Geology: Igneous Rocks

Batholiths and Stocks

Batholith: large, usually discordant plutonic mass that covers more than 100 square kilometers at the Earth's surface, with no known base. Erode overlying layers then uplifted through tectonic movement. Enchanted Rock - millions to be cooled and billions of years to be exposed. Don't know how deep they go because there's no money to find that out. Stock: A plutonic mass that covers less than 100 square kilometers at the Earth's surface, that is usually discordant but not always. Smaller, frozen igneous rock - pluton. Right of enchanted rock, usually connected to batholith deep underground.

Calbuco

Calbuco in Southern Chile erupted most recently in April 2015, followed by two smaller eruptions. Calbuco is an andesite volcano with 36 confirmed eruptions in recorded history. 2 eruptions (1 major eruption) ejected over 210 million m^3 of ash up to 10km into the sky. Very explosive rained down ash on large area. Calbuco is associated with both pyroclastic flows and lahars. Pyroclastic flows are ash and lava fragments in the air, buoyantly supported by the vapor. Lahars are a mix of water, mud, ash, and pyroclastic material. Ash mobilized by water - rain on volcano mixes with ash and forms extremely high viscosity flows causing lots of damage/danger.

Igneous Rock Classification

Categorization based on texture and composition of rock. Explosive eruptions are felsic, watery lava flows are mafic. Granite is an intrusive variety of Rhyolite. Darkness and specific gravity increases from felsic to mafic. Silica increases from mafic to felsic.

Intrusive Igneous Bodies: Pluton

Collective term for intrusive igneous bodies that may be tabular, cylindrical, or massive (usually), including: dikes, sills, laccoliths, volcanic pipes, batholiths, and stocks. The 2 categories of plutons are: Concordant plutons: boundaries parallel the layering of the host country rock (following along with pre-existing structure). Discordant plutons: Boundaries cut across the host country rock (not parallel). Rock holds it down underneath surface, don't see them while they're forming, cooling and crystallizing. Plutons never make it to the surface. Tend to look like giant balloons.

Magma Origin: Subduction Zones

Crust is partially melted as it subsides into the mantle. Ocean sediments and Si-rich continental crust are incorporated into the melt, making the magma more felsic. Plates are coming together, they collide, one plate slides under another, this plate is the oceanic crust (the one that dives beneath). Water-rich oceanic crust melts and rises to the surface. In Andes, subducting beneath South America, squeezing continental crust to form mountains. Continental crust mixes with ocean crust to form Andesite. Subducting plate dives beneath North America and volcanoes need to erupt soon from Washington.

Dikes, Sills, and Laccoliths

Dikes are discordant, tabular igneous body that cuts across the structure of the adjacent rocks or massive rocks. Forms sheets of igneous material slicing its way through pre-existing rocks. Feeder dikes feed material to the surface. Dike cuts through horizontal layers of rocks. Sill: Concordant tabular igneous body that parallels the structure of surrounding rocks (hot rock bakes rock and slightly metamorphose rocks next to it). Laccolith (mushroom rock): A concordant igneous body that has a domed upper surface, a flat lower surface parallel to the surrounding rock and is fed by a dike at its thickest point. Subset of sill, domed sill, deforms surrounding layers.

Igneous Rock Classification: Primary Minerals

Felsic All silicates. Quartz: SiO2 K Feldspar: (more pink): Microcline/Orthoclase - KAlSi3O8 Plagioclase Feldspar (white): Arbite - NaAl3SiO8 Anorthite - CaAl2Si2O8 Biotite (found in Granite): K(Mg,Fe)3(AlSi3O10)(OH)2 Hornblende: (Ca,Na)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH)2 Pyroxene: Enstatite - MgSiO3 Ferrosilite - FeSiO3 Olivine: (Mg,Fe)2SiO4 (breaks rules in its green color, instead of black) Mafic, Ultramafic

Felsic Rocks

Felsic Rocks have a high Si content (>65%), light color (pink, grey), and lower density. They are common and easily recognizable. Rhyolite is an extrusive igneous and Granite is an intrusive igneous. Aphanitic (Rhyolite)/Porphyritic (Llanite). Cooled deep in the subsurface. Form the continental crust - basement rock (Granite).

