

Magma viscosity

these factors control the resistance of flow

Higher temperature

lower viscosity (flows easily)

Cooling magma

less mobile, flow halts

Higher viscosity

More silica means ???

Felsic (Rhyolitic)

very viscous, short thick flows

Mafic (Basaltic)

low viscosity, fluid flows, can travel > 150km

Dissolved Gases

• water reduces viscosity (breaks silicate bonds)

• gas loss increases viscosity

Formation of Magma

• most magma forms by partial melting in the upper

  • usually basaltic in composition

• Because magma is buoyant, it rises, and may collect in a magma chamber

• As it cools:

  • high-melting-point minerals crystallize first

  • remaining melt becomes enriched in silica

• only a fraction of magma ever reaches the surface

Triggering Hawaiian-Type (Quiet) Eruptions

• Basaltic Magma = hot, fluid, low silica

• Triggered when new melt enters a magma chamber, inflating it and fracturing overlying rocks

• Magma rises easily → produces long-lasting lava flows and sometimes lava and fountains

Triggering Explosive Eruptions

• silica-rich magma = viscous, sticky

• contains abundant dissolved gases held under pressure

• As magma rises:

  • Pressure drops → gases form bubbles (like opening soda)

  • Viscosity prevents easy escape of gases

  • Gas buildup causes fractures and violent explosions

• Products

  • Eruption columns (ash + pumice blown into atmosphere)

  • Pyroclastic flows (hot gas + fragments racing down slopers)

Key Control Factors

• viscosity of magma (low in basalt, high in silica-rich magmas)

• amount of dissolved gases

• ease of gas escape

Gentle Eruption

Basaltic, hot, fluid magma

EX: Hawaii

Explosive eruptions

Silica-rich, cooler, viscious magma
EX: St. Helens, Pinatubo

Lava
Gases
Pyroclastic Materials

volcanoes eject three main products

Pyroclastic Materials

examples of these are broken rock, lava bombs, fine ash, and dust

Form and behavior

their ??? depend on composition, temperature, and gas content

Basaltic Lava`

> 90% of all lava (fluid, fast-moving, broad sheets)

• this can travel 30 km/hr on steep slopes; usually slower (10 - 300 m/hr)