What is a rock? A purely descriptive definition is that a rock is - A naturally occurring aggregate of minerals and other solid material. - Usually, there are several minerals in the aggregate, though some rocks may have only one. The other materials may include:
- natural glasses
- organic material (lignite, coal, or petroleum)
- fossils.
Geologists usually think of rocks in a second important way, however. Please and recite it like a mantra:
A rock is a record of the environment in which it formed.
The Rock Cycle: Consider the three basic rock types and how they form:
- Igneous - solidified from molten material
- Metamorphic - recrystallized by heat and/or pressure.
- Sedimentary - composed of remains of preexisting rocks.
The material that makes up any rock might have a complex history.
- The grains of quartz sand in a sandstone might have been wethered from a quartz vein in a metamorphic rock that prior to being metamorphosed, had been an igneous granite.
- That same sandstone may, in the future, become buried deep in the crust, undergo metamorphism, melt, and resolidify as an igneous rock.
Geologists describe this range of possible histories as the Rock Cycle. As the schematic shows, it actually encompasses many possible cycles.
Key concepts and vocabulary:
- Rock textbook definition
- Rock practical definition/mantra
- major rock types:
- Igneous
- Sedimentary
- Metamorphic
- The rock cycle
Igneous Rocks
Definition review: Igneous rocks are rocks that form through the solidification of magma.
Heat in the Earth. Various lines of evidence allow geologists to estimate the geothermal gradient or geotherm - the temperature/depth curve. Note, however that the curve is not linear.
- The core is considerably hotter than the adjacent mantle
- The upper mantle and asthenosphere are considerably hotter than the lithosphere.
Q: Why should this be?
A. Consider convection and conduction. Convection occurs in the mantle but not between the core and mantle or asthenosphere and lithosphere (except at sea-floor spreading zones). Thus, at these transitions, heat must travel by conduction alone.
Convection This is the process by which material circulates through a region that is unevenly heated. In a tea kettle, for instance:
- Water is heated at the bottom.
- It rises.
- Surface water radiates its heat into the air and cools.
- Cooler water sinks into the space evacuated by the rising warmer water and begins to warm, while the warmer water reaches the surface and cools.
- The process repeats, yielding a top to bottom circulation of water.
The tops of convection cells (units of convective circulation) can often be seen in cups of tea or black coffee. Condensing water vapor marks to top of rising columns of warm water. Dark lines separating them marks the location os sinking cooler water.)
In the Earth, convection in ductilely deforming solid rock brings heat from the interior to the surface, powering the movement of plates. Exactly how big the convection cells are and how quickly their material moves is enigmatic.
Even though we don't see this convection directly, we know it happens from thermodynamics: There are two modes of heat transfer:
- Conduction: The direct transfer of heat from one molecule to an adjacent one
- Convection: The bulk transport of material
Suppose the Earth had cooled from conduction only. Thermodynamic calculations show that a given parcel of heat could only have moved only 400 km in 5 g.y. by conduction alone. That means that below 400 km, everything must be molten. That is isn't shows that convection must be at work, also.
The geotherm is a useful means of visualizing the processes the cause rocks to melt. In the following discussion, we use graphs that track
- The geotherm only to the depths of the asthenosphere.
- The melting curve for peridotite. Peridotite is the rock type that mostly makes up the mantle. The melting curve shows the boundary of temperature and pressure beyond which peridotite melts.
How do magmas form? Three factors influence melting point:
- Temperature
- Pressure
- Volatiles
In detail:
- Temperature. All other things being equal, every mineral has a distinct melting point. In the mantle, heat is brought upward by convection. As hot rocks convect upward they transfer heat to cooler rocks lying above them, which may melt.
- Pressure: All other things being equal, the greater the pressure, the less likely materials are to melt. (This explains why the asthenosphere is limited to a shallow region of the mantle and the inner core is solid despite being hotter than the liquid outer core.) When rocks experience decompression without losing their heat, they can experience decompression melting. Consider the fate of hot rocks rising through the mantle from a hot spot.
- Volatile substances: Generally, the addition of substances like water or CO2 to a mineral lowers its melting point. In this case, the shape of the melting curve for peridotite changes.
