Volcanic Eruptions: Magma, Lava, and Flows

Volcanoes

• All eruptions involve magma
• Magma: molten rock with crystals and dissolved gases.
• Lava: erupted magma.

The Nature of Volcanic Eruptions

• Viscosity: a liquid's resistance to flow; increases as temperature decreases.
• Magma viscosity is controlled by composition, temperature, and dissolved gases.
• Quiescent eruptions involve fluid basaltic lavas with lava outpourings lasting weeks to years.
• Explosive eruptions: involve highly viscous magmas that expel fragmented lava and gases at supersonic speeds, forming eruption columns.

Lava Flows

• Lava flows: molten rock that flowed onto Earth's surface.
• 90% of lava: basaltic.
• Less than 10% of lava: andesitic.
• About 1% of lava: rhyolitic.

Materials Extruded During an Eruption

• Volatiles (dissolved gases): 1–6% of magma's weight.
• Gases expand and escape as magma reaches the surface and pressure decreases.
• Gas composition: 70% H2O, 15% CO2, 5% N, 5% SO2, and 5% others.
• Pyroclastic materials: pulverized rock and lava fragments ejected by volcanoes.
• Pyroclastic particle sizes range from fine dust to very large rocks.

Pyroclastic Materials

• Tephra: pyroclastic materials.
• Volcanic ash: fine glassy fragments.
• Lapilli: walnut-sized material.
• Cinders: pea-sized material.
• Blocks: hardened or cooled lava >2.5 inches diameter.
• Bombs: ejected as hot lava >2.5 inches diameter.

Types of Volcanoes

• Three main types of volcanoes: shield volcanoes, composite cones, and cinder cones.
• Shield volcanoes: largest, can be long-lived.
• Composite cones: can be long-lived.
• Cinder cones: smallest, usually short-lived (single eruption).

Shield Volcanoes

• Shield volcanoes have fluid basaltic lava with low viscosity and low volatiles.
• Shield volcanoes are broad and slightly domed.
• Example Mauna Kea; base 6000 m below sea level; height 4200 m.
• Example Mauna Loa is only 35m or 120 ft lower than Mauna Kea.
• Shield volcanoes have thin, basaltic lava flows and little pyroclastic material.
• Shield volcanoes commonly develop steep summit calderas, which collapse when the roof above a magma chamber caves in.
• Shield volcanoes may also develop lava tubes that lead to eruptions along the volcano's flank.

Cinder Cone

• Cinder Cones are built from ejected loose material.
• Lava flows may originate near the base of cinder cones.
• Cinder Cones are often the result of a single, short-lived eruption

Composite Cones

• Composite cones have high viscosity and high volatiles (mostly andesite or rhyolite).
• Composite cones have interbedded lava flows and pyroclastics.
• Composite cones mostly exist in subduction zones (convergent plate boundaries).

Calderas

• Calderas are circular, steep-sided depressions with a diameter >1 km.
• Crater Lake-type calderas: form from the collapse of a composite volcano summit after an eruption and eventually fill with rainwater.
• Hawaiian-type calderas: form gradually from the collapse of a shield volcano summit due to subterranean drainage of the central magma chamber.
• Yellowstone-type calderas: form from the collapse of a large area after the discharge of large volumes of silica-rich pumice and ash; they often have a complex history.

Flood Basalts

• A rising mantle plume is thought to generate Earth's large basalt provinces.
• Rapid decompression melting of the plume head causes extensive flood basalt outflows over a relatively short time.
• Plate movement causes volcanic activity from the plume's tail, generating a linear chain of smaller volcanic structures.

Intrusive Igneous Structures

• A volcanic neck is the remains of magma that solidified in a volcanic conduit, Shiprock, New Mexico, is an example
• A pipe is a rare type of conduit that originated in the mantle at depths exceeding 150 km, Kimberlite pipes, for example

Pacific "Ring of Fire"

• Most volcanoes are along plate boundaries (approximately 90%):.
• The majority of volcanoes are along ocean ridges (divergent boundary, approximately 80%).
• Some volcanoes are along subduction zones (convergent boundary, approximately 10%).
• About 10% of volcanoes are NOT along plate boundary: occurring at hotspots (e.g., Hawaii, Yellowstone).

