ES10 - EARTH

In the last lecture, we zoomed in on the weathering/sedimentation part of the rock cycle. Today, we will consider what happens when stuff goes down the tubes, gets metamorphosed and ultimately gets remelted and comes up as igneous rocks once again.

Metamorphism - what is it?

Metamorphism means to change form. In the geologist's sense it refers to changes in rocks (protoliths) in the solid state (i.e. not by melting the rock wholesale). One can view metamorphism as similar to cooking. Ingredients are mixed and placed at a different temperature (and/or pressure) and changes occur. What was a batch of goo, turns into a lovely cake!

Change occurs in order to maintain equilibrium conditions with new states of heat, pressure, or fluids. Thus, major changes in any of these three environmental variables can result in metamorphism. Because of the great conveyor belt of plate tectonics, rocks can encounter a variety of environmental conditions riding around in pressure, temperature, fluid space. As they go, they may metamorphose and may have a tale to tell of where they've been.

The fingerprints of metamorphism are growth of new minerals stable at the new PTF conditions and changes in texture reflecting the state of stress.

Temperature

There are several sources of the thermal energy that drives metamorphism. Obviously, when and igneous body (some 1200 degrees C) intrudes into unsuspecting host rock, the contact zone heats up considerably. This causes a baking of the neighboring rock called contact metamorphism.

The temperature in the Earth goes up with depth. Near the surface in ordinary crust, the temperature rises some 30 degrees per kilometer. This is partly due to radioactive decay within the crust, but also comes from the fact that the Earth is still very hot from its initial formation (recall that the core is largely molten iron!). So if rocks are taken from the surface "down the tubes" either by burial or on a lithospheric nose dive, they will heat up and metamorphose.

hornfels

Pressure

Pressure changes with depth in the Earth as a result of the increased weight of the overlying rock. Increased pressure drives minerals to form more compact phases, driving for example coal to change into diamonds, and clay to rubies. Pressure also changes the state of stress. Crystals will grow or deform by cracking or flowing in response to the change in stress and show either preferential alignment, or evidence of squashing that reflects in some way the stress regime of the new environment. Deformation near fault zones, for example, results in cracking and grinding of rocks and is referred to as cataclastic metamorphism, while deformation deep in the crust occurs more plastically over a wide area producing regional metamorphism.

Fluids

Changes in the chemistry of the fluids in the pore spaces of rocks also induces change. In its lowest temperature/pressure form, this change is called diagenesis, including weathering discussed in the last lecture. Diagenesis is not usually considered part of metamorphism although the distinction is a pretty subtle one if you ask me.

One common cause of changes in the fluid chemistry is the proximity of something hot - take volcanic activity for example. The heat induces convection currents in the surrounding fluids. The heated water reacts locally with the hot rock and carries a load of dissolved matter (rich in metals and sulfer) from the region of high heat into a region of cooler rock (or water). Here, the fluids tend to "dump their load". This form of metamorphism is known as hydrothermal alteration and is the way most metallic ore bodies formed. When the super-charged fluids come out of brand-new ocean crust and hit 2C water, they drop their load and form what is known as a black smoker. Lots of animals (if you can call them that) live off the stuff. Check these out:

pictures:

Metamorphic textures

Increasing pressure causes a reduction in volume of the rock by first compaction and then recrystallization of minerals to denser forms. Compaction produces a more closely packed arrangement of grains. This is exmplified by the transformation of clay to slate. Recrystallization involves growth of new minerals. The bulk chemistry need not change (unlessfluids are involved).

If the pressure is higher in one direction than the others, minerals will tend to align themselves such that the fabric formsperpendicular to this axis. A planar fabric is known as a foliation while alignment of crystals causes a lineation.

Micas (which are platey) will grow with their plates along the foliation. Increasing metamorphism resultsin different types of foliation.