Composition:
Here's a paradox: Even though peridotite is typically what melts to make magma, hardly any magma ever solidifies into peridotite. WTF?
- In fact, Peridotite is a composite of different minerals, (mostly pyroxene and olivine) each with different melting points. Thus, we get fractional melting, in which minerals with the lowest melting point melt first, leaving the more more refractory minerals as solids. Thus, at the point of its birth, the magma is slightly different from the parent rock.
- As magma rises, it melts its way through overlying rocks of many different types. Thus, the minerals of different rock types also get to add their fractionally melted contributions to the magma.
Silica (SiO4) rich minerals like quartz usually have lower melting points than Fe and Mg rich minerals like olivine. Thus, a relatively low temperature magma will be rich in silica and poor in Fe and Mg.
Where does magma form?
- Mid-ocean ridges: Rising rocks in mantle convection cell bring heat near the surface, transfering heat to overlying rocks. At the same time, the hot rising mantle rocks experience decompression melting. The motion of lithospheric plates away from the mid-oceanic ridge further diminishes pressure yielding more melting.
- Mantle plumes: Those enigmatic localized upwellings of hot mantle rock from hot spots very deep in the mantle, expressed on the surface as hot spots. As in mid-ocean ridges, hot spot rocks transfer heat to overlying rocks and experience decompression as they come up.
- Subduction zones: As oceanic crust sits at bottom of ocean, it becomes charged with sea water. Subduction slab, although relatively cold, dives into hot surrounding rock. The slab acts as conveyors drawing water into the hotter, drier asthenosphere. When the water percolates into the surrounding hot rocks, melting due to the infusion of volatiles occurs. This leads to some interesting consequences:
- Subduction zone magmas tend to be low temperature magmas compared to those from the other regions. Because they are, they are compositionally enriched in SiO4. These magmas are the ultimate origin of continental crust.
- We said earlier that Venus apparantly has no subduction zones. Even if it did, this kind of melting would not occur there because the subducting slab would be carrying no water downward. Hence, no continental crust on Venus.
How does magma behave? When melting first occurs, it happens mineral grain by grain, yielding tiny pockets of magma. Being liquid, magma tends to be lighter than surrounding material from which it has melted. Thus, it tends to percolate upward by any available means. As this happens, droplets coalesce, eventually forming large magma chambers that can be relatively small (Enchanted Rock, Texas) to very large (Sierra Nevada Batholith).
Rocks from magma
Igneous rocks differ from one another in.
- Emplacement process
- Texture
- composition
Process differences in igneous rocks:
- Intrusive (plutonic) rocks: Definition: An intrusive (a.k.a. "plutonic") rock is an igneous rock that is formed by the cooling of magma that has forced its way into surrounding rock (a.k.a. Country rock) but not reached the surface:
- Large interlocking crystals
- Crystals usually have no preferred orientation - i.e. a piece of intrusive rock looks the same no matter which way you turn it.
- Extrusive (volcanic) rocks: Definition: An volcanic rock is an igneous rock that is formed by the cooling of magma that has erupted onto the Earth's surface.
Texture: Depending on how quickly they cool, igneous rocks can show two basic textures:
- Phaneritic: Rocks which show large visible interlocking crystals. Most intrusive rocks which cool slowly have this texture.
- Aphanitic: Rocks which show small or microscopic interlocking crystals. Most extrusive rocks which cool quickly have this texture.
Fine points of texture:
- Glassy: Rocks which have no crystals but consist, instead, of volcanic glass. These cool almost instantaneously.
- Pyroclastic rocks: Rocks formed from the deposition of volcanic ash. This, in turn, forms when magma erupts as an aerosol of fine particles. Often, ash fragments are still slightly sticky when they fall, sticking together to form welded tuff. Of course, tuff is a pyroclastic rock.
- Vesicular: Magma typically has some gas in solution. During eruptions, that gas effervesces out of solution, like CO2 bubbling out of a soda. As the magma freezes, these bubbles are preserved as vesicles, and rocks containing them are called vesiclular.
- Porphyritic: What happens when a magma chamber cools slowly underground to the point of being a slurry of large crystals and liquid, then erupts so that the liquid chills quickly. The result is porphyry, a rock with both phaneritic and aphanitic aspects.