Rock Types & Formation

• Metamorphic Versus Sedimentary and Igneous Environments; Sedimentary transitions to Metamorphic upon increasing temperature and pressure, before becoming partial, then complete melting into the Igneous state.

Metamorphism vs. Metamorphism

• Metamorphism: Rock remains solid throughout the entire process; it produces distinct rock textures and mineralogy; changes are gradual.

The 'drivers'

• Heat is the most important driver; unstable minerals when buried; New minerals grow; Grain size increases
• Confining Pressure increases with depth and is equal in all directions
• Differential stress is Directed pressure
• Chemically active fluids are Mainly water; that allow for Easier migration of ions

Metamorphic Minerals

• Certain mineral content are sensitive indicators of metamorphism
• They grow a variety of distinctive minerals change with changing metamorphic conditions (e.g. of pressure and temperature).

Metamorphic Rocks

• Metamorphic grade A is low grade metamorphism illustrated by the transformation of sedimentary rock shale to the more compact metamorphic rock slate.
• Metamorphic grade B is high grade metamorphism that obliterate the existing texture and often change the mineralogy of the parent rock.
• High-grade metamorphism occurs at temperatures that approach those at which rocks melt.

Metamorphic Rocks

• Parent rock is important because it controls what is actually produced during metamorphism.
• Starts out to closely resemble original rock, but strongly changes as metamorphism increases

Classification

• Examples of Metamorphic Rock include; Slate; Phyllite; Schist and; Gneiss

Metamorphic settings

• Temperatures and pressures are typically associated with the major types of metamorphic environments.

Contact metamorphism

• Contact metamorphism occurs Near magma intrusions
• Host rock is baked
• Ore deposits are abundant
• Shale becomes hornfels and; Quartz sandstone becomes quartzite and; Limestone becomes marble and;

Regional metamorphism

• Regional metamorphism Occurs during mountain building and;
• Produces the greatest volume of metamorphic rock
• (Sedimentary) Shale transitions to low grade, Slate then high grade, Phyllite and Schist before Partial melting (Migmatite)

Metamorphic settings

• Hydrothermal metamorphism occurs along a mid ocean ridge

Magma Generation

• Magma consists of three components, Melt which is the Liquid portion; Solids such as Silicate minerals that have already crystallized from the melt and; Volatiles which are Gases dissolved in the melt.

Magma Generation

• Melting rocks occurs in three ways; By Exceeding the melting temperature of material or; By Decompression or; By Adding water.

Igneous rocks

• Igneous rocks Form as magma cools and crystallizes
• Plutonic (intrusive) rocks form inside Earth
• Volcanic (extrusive) roacks form on the surface.
• All magmas are silicate melts composed of silica and oxygen coming from below the crust
• Because its Density is less than surrounding rock the magma will rise

Magma Evolution

• A magma evolves as the earliest-formed minerals (those richer in iron, magnesium, and calcium) crystallize and settle to the bottom of the magma chamber
• This leaves the remaining melt richer in sodium, potassium, and silica (SiO2)

Magma Evolution

• Assimilation is when Magma rises through Earth's brittle upper crust, dislodging and incorporating the surrounding host rocks, changing the rising magma body
• Magma Mixing occurs when two chemically distinct magma bodies ascend, convective flow will mix the two magmas, generating a mass that is a blend of the two magma bodies.

Weathering

• Weathering involves the physical breakdown and chemical alteration of rock at or near Earth's surface.
• Mechanical weathering: physical forces breaking rocks into smaller pieces.
• Chemical weathering: chemical transformation of rock into new compounds.
• Both work simultaneously and reinforce each other.

Mechanical Weathering

• Frost wedging has involves two methods.
• Water works its way into cracks in rocks, and freezing enlarges the cracks.
• Lenses of ice in soil grow larger as they attract liquid water from surrounding areas.

