Rocks

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
First-time Visitors: Please visit Site Map and Disclaimer. Use "Back" to return here.

What Rocks Tell Us

What Rocks Mean

Sedimentary Rocks

Sedimentary Rocks preserve the record of the earth's ancient surface environments, and of life. Undisturbed sedimentary rocks tell us of long quiet conditions. Tilted and fractured sedimentary rocks are a record of crustal disturbance.

Igneous Rocks

Igneous Rocks form in a limited number of settings. Most are formed where plates converge. A few form where the crust splits apart. Almost all are a record of crustal disturbance. Most of the mantle is solid, so igneous rocks tell us there was unusual heat in the crust or mantle.

Metamorphic Rocks

Metamorphic Rocks formed kilometers below the surface. They tell us that the crust has undergone kilometers of erosion. The vast majority of metamorphic rocks formed during orogenic (mountain-building) events and are a record of crustal disturbance.

Igneous Rocks

Cool from the Molten State

• Volcanic -- Erupted on Surface -- Fine Grained
• Plutonic -- Solidify Within Earth -- Coarse Grained

Porphyritic Texture:
Large Crystals in Fine-grained Setting

• Slow Initial Cooling
• Rapid Final Cooling

Igneous Rock Classification

1. How Much Silica?
   Account for Si
   • Excess - Rock Has Quartz
   • Just Enough to Form Other Silicates
   • Deficient - Silica - Poor Minerals (Like Olivine)
2. What Feldspars?
   Account for Al, Ca, K, Na
   • Potash Feldspar KAlSi3O8
   • Plagioclase Series
     NaAlSi3O8......CaAl2Si2O8
3. What Other Minerals Are Present?
   Account for Fe, Mg

Feldspars

K - Feldspar:

KAlSi3O8: Several Slightly Different Forms:

• Microcline
• Orthoclase

Plagioclase (Solid Solution)

• Albite: NaAlSi3O8
• Anorthite: CaAl2Si2O8
• Any Mixture of the Two Is Possible

Bowen's Reaction Series

The geologist N.L. Bowen found that minerals tend to form in specific sequences in igneous rocks, and these sequences could be assembled into a composite sequence.

No igneous rock ever displays the whole sequence. Igneous rocks display a slice across the sequence. Basalt, for example, typically has olivine and calcium plagioclase forming first, followed by pyroxene and more sodium-rich plagioclase. In granite, sodium plagioclase and biotite typically form first, followed by muscovite, potassium feldspar, and last of all quartz. The sketch below turns the series on its side. It's actually a more realistic view since successive minerals often form simultaneously.

Bowen's Series and Igneous Rocks

Some Igneous Rocks Are Named on Textural Criteria:

• Scoria: Porous
• Pumice
• Obsidian - Glass
• Tuff - Cemented Ash
• Breccia - Cemented Fragments
• Pegmatite - Extremely Large Crystals
• Aplite - Sugary Texture, Quartz & Feldspar
• Porphyry - Fine Matrix, Large Crystals

What Igneous Rocks Mean

Basalt

What you get if you melt average planetary material. Basalt is found on the moon and other planets and we can expect it to be a very abundant rock in the universe. On earth it forms by melting mantle material. The ocean floors are made of it. On land it often forms where the crust is splitting apart.

Andesite

What you get when mantle material mixes with continental crust. Usually it marks the site of volcanoes on converging plate boundaries, like the present Andes. This means it is geologically very significant even though it can be hard to tell from basalt by eye.

Rhyolite

Forms from melting continental crust. Forms in two settings: the final stage of evolution of converging plate boundaries, or places where the crust has been slowly extended. Either way it usually means a long period of crustal heating.

Granite

The main component of continental crust. There is nothing in the earth that can melt directly to yield granite. It forms by repeated melting of rocks and accumulation of certain elements, like potassium, in the melt. Granite is the result of of long continued crustal activity and re-working of rocks. If we ever find granite on another planet, we can be sure that planet has a complex crustal history.

Sedimentary Rocks

Deposited on or Near Surface of Earth by Mechanical or Chemical Processes

Clastic Rocks

• Made of Fragmentary Material
• Deposited by
  • Water (Most Common)
  • Wind
  • Glacial Action
  • Gravity

Biohemical Sedimentary Rocks

• Evaporation
• Precipitation
• Biogenic Sediments

Environmental Clues in Sedimentary Rocks

• Grain Size - Power of Transport Medium
• Grading - Often Due to Floods
• Rounding
• Sorting
• Transport, Reworking
• Cross-bedding-wind, Wave or Current Action
• Fossils
  • Salt Water - Corals, Echinoderms
  • Fresh Water - Insects, Amphibians
  • Terrestrial - Leaves, Land Animals
• Color And Chemistry
  • Red Beds - Often Terrestrial
  • Black Shale - Oxygen Poor, Often Deep Water

Bedding or Stratification

• Almost Always Present in Sedimentary Rocks
• Originally Horizontal
• Tilting by Earth Forces Later
• Variations in Conditions of Deposition
• Size of Beds (Thickness)
  • Usually 1-100 Cm
  • Can Range From Microscopic to 50m

Clastic Rocks

Classified on

• Grain Size
• Grain Composition
• Texture

Sediment Sizes and Clastic Rock Types

• Shale - Clay less than 0.001 Mm
• Siltstone - Silt .001-0.1 Mm
• Sandstone - Sand .01-1 Mm
• Conglomerate - Gravel 1mm +

Sedimentary rocks made of silt- and clay-sized particles are collectively called mudrocks, and are the most abundant sedimentary rocks.