Bowen's Reaction Series (Figure)

Highest Temperature - Lowest Temperature Magma goes from Mafic - Intermediate - Felsic Discontinuous branch: Olivine, Pyroxene (augite), Amphibole (hornblende), Biotite mica, Potassium feldspar, Muscovite mica, and Quartz. Continuous branch: Calcium-rich plagioclase, Sodium-rich plagioclase, Potassium feldspar, Muscovite mica, and Quartz.

Igneous Rocks

Igneous rocks are born from fire. They are the first rocks to form on Earth.

Igneous Rock Composition

Igneous rocks are composed mainly of the most abundant elements (Oxygen, Si). Felsic Rocks make up the continental crust. They are more than 65% silica (Si) minerals, have abundant Sodium (Na), potassium (K), and aluminum (Al) and little Calcium (Ca), Iron (Fe), and Magnesium (Mg). They are silica-rich, sticky, don't want to flow, lightly colored, low-density, with light coloration. Mafic Rocks have less Silica at <52% Si. They are abundant in Ca, Mg, and Fe, and have little Al, K, and Na. More Ca, Fe, and Mg. They are dark, higher-density, black, brown to greenish color. They are Mg-Fe rich, form the oceanic crust. Intermediate Rocks are a composition between Felsic and Mafic. They are a mix between 2 materials. For example, Diorite is 1/2 dark and 1/2 light, dalmatian, with a middle density. Ultramafic are very black and dense. Don't see them very often because they are in the mantle. CAN use color for igneous rocks.

The Rock Cycle

Igneous rocks are the starting point of the Rock Cycle. Igneous rocks are formed through the cooling and crystallization of molten rock. Molten rock begins as magma beneath the surface, down in the mantle. When exposed at the surface, molten rock is lava. Temperatures needed to melt rock range from 600 degrees to 1200C. Start out as liquid material and the temperature cools them to form solids. At surface, lava cools and crystallizes quickly. Where it's found determines the difference between lava (surface) and magma (subsurface). Density of the liquid rock is lower in the heat. Solid form has a higher density. Ice is the only one that is lower density in solid form. Low-density liquid rock surrounded by high-density solid rock. Magma tries to rise through the surface.

Igneous Textures I

Igneous textures are based on the size of mineral crystals. Aphanitic does not have visible crystals. It has fine-grained crystals (fast-cooling). Crystal nucleation < crystal growth. Phaneritic has visible crystals. It has coarse-grained crystals (slow-cooling). Cystal nucleation > crystal growth. Porphyritic means purple, but that doesn't apply. Variable crystals size. Phenocrysts are the larger, visible crystals. Groundmass is the less visible, finer crystals (smaller stuff in between). Ex: Llanite has 2 cooling periods. Aphanitic light. Diorite is phaneritic. 2-stage cooling periods process. Phanerose has visible mineral crystals. Porphyritic is based on the stuff in between (groundmass). This determines whether it is aphanitic porphyritic or phaneritic porphyritic. Make-up in Llanite is Quartz and Feldspar.

Toba Lake Eruption - 72,000 Bp

In Sumatra, Indonesia, over twice as large as Yellowstone super volcano. It drastically lowered the temperature of the earth for about 1000 years, coincides with a genetic bottleneck with only about 3000-10000 breeding individuals that survived. Very, very similar limited our populations as well as cheetahs and gorillas. Responsible for lack of diversity in humans. Intersected groundwater or surface lake turned to steam causing a massive explosion. Lowered and raised temperatures.