Mechanical Weathering

• Salt Crystal Growth is where Sea spray or salty groundwater penetrates crevices and pore spaces in rocks
• As the water evaporates, salt crystals form and enlarge the crevices
• Sheeting/Unloading is where Large masses of igneous rock are exposed by erosion and concentric slabs break loose due to release of confining pressure
• An exfoliation dome is formed after continued weathering causes slabs to separate and spall off

Mechanical Weathering

• Biological ACTIVITY is where Plant roots grow into fractures in a rock, causing the cracks to expand (root wedging)
• Burrowing animals break down rocks by moving fresh material to the surface, enhancing physical and chemical weathering
• Human impacts (rock blasting) is very noticeable- can produce effects much like unloading

Chemical Weathering

• The Most Important Agent Is Water
• Responsible for transport of ions and molecules involved in chemical processes

Chemical Weathering

• Dissolution occurs when Certain minerals dissolve in water
• Halite is one of the most water-soluble minerals
• A small amount of acid in water increases the corrosive force of water, causing dissolution

Chemical Weathering

• Carbonic acid is created when carbon dioxide dissolves in raindrops
• Calcite is easily attacked by weakly acidic solutions
• This process is responsible for the formation of limestone caverns
• Oxidation occurs when Essentially the rusting of iron-rich minerals

Chemical Weathering

• When Oxygen combines with iron to form iron oxide
• Process is slow in dry environments
• Water increases the speed of the reaction
• Important in decomposing ferromagnesium minerals like olivine, pyroxene, hornblende, and biotite
• Oxidation can only occur after iron has been freed from the silicate structure by basis

Chemical Weathering

• Hydrolysis occurs when The reaction of any substance with water
• A hydrogen ion attacks and replaces another ion
• Silicates primarily decompose by hydrolysis
• Clay minerals are the most abundant product of weathering
• Clay minerals are very stable under surface conditions
• Acid greatly accelerates hydrolysis

Chemical Weathering

• Spheroidal Weathering attacks edges from two sides and corners from three sides
• Sharp edges gradually wear down and become rounded Granite

Rates of Weathering

• The rate of weathering is influenced by Rock Characteristics, dependent of mineralogy.
• Silicate minerals weather in the same order as crystallization (Bowen's reaction series)
• Carbonates and halides weather more quickly than silicates
• Climate is influencial
• Temperature and precipitation are crucial
• Frequency of freeze-thaw
• Moisture available for dissolution
• Conditions favoring vegetation growth

Rates of Weathering

• Variations in local climate and the composition of the rock formation will produce uneven weathering of the rock called differential weathering

Soil and soil profile

• Soil is "the bridge between life and the inanimate world"
• The bridge between the various Earth systems Earth's land surface is covered by a layer of rock and mineral fragments produced by weathering, called regolith Soil is a combination of mineral and organic matter, water, and air and is the portion of the regolith that supports the growth of plants

Soil Texture

• Soil Texture and Structure
• Most soils are far from uniform
• Soil texture refers to the proportions of different particle sizes
• This property strongly influences the soil's ability to transmit and retain water and air
• Four basic soil structures are recognized
• Platy, prismatic, blocky, and spheroidal
• Influences how easily the soil can be cultivated, how susceptible it is to erosion, porosity and permeability

Controls of Soil Formation

• The controls of soil formation are; Parent material or; Climate or; Plants and animals or; Time or; Topography

The Importance of Sedimentary Rocks

• Sediments and sedimentary rocks cover approximately 75% of land and virtually ALL of the ocean basins
• However, those only comprise about 5 percent (by volume) of Earth's outer 10 miles
• Those contain evidence of past environments
• Those contain important economic resources
• Coal, oil, and other fossil fuels
• Uranium, iron, aluminum, manganese, phosphate
• Groundwater resources

Origins of Sedimentary Rock

• Sedimentary rocks are products of mechanical and chemical weathering
• Sediments and soluble constituents are typically transported downslope by gravity (called mass wasting)
• The sediments are then deposited and subsequently buried
• As deposition continues, the sediments are lithified into sedimentary rocks
• There are three types of sedimentary rocks:
• Detrital, chemical, and organic sedimentary rocks

Detrital Sedimentary Rocks

• Detrital sedimentary rocks form from sediments that have been weathered and transported
• Chief constituents of detrital rocks include clay minerals, quartz, feldspars, and micas
• Particle size is used to distinguish among the various rock types
• It also presents important information about the environment of deposition

Particle Size Categories

• Example sizes are Boulder, Cobble, Pebble, Granule or; Silt and Clay

Detrital Sedimentary Rocks

• Shale is an example.
• Silt- and clay-sized (fine-grained) particles.
• Forms from gradual settling of sediments in quiet, non-turbulent environments.
• It has fissility meaning the rock can be split into thin layers.
• Shale Crumbles easily and tends to form gentle slopes.
• Most abundant sedimentary rock.
• Sediments form in thin layers that are called laminae