In its journey around the tectonic merri-go-round, clay will turn first to shale during diagenesis, then procede to slate. Growth of micas then will contribute a peach fuzz sheen characteristic of phyllite. As metamorphism becomes more intense, schist forms. Schists have large mica flakes in them.

schist

schist (thin section)

Under the most intense metamorphism, minerals segregrate into bands of light and dark minerals, characteristic of gneiss.

gneiss

If the rock easily splits along smooth parallel surfaces, it has what is known as fracture cleavage.

Not all rocks are foliated. For example, contact metamorphism does not generally produce foliated rocks. Also, parent rocks (protoliths) that tend to grow minerals that are not platey or elongated, will produce metamorphic rocks that have no foliation or lineation. These include quartz, and calcite. So sandstones make massive quartzites and limestones make marble, neither of which are strongly foliated. Here is an example of what marble looks like.

Porphyroblast texture

Porphyroblasts are larger crystals in a finer-grained matrix. The metamorphic rock will bear the name of the dominant porphyroblasts, e.g. garnite schist.

Mineral changes in metamorphic rocks

Changing conditions results in phase transformation from one mineral to another. Minerals coalesce or change crystal structure. The particular minerals that form are characteristic of thepressure/temperature conditions. Particularly useful for determining PT conditions are the following metamorphic index minerals

Low metamorphic grade (low temperatures and pressures) - about 200 degrees C
Slates and phyllites are characterized by:

Intermediate metamorphic grade rocks such as schist often have:

High metamorphic grade - 800 degrees C (verging on melting), such as gneiss and migmatite have the high temperature high pressure phase sillimanite.

Staurolite, kyanite and sillimanite all have the same composition but are stable at different PT conditions (like graphite and diamond). Therefore the presence of one particular form documents the PT conditions. A more accurate idea of PT conditions can be gotten by considering a whole suite of minerals. Determining the PT history of a sequence of rocks describes the journey of that particular crustal package up and down the tectonic elevator.

Role of initial composition

Obviously, the composition of the protolith plays a strong role in which minerals will grow. Thus basalts, granites and carbonate rocks each develope into the different metamorphic mineral assemblages leading ultimately to amphibolite, gneiss or marble respectively.

Plate tectonic settings of metamorphism

Hydrothermal metamorphism occurs near mid-ocean ridges driven by the heat of the volcanic activity there. Intrusion of igneous rocks drives contact metamorphism anywhere it occurs. Both of these sorts are metamorphism with high temperatures and low pressures.

Faults associated with plate boundaries create cataclastic metamorphismin the shallow crust.

Here is an example of: cataclastic metamorphism

Cataclasis grades into totally pulverized minerals that are streaked out in bands characteristic of mylonites.

blue-schists. When found on the surface, they are a strong indication that there once was a subduction zone in the neighborhood.

Finally, burial of sediments in a sedimentary basin takes the rocks down the PT road characteristic of the crust, the so-called geotherm . They respond to this by developing the characteristic mineral phases of burial metamorphism. The minerals are a guide to just how deep and hot sediments got.

Where (and why) do igneous rocks form?

Rocks melt when the temperatures, pressures, and/or water content are sufficient to bring the rock to the melting tempertare (duh). This most occurs 1) when rocks have been brought up from deeper in the mantle for example at a RIDGE, 2) rocks with water in them are brought from the surface down into the mantle, for example at a SUBDUCTION ZONE, or there is some "HOT SPOT" arising from deep in the mantle unrelated to a plate boundary. About 90% of all melting occurs at plate boundaries (spreading or subducting). The rest is called "intra-plate" and is typified by Hawaii. For a summary of where melts form, see Figure 4.7 in your book.

When a partial melt forms, it rises and collects in a magma chamber (see Figure 3.2 in your book). In the magma chamber, the melt continues to crystallize thus changing its chemistry. This is a process known as magmatic differentiation. As magmas cool, different minerals will crystallize out of the melt. By studying the crystallization of melts in the laboratory, this process is fairly well understood. If these minerals settle out of the melt to the floor of the magma chamber, (see Figure 4.11 in your book), the chemistry of the remaining melt changes from a more mafic to a more felsic melt; thus, if fractional crystallization is taken to the extreme granite can be gotten from what was originally a basaltic melt.

