Magma and Igneous Rocks Rock: A coherent, naturally occurring, aggregate of minerals or glass Geologists distinguish three main types of rocks 1- Igneous Rocks that form by the freezing or solidification of melt
2- Sedimentary Rocks that form by the cementing of grains or fragments of pre-existing rocks, or by the precipitation of minerals out of a solution
3- Metamorphic Rocks that form when pre-existing rocks change due to temperature or pressure, and/or as a result of squashing or shearing.
Thin Sections • To study rocks in detail, geologists cut thin slices of rock so that they are translucent Geologists cut rocks with a rotating saw
• Geologists can then look at them through petrologic microscopes
and then grind them into thin sections
A hand sample of granite
A magnified thin section of granite
Igneous Rocks- The Basics • Solidified molten rock (which freezes at high temp). – 1,100°C to 650°C. – Depends on composition.
• Earth is mostly igneous rock. – Magma: Subsurface melt. – Lava: Melt at the surface.
• Magma erupts via volcanoes.
Igneous Rock Types • In general, there are two basic types of igneous rocks – Extrusive/Volcanic: Igneous rocks that form due to the freezing of melts above the surface of the Earth • Includes rocks made of volcanic ash (pyroclastics)
– Intrusive/Plutonic: Form by freezing of melts below the surface of the Earth.
Formation of Magma • Remember that the tectonic plates don’t really float on a liquid asthenosphere, rather the asthenosphere is a ductile solid and is only melted in specific locations. • Most magma/lava is not 100% liquid. – Magma/Lava is made of many compounds, all of which have different melting temps. Analogy: a slushy or frozen margarita – Only a few percent of liquid is required to make a melt.
• Other than a rise in temperature, what causes melting of rock within the Earth? Melting happens because of: – Decrease in pressure (decompression) – Addition of volatiles (H2O, CO2, etc…) – Heat transfer from rising magma
Melting due to Decompression The Earth gets hotter with increasing depth due to primordial heat and radioactive decay of elements near the core. The rate at which temperature increases with depth is called the geothermal gradient, or geotherm
Liquids have no organized structure, so to melt a rock, the mineral bonds must be broken (animated gif of atoms)
The geotherm of the Earth
Melting due to Decompression At depth, confining pressure prevents atoms from breaking free of crystals Solidus: The temperature when a rock first begins to melt Liquidus: The temperature where the last solid particle melts
The asthenosphere cools only slightly as it rises (convection) because it is a good insulator (high specific heat)
The solidus and liquidus of peridotite (ultramafic mantle rock)
Melting due to the Addition of Volatiles • Volatiles: A substance that can easily change into a gas at relatively low temperatures (H2O, CO2, etc…). • The addition of volatiles at depth (mainly H2O) seeps into rocks and helps break bonds (aids in melting). • Analogy: Think of putting salt onto ice to lower the melting temperature. Likewise, adding water to rocks changes the melting point of rocks just like adding salt to water.
Melting due to the Addition of Volatiles
Depth (km)
• The addition of H2O into basalt, for example, drastically changes its melting temperature
• In this case, basalts at 60km depth beneath the continents could begin to melt only if they were volatile rich.
The geotherm beneath a continent and the solidus of wet and dry basalt
Melting Due To Heat Transfer • Melting can also occur when rising bodies of hot material essentially bake the nearby rock • Analogy: Think of pouring hot fudge into ice cream. The hot fudge transfers heat to the ice cream and melts it
What is Magma Made of ? • All magmas contain Si and O – Upon cooling, bond together into silicon-oxygen tetrahedrons • More silica (i.e. felsic), more viscous (harder to flow, thicker)
• Also contain varying amounts of other elements like Na, K, Al, Ca, Mg, Fe, etc… • Dry magmas – no volatiles • Wet Magmas – up to 15% volatiles • Volatile content strongly effects the viscosity (ability to flow) – More volatiles, less viscous (easier to flow or more fluid)
Increasing Fe, Mg
Increasing SiO2
Types of Magma - Composition Like rocks, not all magma is made of the same stuff • We divide magmas into groups by their composition – Felsic (Silicic): 66-76% Silica (SiO2) • Most viscous, Least dense (~2.5 gm/cm3), melting point 650-800oC
– Intermediate: 52-66% SiO2 – Mafic: 45-52% SiO2, lots of MgO, FeO, and Fe2O3 – Ultramafic: 38-45% SiO2, abundant MgO, FeO, and Fe2O3 • Least viscous, Most dense (~3.5 gm/cm3), melting point up to 1300oC
Magma Compositions • Composition controls density, T, and viscosity. – Most important is the content of silica (SiO2). • Silica-rich magmas are thick and viscous. • Silica-poor magmas and thin and “runny.”