Diagenesis

Compaction

Cementing

• Quartz
• Iron Oxide

Alteration

• Limestone - Dolomite

Recrystallization

• Limestone

Chemical Sediments

Evaporites -Water Soluble

• Halite
• Gypsum
• Calcite

Biogenic Sediments

• Limestone - Shells, Reefs, Etc.

Organic Remains

• Coal
• Petroleum

Fossil Fuels

Coal

Coal is a slam-dunk. It's carbonized wood. We know that because the actual wood fragments are easily visible in low-grade varieties of coal, fossilized wood is often found in adjacent rocks, the overall environment is typical of coastal swamp or delta settings, and ancient soils are sometimes found beneath the coal beds. Organic matter goes through a variety of changes as it becomes coal:

Peat

Compacted and partially decomposed organic matter. About 50% carbon.

Lignite

Brown or gray brittle coal with lots of impurities, and often with easily visible plant fragments. About 80% carbon.

Bituminous

Black with banding. Some bands are dull, others shiny. These bands reflect different types of processed plant matter, which are still visible under the microscope. About 90% carbon.

Anthracite

Black or dark gray, metallic luster and conchoidal fracture. A true metamorphic rock, since it's heated beyond the temperatures found in normal sedimentary burial. About 95% carbon

Graphite

Dark gray and metallic, 100% carbon but unburnable in normal flames.

Diamond

Contrary to popular misconception, diamond is NOT the final stage in coal metamorphism! Coal is never buried deeply enough to reach the pressures needed to form diamond. Diamond form in the earth's mantle from carbon that was always in the earth's interior.

Petroleum

The problem with petroleum is that it's a fluid and moves, so it may migrate far from its source. A typical petroleum molecule looks like this:

So if petroleum is the remains of living things, what sorts of organisms make these molecules? Answer: NONE. If it were that easy, we wouldn't have to look for oil, we'd just toss our garbage into a vat of the right microbes and skim off the petroleum. But lots of organisms make molecules that look like this:

This is called a fatty acid (octanoic acid to be exact). Most petroleum occurs in marine sedimentary rocks, so we want organisms rich in fatty acids that live in the sea, in huge quantities. And we have them. They're called plankton. Marine plankton, not dinosaurs, are the precursors of petroleum.

Being fluid, petroleum moves, and since it's lighter than water, it floats upward. Left unconfined, it will reach the surface and evaporate or be oxidized. So it has to be confined somehow. Contrary to the popular term "oil pool," oil does not collect in pockets in the rock. It floats upward on water until it either reaches the surface or is trapped from above. The most important economic application of an understanding of rocks in three dimensions is the search for petroleum traps.

In the diagram, light blue represents water-soaked porous rock, dark gray represents petroleum and light gray represents natural gas. Like water in an aquifer, the petroleum and natural gas fill pore spaces in the rocks. All other colors represent impervious rocks.   Structural Traps (top) Traps that result from deformation of the rocks by outside forces. Startigraphic traps (bottom) Traps formed by variations within the sedimentary rocks themselves.

Since oil weighs a lot less than rock, the oil in a well weighs far less than the same volume of rock next to the well. Thus in many cases oil is still under pressure when it reaches the surface. In old-time movies, it was common to see the climax come when oil drillers on the verge of quitting hit oil, got a "gusher" and celebrated in the resulting rain of oil.

Fact: the absolute last thing anyone in the oil business ever wants is a gusher. They are incredibly dangerous to get under control. In fact, when PBS did a series on oil, they could not locate a sound recording of a gusher anywhere and had to interview a few surviving old-timers who could remember what one sounded like. It's been that long since one happened.

However, that pressure is extremely valuable because not only does it get the oil to the surface, but it helps move oil through the rocks to the well. Get greedy and drill too many wells, and you bleed off the pressure, and in the long run you get less oil, not more.

Landforms Associated with Sedimentary Rocks

Mesa Flat-topped hill capped with hard hock Cuesta Gently-tilted layer of hard rock: Door Peninsula. The gentle upper slope, on top of the layer is called the dip slope Hogback A sharp ridge of hard rock, edge of a steeply-dipping layer

What Sedimentary Rocks Mean

Clastic Rocks

How powerful was the transport mechanism? Coarse material requires a powerful agency: floods, glaciers, landslides. Sand and silt mean fairly gentle transport.