Intermediate Rocks

Intermediate rocks have a moderate Si content (53-65%), wide range of colors (salt-and-pepper look), and moderate density. Mix of mafic and felsic material, some found in the Andes Mountains along the South American West Coast. Felsic and mafic mix found on the surface are considered porphyritic (Extrusive Andesite), otherwise aphanetic. Inside the ground there is intrusive igneous - Diorite. Diorite means distinguishable (Dalmatian) light and dark minerals both.

Peru, Andes

Lahar: flash-flood with rocks and ash. Ash-water mixture mobilizes large boulders carved out channel for itself.

Magma and Lava

Lava flows are channels and sheets of lava that flow like rivers. They are a benign igneous feature, easily avoided, and can flow very slowly like water. Pyroclastic event are fast-moving currents of both hot gas and rock that can reach speeds of 450mph with temperatures of around 1000C. They are dangerous, unpredictable, fast, and occur over an extremely large area. They pulverize surrounding rock and heat ash; very destructive. Magma does not always reach the surface, it can be trapped beneath the surface where it slowly solidifies over time, but it wants to rise. It is stuck in the subsurface and becomes Granite which cools in the subsurface.

Mafic Rocks

Low Si content (45-52%), dark color, usually green, gray, or black and high density. They form the crust of the Earth. Dark, dense, black-colored, mafic rocks. Crystal faces reflect light are found in Gabbro with phaneritic cooled underground with visible mineral crystals. Basalt forms most of ocean floor with aphanitic texture - faces too small, don't wink at you. Basalt is an extrusive igneous and Gabbro is an intrusive igneous.

Magma Evolution: Bowen's Reaction Series

Mafic, intermediate, and felsic rocks can all arise from the same source magma. Discontinuous series contains ferromagnesium silicates. Continuous series contains only non-ferromagnesium silicates. Higher melting temperature for the minerals higher on the list. Lower melting temperature for the minerals lower on the list. Translates to mineral stability. Need felsic (granite) headstone because Olivine is not stable at the surface of the Earth - ephemeral. Temperature at which they crystallize is also their melting temperatures. Mafic cool at cooler temperatures then form certain minerals. The first to cool is Olivine and Ca-Plagioclase. Higher temperature minerals are less stable at the surface and will react with other chemicals. Quartz, Muscovite Mica, and Potassium feldspar are the last minerals to come out of molten solutions. They also melt at low temperatures (600C). Most stable at temperatures at which they form. Quartz at the surface stays in form of Quartz, doesn't change its chemical composition. Melting temperature stays the same as forming (crystallization) temperature. Unstable, degrade quickly at higher temperatures.

Source Changes (Magma Evolution)

Magma can change through time as the source composition changes. Assimilation: Country Rock - the surrounding rocks in which the magma intrudes. Country rock is consumed/melted by magma. Mixing with surrounding rock as it rises, falls off into magma chamber and melts adding its elements. Crystal Setting/Fractional Crystallization: Crystals form and sink to the bottom of the melt because they are more dense, specifically the minerals higher on the Bowen's Reaction Series settle first. Certain minerals come out of melt earlier than others, crystallize first. Olivine, Pyroxene, Amphibole to be specific. Settle at the bottom of the melt - rain out of magma chamber - form bottom layer. Decrease Fe and Mg by taking out the mafic material. Magma Mixing: This is seen when 2 plates collide. Different magmas may converge into one chamber and mix. Mixing of mafic magma with a felsic magma would create an intermediate magma.

Phreatic Eruptions

Mt. Ontake's phreatic eruption: Moisture from snow and rain seeps into the geothermal system. Heat sources can flash vaporize the water into steam that expands violently. Ash plume has not glass fragments (chilled magma), indicating that the eruption was steam-driven. Phreatic eruptions are incredibly explosive eruptions that occur when rising magma reaches groundwater and vaporizes the water into steam quickly. Water expands by a factor of around 1600 when it turns to steam. 2014 Eruption of Mount Ontake in Japan that killed 30 hikers through suffocation, not steamy hot volcano. In Japan, massive steam explosion built up pressure and exploded. Yellowstone/Indonesia - loads of water in sediments in subsurface, lava interacts/turns into steam. Phreatic eruption interacts with water.