– These characteristics govern eruptive style.
Type
Density
Temperature
Viscosity
Felsic
Very low
Very low (600 to 850°C)
Very High: Explosive eruptions.
Intermediate
Low
Low
High: Explosive eruptions.
Mafic
High
High
Low: Thin, hot runny eruptions.
Ultramafic
Very high
Very high (up to 1,300°C) Very low
Bowen’s Reaction Series • • •
In order to understand the melting and solidifying of magma we need to understand Bowen’s reaction series. – Bowen figured this out by melting rocks in an oven, letting them cool, and watching what minerals crystallized This series outlines the order in which minerals form in a cooling melt Also applies in reverse order to rocks that are partially melted
• •
Discontinuous series (different minerals form) and Continuous series (Plagioclase only) So, a melt gets less mafic as it cools; In heating, the first minerals to melt are felsic.
Why are Magmas so Variable in Composition? Differences in Magma composition occur due to 5 main reasons…
1.
Different source rock compositions melt a felsic rock = felsic magma
2.
Magma mixing mix felsic magma with mafic magma = intermediate magma
3.
Partial melting
4.
Assimilation
5.
Fractional crystallization
Partial Melting • Most magmas are not 100% liquid – Commonly 2-30% melt; called a crystal mush
• According to Bowen’s reaction series, rocks that are partially melted become more mafic, because the silica-rich felsic minerals are melted first. • The melted part of the partial melt is thus more felsic than the remaining rock.
The felsic mineral, quartz, is a common cement in many rocks
Assimilation • As magma sits in its chamber, it may incorporate minerals from the surrounding wall rock – Called assimilation
• Occurs when wall rocks fall into the magma and melt (stoping) or when the magma partially melts minerals from the wall rock • Degree of assimilation depends on composition of wall rock, temp of magma, amount of H20 present, amount fractures in and strength of the wall rock, and residence time
Stoping & Xenoliths • Stoping: The process of incorporating chunks of wall rock into a magma body • Xenolith: A non-melted chunk of wall rock incorporated into a magma body – May have a very different composition than the magma
Xenolith • A xenolith in granite in the Mojave desert • Usually recognized because they may have a different texture (grain size) and composition than the rest of the rock
Fractional Crystallization • Not all minerals crystallize at the same temperature – This is fractional crystallization • As magmas cool, they become more felsic. • Mafic minerals crystallize first and are more dense than the melt, so they sink to the bottom
Bowen’s reaction series is an example of fractional crystallization
Magma Movement • If magma did not move, no extrusive/volcanic rocks would ever have formed • Magma rises because: – hotter and less dense than the surrounding rock and therefore buoyantly rises. – the weight of the overlying rock (lithostatic pressure) literally squeezes the magma out. • Analogy: Think of stepping on a tube of toothpaste to force it out, or mud squishing through your toes when you step in a puddle
• Viscosity affects a magma or lava’s ability to flow – Controlled by: • Temperature (high temp - low viscosity) • Volatile content (more volatiles – less viscous) • Silica content – silica tends to form silica-oxygen tetrahedrons that bond with each other to make long chains that ultimately resist flow (more silica – more viscous)
Extrusive Igneous Rock Environments •
Explosive eruptions generally occur when source magma is: – – –
•
Effusive eruptions generally occur when source magma is: – – –
High in silica (felsic-intermediate) Low temp High in volatiles
•
These volcanoes form – –
•
These volcanoes form – –
Lava domes Ash clouds and ash flows
Hawaii
Cascades NW USA
Low in silica (mafic) High temp Low in volatiles Fluid lava flows Fire fountains (if volatiles), lava tubes
Intrusive Igneous Rock Environments • Magma rises by percolating between grains and/or by forcing open cracks in the subsurface • The magma that doesn’t reach the surface of the Earth cools into intrusive igneous rocks – Country rock or wall rock: The pre-existing rock that magma intrudes into – Intrusive contact: The boundary between the igneous intrusion and the wall rock
• Tabular intrusions: Dike, Sill, Laccolith (pseudo-tabular, or sheet-like) • Non-tabular intrusions: Pluton, Batholith, Stock
Mt. Rushmore is carved out of a granitic igneous intrusion
Dikes and Sills • Dikes: igneous intrusions that cut across layering, i.e. discordant • Sills: igneous intrusions that follow layering, i.e. concordant
Dikes in the Sierra Nevada Batholith
•
Near Ruby Lake, CA @ ~12,000 ft
Laccoliths • Laccolith: a dome-like sill that bends the layers above it into a dome shape
Non-Tabular Intrusions: Plutons • Pluton: Irregular blob-shaped discordant intrusions that range in size from 10’s of m, to 100’s of km
• Batholith: A pluton that is 100 km2 in surface exposure • Stock: A pluton that is <100 km2 in surface exposure
The Sierra Nevada Batholith
The Sierra Nevada Batholith •
At ~100 Ma the west coast of the US, was a subduction zone with numerous volcanoes
•
The magma chambers cooled and the rocks above were eroded away leaving a large batholith exposed.