Mineral Makeup of Clastic Rocks

Easily weathered minerals mean short transit time from source to destination, typically rugged terrain. Presence of only stable minerals means lengthy working and reworking in stable environments.

Carbonate Rocks

Virtually all limestones are biological in origin. Dolostone began as limestone but was altered by removal of calcium and addition of magnesium after deposition.

Evaporites

Indicate high evaporation settings, typically deserts or hot tidal flats (like the present day Persian Gulf)

Locale for Petroleum

Source Rocks: typically shallow marine sandstones
Reservoir Rocks: porous rocks like sandstone, fractured limestone or fossil reefs
Trap Rocks: mudrocks or massive carbonates.

Metamorphism

• Changes in Rock Composition or Texture
• Due to Heat, Pressure and Action of Fluids

Where Does the Heat Come from?

• Uranium and Thorium to Other Elements + Lead + Radiation
• Potassium-40 to Calcium-40 or Argon-40 + Radiation

Where Does the Pressure Come from?

• Air Pressure = 14 P.s.i. (1 Atmosphere or 1 Bar = 100,000 pascals)
• Pressure Beneath 10 Meters (33 Ft.) Of Water = 1 Atm. = 1 Bar.
• Same Pressure Beneath 3.5 M (10 Ft.) Of Rock
• Pressure in Deepest Part of Ocean = 1000 Bar.
• Pressure under One Mile of Rock = 500 Bar.

1000 Bars (2 Mi. or 3 km Of Rock) = 1 Kilobar (Kb.)

How Do Metamorphic Rocks Get to the Surface?

• Tectonic Uplift  and Erosion
• Isostasy (crust floats upward) and Erosion

Types of Metamorphism

Contact

• Around Intrusions
• Shallow: 0-6 Km
• Low Pressure
• Local

Regional

• Wide Areas
• 5-20 Km, Sometimes 30+
• High Pressure
• Usually Accompained by Deformation

What Happens During Metamorphism

Minerals React to Form New Minerals

• 2SiO2 + CaMg(CO3)2 == CaMgSi2O6 + 2CO2
• Quartz + Dolomite == Pyroxene

Minerals Change Form

• Al2SiO5 == Al2SiO5
• Andalusite == Kyanite

New Materials Are Added (Metasomatism)

• CaMg(SiO3)2 + 2CO2 == CaMg(CO3)2 + 2SiO2
• Pyroxene + CO2 == Dolomite + Quartz
• Minerals in Solution == Ore Bodies

Recrystallization

Why Don't Rocks "De-metamorphose"?

• Reactions Can't Reverse Because Ingredients Lost
  • 2AlSi2O5(OH) == Al2SiO5 + 3SiO2 + H2O
  • Clay Mineral == Andalusite + Quartz + Water (Lost)
• An example of carbonate metamorphism:
  • CaMg(CO3)2 + 2SiO2 == CaMgSi2O6 +2CO2
  • Dolomite + Quartz == Pyroxene + CO2 (Lost)
• Reactions "Freeze"
• Sometimes it Does Happen
  • Retrograde Metamorphism

Grade - Degree to Which the Rock Has Changed Composition

• Can Often See Original Bedding
• Can Sometimes Even See Deformed Fossils
• At High Grades, Rocks Can Often Lose All Trace of Their Original Appearance

Major Metamorphic Rock Types

Polymorphism

• Al2SiO5
  • Kyanite
  • Sillimanite
  • Andalusite
• Ice - 6 high pressure forms
• Diamond - Graphite
• Calcite - Aragonite
• Quartz -
  • - Tridymite - Cristobalite (increasing temperature)
  • - Coesite - Stishovite (increasing pressure)

Metamorphic Facies

Not all changes in rocks are metamorphism:

• Surface temperature and pressure --- Weathering
• Sedimentary rocks, surface to 250 C --- Diagenesis

The different rocks and minerals that form during metamorphism indicate different temperature and pressure conditions.

What Metamorphic Rocks Mean

All metamorphic rocks were once far below the surface and got to the surface by uplift and erosion.

Greenschist Metamorphism

Mild metamorphism at moderate depths. Typical on the margins and upper levels of mountain belts

Amphibolite Metamorphism

Strong metamorphism at considerable depths. Typical of the cores of mountain belts

Granulite Metamorphism

Extreme metamorphism. Almost all water-bearing minerals have broken down. These are rocks of the lower crust and signify extreme uplift (30 km and more).

Blueschist Metamorphism

Moderate to great depth but unusually low temperature. Somehow these rocks got very deep and then back to the surface quickly. Typical of plate collision boundaries.

Return to Physical Geology Visuals Index
Return to Professor Dutch's home page

Created February 26, 1997, Last Update 10 September 2003