Detrital Sedimentary Rocks

• Sandstone forms Sand-sized particles in a variety of environments
• Sandstone is the Second most abundant sedimentary rock
• Quartz is the most abundant mineral
• It includes Quartz sandstone which is predominately composed of quartz or; Arkose sandstone which contains appreciable quantities of feldspar and; Graywacke which contains rock fragments and matrix, in addition to quartz and sandstone

Detrital Sedimentary Rocks

• Particles are classified by sorting and shape
• Sorting is the degree of similarity in particle size
• If all the grains in a rock are of similar size, the rock is well sorted
• If the grains in a rock are different sizes (both large and small grains), the rock is poorly sorted
• Sorting can help decipher the depositional environment of the rock

Detrital Sedimentary Rocks

• Particle shape varies from rounded to angular
• The degree of rounding is indicative of how far the sediments have been transported
• Rounded sediments are typically transported to great distances
• Angular sediments are only transported a short distance

Detrital Sedimentary Rocks

• Conglomerate and Breccia
• Conglomerate consists of rounded, gravel-sized (or larger) sediments
• Breccia consists of angular, gravel-sized (or larger) sediments
• Both types of rocks are usually poorly sorted

Chemical Sedimentary Rocks

• Chemical sedimentary rocks form from precipitated material that was once in solution
• Precipitation of material occurs by:
• Inorganic processes: evaporation or chemical activity or;
• Organic processes: from water-dwelling organisms form biochemical sedimentary rocks
• Examples include: Limestone, chert, rock salt

Chemical Sedimentary Rocks

• Limestone is the Most abundant chemical sedimentary rock in that it's
• Mainly composed of the mineral calcite
• Can form from inorganic and biochemical origins
• It has economic value

Chemical Sedimentary Rocks

• Biochemical limestone originates from the shells of marine organisms
• Large quantities of marine limestone are formed from corals
• Corals secrete a calcium carbonate skeleton and create reefs
• Coquina is composed of cemented fragments of shell material
• Chalk is composed of the hard parts of microscopic marine organisms

Chemical Sedimentary Rocks

• Inorganic Limestone forms when chemical changes increase the calcium carbonate content of the water until it precipitates
• Travertine is a type of limestone found in cavesIt is precipitated when the water in the cave loses carbon dioxide
• Oolitic limestone is composed of small spherical grains called ooids
• Ooids form as tiny “seeds” roll in shallow marine water supersaturated with calcium carbonate

Chemical Sedimentary Rocks

• Dolostone is similar to limestone but contains magnesium, its origin is unclear
• Significant quantities are created from magnesium-rich waters circulate through limestone
• Chert is Composed of microcrystalline quartz; Forms when dissolved silica precipitates
• Examples include Flint jasper varieties of chert

Chemical Sedimentary Rocks

• Evaporites Form when restricted seaways become over-saturated and salt deposition starts
• Rock salt and rock gypsum are two common evaporites
• Occasionally, evaporites form on salt flats when dissolved materials are precipitated as a white crust on the ground

Coal: An Organic Sedimentary Rocks

• Coal is different from other sedimentary rocks
• Organic sedimentary rocks are formed from carbon rick Remains of organisms
• Occasionally, plant structures (leaves, bark, and wood) are identifiable in coal
• There are Four stages of Coal Formation, these involve: Accumulation of plant remains, then Formation of peat and lignite, followed by Formation of bituminous coal prior to Formation of anthracite coal

Turning Sediments into Sedimentary Rock:

• Diagenesis and Lithification describe the processes that cause Many changes to occur to sediment after it is deposited
• Diagenesis-chemical, physical, and biological changes that take place after sediments are deposited
• Occurs within the upper few kilometers of Earth's crust

Turning Sediments into Sedimentary Rock:

• Diagenesis and Lithification are the processes that describe how uconsolidated sediments are transformed into solid sedimentary rocks
• This involves Compaction where as sediments are buried, the weight of the overlying material compresses the deeper sediments and; Cementation where the process involves the crystallization of minerals among the individual sediment grains

Classification of Sedimentary Rocks

• Sedimentary rocks are classified according to the type and texture of material
• Two major groups are Detrital or; Chemical/organic
• Detrital has clastic texture while;
• Chemical/organic Has nonclastic or crystalline texture and its The minerals form patterns of interlocked crystals