Effects of Intrusions • Dikes form in regions of crustal stretching
• Sills may cause uplift at the surface of the Earth
Effects of Intrusions • Dikes form in regions of crustal stretching
Scotland was stretched during the Cenozoic
• Sills may cause uplift at the surface of the Earth
La Sal Mountains, Utah were uplifted by a laccolith
Effects of Intrusions • Plutons disrupt the surrounding layers of rock and may cause crustal stretching above • Plutons grow by stoping: opening cracks and assimilating xenolithic blocks in the melt
Cooling of Magma and Lava • Magma cools for several reasons – Removal of volatiles – It rises to a cooler location and has time to cool • Cooling depends very much on the geometry (surface area) of the intrusion. Tabular-shape = fast cooling Spherical shape = slow cooling
– Cooling times vary from days minutes to millions of years
Igneous Textures • Glassy Texture: A solid mass of glass or tiny crystals surrounded by a glass matrix – Matrix: the smaller stuff in a rock (relative term)
• Interlocking Texture (Phaneritic): Rock made of interlocking crystals that grew as the melt solidified. Commonly called crystalline igneous rocks – Crystals fit together like pieces of a puzzle
Igneous Textures • Fragmental Texture: Volcanic rocks that are made of various types of fragments that form from volcanic eruptions. – Fragments can be: • Crystals • Xenoliths (from volcano walls) • Glass Volcanic Breccia – angular pieces of fragments entrained in the eruption
A Welded Tuff – white specks are fragments, grey is ash
Crystalline Igneous Rocks
Glassy Igneous Rocks • Obsidian: Mass of solid felsic glass; conchoidal fracture •
Tachylite: mafic, bubble-free mass of >80% glass (very rare)
• Pumice: glassy felsic volcanic rock that contains abundant open pores called vesicles (lt grey to tan in color). Occasionally less dense than water (it floats!) – Vesicle: a open space left over from a gas bubble in a lava or magma
• Scoria: glassy mafic volcanic rock with abundant vesicles (>30%). Grey, black, or red in color. – Typically has larger and rounder vesicles than pumice
Fragmental Igneous Rocks { Rocks blasted out of volcanoes…commonly called pyroclastic rocks }
• Tuff: Fine-grained rock, composed of lithified volcanic ash and/or fragmented lava and pumice. Formed from ash fall from the air, or from hot material that avalanches down the side of a volcano. – If material is still very hot (gooey) it may get squished upon landing and weld with other particles forming a welded tuff
•
Volcanic Breccia: Large angular chunks of material from either volcanic debris flows (blocky lava flow) or air fall (bombs).
• Hyaloclasite: formed when lava erupts under ice of water and cools so quickly that it shatters into fragments that weld or cement together.
Where Does Igneous Activity Occur?
Most volcanoes occur at plate boundaries or Hot Spots Most subaerial (above sea level) occur in volcanic arcs Subduction-related volcanic arcs are responsible for “the ring of fire”
Subduction and Volcanism Subduction creates volcanism 1- The down-going slab has lots of volatiles (e.g. H2O). At depth, these volatiles are heated and are squeezed from the rock and migrate into the asthenosphere above the plate. 2- The addition of volatiles, as we now know, changes the melting point of rocks and causes the asthenosphere to melt above the sinking plate. 3- The sinking plate may partially melt too, but most melting occurs in the asthenosphere above the slab.
Hot Spots and Volcanism
• There are many hot spots throughout the world, including Hawaii and Yellowstone. • Many pacific islands are or were hot spots
Large Igneous Provinces (LIPs)
• LIP: a region of particularly voluminous eruptions of magma/lava – May be a consequence of a super plume
Flood Basalts
• Flood basalts are a type of LIP that emits a large amount of basalt flows. • The Columbia River flood basalts (above) and the Deccan Traps (India) are two examples.
Formation of Igneous Rocks at Mid Ocean Ridges • ~70% of the Earth’s surface (including the underwater surface) is oceanic crust, so most igneous rocks form at mid ocean ridges
• Mid Ocean Ridge lavas are compositionally similar to Oceanic Hot Spots (basalt, mafic) • Underwater flows for Pillow Basalts – Pillows have glassy outer rim and more crystalline center