Huge volcano sleeps under Yellowstone

Reading the geothermal fine print found in tiny crystals of zircon and quartz, scientists are forming a new picture of the life history - and a geologic timetable - of a type of volcano in the western United States capable of dramatically altering climate sometimes within the next 100,000 years. These are volcanoes that occur over "hot spots" in the Earth and they erupt in catastrophic explosions, sending hundreds to thousands of cubic kilometers of ash into the atmosphere and wreaking climatic havoc on a global scale. By comparison, the eruption of Mount St. Helens sent a mere two cubic kilometers of ash skyward. Could it erupt again? The near-clockwork timing of eruptions at Yellowstone - 2 million years ago, 1.3 million years ago and 630,000 years ago - show a regular periodicity of cataclysmic eruptions, and suggest a high probability of a future catastrophic eruption. Yet, the zircon and quartz data show the geochemical signature of a waning cycle. Mt. St. Helens produced only a few eruptions, blacked out the sky, plants die, organisms die. Encompasses a very large area, superheated, compressed air would kill us. Almost 1/2 of the U.S. will be covered. Lava Creek and Huckleberry Ridge produced tuff, vesicular ashy material.

Magma Origin: Hot spots

Regions where mantle plumes breach the crust (oceanic or continental) and form deposits. Mafic rocks with no association to plate boundaries. Air units NOT adjacent to tectonic boundaries. Magma in Yellowstone came into contact with groundwater (giant explosion in crust - supervolcano). Oceanic hotspots give you mafic material. Continental crust - felsic to intermediate. These hotspots are worse; global climate changes Yellowstone National burning hole in crust. These are poorly understood. Originate at the core-mantle boundary. Burn a hole in crust, driving material to the surface. Lurking under continents/ocean bases. Explain island of Hawaii (formed in middle of plate, not at boundary). Underwater volcano rises above surface of ocean to become volcanic islands.

Magma Origin: Spreading Zones

Spreading Zones split the Atlantic Ocean pushing Europe and America apart. Cracks in the ocean floor where magma breaches and is quenched by the ocean water. Melting temperature is lower because of presence of water. Melting temperature is lower because of lower pressure. Gases want to come out through a steam explosion. Water increases reaction rate. Pull water-rich rock apart; melts very easily. Pillow basalts are mafic material that cools and crystallizes underwater, coming from a central crack called a mid-ocean ridge. Pillow basalts. New oceanic crust is created. Actual ocean is spreading. Material melting is deep mafic. Thin layer of basalt gets stuck in crust and becomes Gabbro.

Extrusive vs. Intrusive

These are the two types of origins. Extrusive igneous rocks form by lava on the surface. Rocks tend to crystallize rapidly. Small crystals are formed. aka Volcanic Rocks. Thermally conductive atmosphere. Minerals are not visible to the naked eye. No growth of large minerals (takes long time to grow, cools quickly). Intrusive igneous rocks are rocks formed from magma that cools in the crusts (never reaches the surface). Rocks tend to crystallize slowly (hotter). Larger crystals are formed (take 100 - million years to grow). aka Plutonic Rocks. Magma insulated by surrounding rocks. Every inch formed is a million years. Plutonic named after Pluto (God of Underworld), cools in his domain/realm.

Ash and volcanic material

They take the quickest path from high to low gravitational forces - follows same path as water - flows down valley where water is already flowing. Andes, Japan.