- Vesicular: Magma typically has some gas in solution. During eruptions, that gas effervesces out of solution, like CO2 bubbling out of a soda. As the magma freezes, these bubbles are preserved as vesicles, and rocks containing them are called vesiclular.
Chemical and Mineral composition: The chart below shows the important mineral components of common igneous rocks. Its x axis shows the percentage of silica in the rock, the y axis shows the relative abundance of different minerals in the rock. Remember, for each composition there are intrusive (phaneritic) and and extrusive (aphanitic) textural versions.
For example, we see that granite might have 70% silica and be composed of 50% orthoclase, 25% quartz, and 25% plagioclase, muscovite, biotite, and amphibole. Its volcanic equivalent is rhyolite. This figure summarizes the information you need on igneous rock compositions. GEOL100 students should it.
The silica concentration continuum: Silica content is the key to understanding igneous rocks. In the modern world, igneous rocks range from about 70% to about 40% silica. In the crust, they usually don't go below about 50% silica. We use the terms felsic and mafic to describe silica content.
- Felsic: (Felsic = Feldspar + silca.) High quantities of orthoclase and quartz, small amounts of plagioclase, muscovite, biotite, and amphibole.
- Intermediate:
- Granodiorite (Intrusive) - Dacite (Extrusive): Small amounts of orthoclase and larger of plagioclase.
- Diorite (Intrusive) - Andesite (Extrusive): No orthoclase and little or no quartz. The major component of subdiction zone Extrusive arcs.
- Mafic (Mafic = Ma + Fe): High quantities of olivine and pyroxene, smaller amounts of plagioclase.
- Ultramafic: Very rare on surface, often found as mantle xenoliths. Low silica content, with rocks primarily made up of olivine with some pyroxene.
- Intrusive: Peridotite, the dominant rock of the mantle. Seen in mantle xenoliths.
- Extrusive: Komatiite. No extrusive ultramafics exist in the recent world. Komatiite, an ultramafic extrusive rock, was erupted during roughly the first half of Earth's history, in the Archean and early Proterozoic eons. You should be able to figure out why there's no new komatiite today
- General compositional trends:
- Felsic:
- more silica
- more Na (in plagioclase)
- More K
- Mafic:
- More Ca (in plagioclase)
- More Mg
- More Fe.
- Felsic:
- Magma terminology: When we describe magma composition, by convention, we do so with reference to the kind of extrusive rock it would form, thus we have basaltic, andesitic, dacitic, and rhyolitic magmas. Depending on their composition, magmas behave differently:
- More felsic magmas (such as rhyolitic) are:
- viscous
- often have large quantities of water vapor
- relatively low temperature
- tend to erupt explosively, as in Mt. St. Helens in 1980.
- More mafic magmas (such as basaltic) are:
- less viscous (i.e. more fluid)
- dry (i.e. little water vapor)
- relatively high temperature
- ttend to flow as a liquid after eruptions as in Kilauea.
- More felsic magmas (such as rhyolitic) are:
Key concepts and vocabulary:
- Geothermal gradient or Geotherm
- Convection
- Conduction
- Melting curve
- Peridotite
- Factors influencing magma formation:
- Temperature
- Pressure
- Volatile substances
- Fractional melting
- Sites of magma formation:
- Mid-ocean ridges
- Mantle plumes/hot spots
- Subduction zones
- Magma chamber
- Emplacement types:
- Extrusive igneous rock
- Intrusive igneous rock
- Igneous rock textures:
- Phaneritic
- Aphanitic
- Glassy
- Pyroclastic
- Vesiscular
- Porphyritic
- Memorize the rock chemistry chart
- Felsic
- mafic
- Felsic rocks:
- Granite (intrusive)
- Rhyolite (extrusive)
- Intermediate rocks:
- Granodiorite (intrusive)
- Diorite (intrusive)
- Dacite (extrusive)
- Andesite (extrusive)
- Mafic rocks:
- Gabbro (intrusive)
- Basalt (extrusive)
- Ultramafic rocks:
- Peridotite (intrusive)
- Komatiite (extrusive)
- Felsic - mafic compositional trends
- Magma chemistry terminology:
- Rhyolitic
- Dacitic
- Andesitic
- Basaltic