The magma chamber may erupt from time to time. If the melt doesn't make it to the surface, it forms an intrusive rock. (see Figure 4.16 in book). Intrusive bodies can be big balloon shapes (plutons), sub-horizontal slabs (sills) or sub-vertical walls (dikes).

If it does make it, it becomes an extrusive rock. Extrusives can flow out over the ground (lava flows) or be blasted into the air to form ash falls and pyroclastics.

Classification of igneous rocks

Igneous Rocks have a two-dimensional classification scheme based on chemistry, grain size and texture.

chemistry:

The key to chemical classification in igneous rocks is the amount of Silica (SiO₂) in the magma. (Of course people who study this make a much bigger deal out of it! If magmas don't have much silica, their minerals are dominated by magnesium and iron (Fe) - hence the term MAFIC (MA- from the magnesium and FIC from the Fe), or even ULTRAMAFIC for the really silica poor varieties. Silica rich magmas have a mineral named feldspar in them (see book) and are called FELSIC as a result. You will also see the words "acidic" and "basic" used for felsic and mafic respectively and you should be aware that this has nothing to do with pH! One can often tell about how much silica is in a rock just by its color. The more silica, the lighter the color.

grain size:

The main control of grain size is how fast the rock cooled from the molten state. Slow cooling allows bigger crystals to form, and fast cooling makes smaller crystals and even glass (no crystals). So the second dimension of igneous rock classification is whether the rock was formed by cooling on the surface as an extrusive rock. or in the crust as an intusive rock. Magma can either be erupted (extruded) as ash to make pyroclastic rock or as lava to make volcanic rocks.

IGNEOUS COMPOSITIONAL NAMES AND MAGMA TYPES

SiO2 (wt. %)	<45	45 -52	52 - 57	57 - 63	63 - 68	>68
Compositional or Chemical Equivalent	Ultrabasic	basic	basic to intermediate	intermediate	intermediate to acidic or silicic	acidic or silicic
Magma Type	ultramafic	mafic	mafic to intermediate	intermediate	intermediate to felsic	felsic
Extrusive Rock Name	komatiite	basalt	basaltic andesite	andesite	dacite	rhyolite
Intrusive Rock Name	peridotite	gabbro	diorite	diorite or quartz diorite	granodiorite	granite
Liquidus Temperature
Mafic Mineral Content
Water Content
Mg/Fe
Ca/Na or Ca/K

Texture

Igneous textures are classified by the presence or absence of crystals, the size of the crystals, and the size and density of vesicles (holes). Check out this page for a nice summary of igneous textures.

Extrusive rocks

Pyroclastic rocks:

Pyroclastic rocks are classified by grain size from BOMBS (>64mm) to ash (<2mm). Lapilli are pea-like grains often in a finer matrix.

Here is a nice picture I found to illustrate the classification scheme of pyroclastics:

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Volcanic Rocks:

Volcanic rocks are mainly classified by the amount of silica. There are four main categories with increasing silica: basalt, andesite, dacite and rhyolite.

Intrusive Rocks

Intrusive rocks cool slower and have coarser grain sizes than their extrusive counterparts. The big four of intrusive rocks are with increasing silica: gabbro, diorite, granodiorite, and granite.

A few other points

Viscosity: Increasing silica not only makes magmas lighter (in color), but makes them more viscous (stiffer), so the beautiful movies of flowing glowing lava flows from Hawaii are of basaltic lavas with little silica.

Melting temperature: Increasing silica lowers the melting temperature, so that granites melt at about half the temperature that basalts do.

Water: Adding water also lowers melting temperature and decreases viscosity. Also, water and other gases make bubbles in the magma, contributing to the explosive power of some eruptions and also leaving holes in the rocks (vesicles).

Melting: While we're on the subject of melting, it is very important to note that each mineral has a different melting temperature, so rocks do not melt all at once, but bit by bit. Thus most magmas have floating bits of crystals in them (phenocrysts) and are not 100% melted.

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Lisa Tauxe
ltauxe@ucsd.edu