Ultramafic Rocks

Very low Si content (<45%). Color is generally green (Olivine, especially subset Forsterine) or black. Very high density (very dense) meaning they should be found deep in the Earth. Peridotite is an extrusive igneous and is slightly greener than Gabbro, it is a collection of gemstone versions of Olivine. Most common gas expelled from volcanoes is water vapor. Pummous raft is formed by particles on the surface of water. Igneous rocks are composed of primarily silicon and oxygen. Rarely see these at the surface. Chlamadeite spews ultramafic material to surface with aphanitic texture. Phaneritic has visible minerals cooled in the sub-surface. Most dense magmas from ultramafic material. A lot more Fe and Mg than Si.

Viscosity

Viscosity is the resistance to flow. Water has a LOW viscosity (meaning it doesn't resist flow). Maple syrup has a high viscosity, so it does resist flow. Viscosity controls composition - high silica increases viscosity (silicon tetrahedrons grab onto each other, resist more). Temperature - increased temperature results in lower viscosity. The hotter it is the better flow it has. Pressure - low pressure, low viscosity. Dissolved Gases - more gasses less viscosity. CO2, CH4, H2S, H2O stay dissolved in the rocks. If more gases foam at the surface then it is less viscous. Lava flows are mafic with low viscosity (flow well and predictable). Pyroclastic materials have a high viscosity. Events are felsic, nasty eruptions, silica blocks flow. Plug up with felsic material then the magma still wants to escape, represents-pulverizes surrounding material. Felsic-rich lava has bad flow, mafic acts like a watery material. Felsic material leads to massive eruptions where people die from the super heat or from inhaling ash (glass pieces). Hawaii has mafic eruptions all the time.

Volcanic Pipes and Necks

Volcanic Pipes: the central portion of a volcano through which magma is transported. Plumbing system of volcanoes where igneous material comes up to surface. Volcanic Neck: vertical pipe-like intrusion that represents a former volcanic vent/pipe. Lacking volcano that surrounded it (eroded and weathered). Magma freezes at subsurface (cools underground). Magma body pushes to surface. Devils Tower is a volcanic neck, it can't erupt because the magma's all frozen, intrusive igneous material being fed to the surface, cools and crystallizes from the top down, cools and contracts see cracking that forms linear joints/cracks in rock itself. Giants causeway is the little interlocking basalt columns. Start removing tough, fragmental, pyroclastic material around the neck. Volcano is built up with ashy, fragmental material. The volcanic pipe leads to the volcanic neck from a feeder dike.

Igneous Textures II

Volcanic Textures: Pyroclastic, Glassy, and Vesicular. Pyroclastic (fire-broken) came from some violent explosion. It is fragmental material of volcanic ash and bombs (blobs of molten lava flying through the air while also cooling take on aerodynamic shape and explode on impact). Bombs are crystallized on outside and molten on inside. Tuffs and welded tuffs won't float in water and are ash hot enough to weld itself to other layers of ash. Glassy has no crystals (not a true mineral) and extremely rapid cooling. Ex: obsidian-glass. Vesicular (more of a descriptor) contains numerous small holes called vesicles which are remnants of escaped gases. Gases trapped during rapid cooling have small bubbles in it where the gases came out of solution. Glassy is lava that flash-froze before atoms could rearrange themselves into crystal structure, amorphous. Scoria is also glassy, with no minerals in it. Pummous is vesicular from frothy lava where the gases have been churned up. It can float on water.

Yellowstone

Volcanism in the American Northwest. Cascade Volcanoes Now active/dormant. Eruptions are massive at the Yellowstone Caldera Chain. 2 million and a 0.6 million years ago were the last ones. It's supposed to go off every 200,000 years.

More Hot Spots

Yellowstone is representative of a hotspot under continental crust. The composition of the magma at the hotspot is basaltic, but melting of continental crust results in rhyolitic magma that is seen at the surface. Entire park is a crater. Melt continental crust, has erupted in past (extremely large eruption event), pressure builds up though it's releasing its gas. Yellowstone is a giant volcano, centered around geothermal activity. West-flood basalts. Hotspot hasn't moved, but North America has moved over it.