Section 4 - Igneous, Sedimentary, Metamorphic Rocks

Section 4
  1. Igneous Rocks
  2. Sedimentary Rocks
  3. Metamorphic Rocks

Igneous Rocks - Lecture Notes

As one descends deeper into the earth's surface, the temperature rises.  This temperature rise is called the geothermal gradient and has an average value of 30oC.  Scientist believe this heat source originates from radioactivity or the initial coalescence of the earth from a gaseous cloud.  The Jovian planets have a heat imbalance mystery.  Such that, they emit more radiant energy than they absorb from the Sun.
    Note:  Many rocks melt at 1500oC (or 2,757oF) and room temperature is about 23oC.

     Molten rock in the ground is called Magma, extruded onto the surface it is called Lava.  If it is blown out of a volcano, it is called pyroclastic debris.  All igneous rocks have been formed from the solidification of molten rock.

As a general rule, both the melting (solid to liquid) and crystallization (liquid to solid) for rocks into magma and magma into rocks are complex.

Bowen's Reaction Series provides us with a glimpse of how molten rock solidifies:

"Differences in end products depend on the rate at which the magma cools and on whether early formed minerals remain in or settle out of the remaining liquid during its crystallization."

There exists a discontinuous reaction series where forming minerals will change their crystalline structure and a continuous reaction series where forming minerals keep the same crystalline structure.

Keep in mind:  First to crystallize (or solidify) have a high temperature melting point.  Last to crystallize have a low temperature melting point.


A Simplified View

Some types of igneous rocks (and magma) "prefer" one tectonic setting as opposed to another.  Basaltic (or mafic) magma has a high iron and magnesium content.  This magma is usually associated with Shield volcanoes and has a dark color.
Andesitic magma has a mixture of iron, magnesium, sodium, and silica (SiO4).  It is intermediate between Basaltic and Granitic magma.
Granitic (or sialic) magma has a high concentration of Silicon and Aluminum.  It is usually associated with composite volcanoes and has a light color. 
(Page 63 in Tarbuck and Lutgens has a good graph.)

The texture of igneous rocks are often used in identification.  The texture is determined by the size, shape, and arrangement of the interlocking mineral grains.  The most important factor influencing the texture of igneous rocks is the rate of cooling for the magma.

Igneous Rock Textures
Phaneritic (or granular) texture has large mineral grains from slow cooling (usually below the earth's surface).  The grains can clearly be seen with the un-aided eye.
Aphanitic texture occurs from rapid cooling and consists of individual minerals so small that they cannot be identified without a microscope. 
Glassy texture consists of ions disorganized as in a liquid but frozen in place by an extremely rapid cooling.
Vesicular (can be aphanitic and vesicular) rocks are very porous because of gas bubbles in the magma or lava when it cooled.
Porphyritic is a mixture of large mineral grains in an aphanitic or glassy goundmass (smaller crystal material).
Pegmatitic textured rocks have exceptionally large granular mass of crystals formed by hydrothermal solutions late in the cooling process.

Igneous Rock Formations
[Terminology: a pluton is igneous rock formation that has solidified underground, tabular means that it is flat (i.e. width is significantly less than the length), discordant implies that the igneous rock formation cuts perpendicular to existing rock layers, and concordant igneous rock cuts parallel to existing rock layers.]
Sill are concordant tabular plutons.
Dikes are discordant tabular plutons.
Lopoliths are concordant tabular plutons shaped like a spoon (sags downward).
Laccoliths are massive concordant plutons with domed tops.
Batholiths are massive plutons (10-15 km thick) having no particular shape.  (El Capitan and Half Dome in Yosemite National Park are part of the Sierra Nevada Batholith.)

Sylvan Lake
Sylvan Lake in Custer State Park, South Dakota.  These formations are mostly granite.  (For more information on the geology of this area see the Geology of South Dakota fact sheet.)

Igneous Rocks - Related Web Links
Dr. Andy Frank's Physical Geology Igneous Rocks
Igneous Rock lecture and Ancient Lava Flows / Plutons from Dr. Pamela Gore
Geology 41 at Duke University - Igneous Activity and Metamorphic Rocks (Part I and Part II)
Dr. Susan DeBari's lecture notes on Igneous Rocks
Google - Search for Igneous Rocks

Sedimentary Rocks - Lecture Notes

About 95% of the outer 10 km is made up of crystalline rock (igneous and metamorphic), 5% is sedimentary.  However, sedimentary rocks make up about 75% of the rocks exposed at the surface.
Note:  Most rocks that can be seen on the Red Cedar trail in Menomonie, WI, are sedimentary of pre-cambrian time.

Sedimentary Rocks are divided into two categories:
Detrital (terigenous or clastic) sedimentary rocks are derived from the weathering of pre-existing rocks which have been transported to a depositional basin.
Non-detrital (or chemical) are produced by chemical or biological processes.

Method of sediment transportation (annual delivery to oceans):
    Rivers - 10 billion tons
    Glaciers - 100 million-1 billion
    Wind - 100 million
    Extraterrestrial - 0.03-.3 tons (meteors, etc.)

Material can be deposited when its agent of transportation no longer has sufficient energy to keep it moving.  Material can also be precipitated out of a solution.  Dissolved material is converted to a solid.

Detrital Sedimentary Rocks have a clastic (or broken, fragmental) texture consisting of

  1. Clasts - large pieces such as sand or gravel
  2. Matrix - mud or fine grain sediments (this surrounds the clasts)
  3. Cement - calcite, iron oxide, silica

The most common detrital rocks (in order of decreasing size of clasts) are Conglomerate or Breccia, Sandstone, and Mudstone or Shale.  The clasts within a breccia are sharp as compared to the conglomerate clasts which are rounded.  This suggests a difference in weathering and/or transporting.

Wentworth Scale of Particle Sizes


>256 mm



Pebble, Gravel



(decreasing in size)






<0.004 mm

Chemical/Biochemical Sedimentary Rocks

Evaporates form from the evaporation of water.  Examples: rock salt of Halite, Rock Gypsum, Travertine (caves and hot springs)
Carbonates mainly composed of limestones and dolostones.  (reacts with HCl)
Siliceous rocks are those which are dominated by Silica (SO2).  They commonly form from silica secreting organisms such as diatoms, radiolarians, or some type of sponges. Examples: Diatomite, Chert

The process that converts unconsolidated rock-forming materials to consolidated is lithification.  Diagenesis is the term describing physical, chemical, or biological changes in the rock before any significant amount of heat or pressure is applied.

Features of sedimentary rocks
Bedding planes - usually horizontal, they represent changes in the depositional environment.
Sedimentary Facies - the characteristics of a unit of sediments which can be used to interpret the depositional environment.
Ripple Marks, Mud Cracks

"Organic" Sedimentary Rocks:  Peat, Lignite, Bituminous (coal, low carbon, sooty), Anthracite (coal, high carbon, not sooty, metamorphic rock?)

Sedimentary Rocks - Related Web Links

Dr. Andy Frank's Physical Geology Sedimentary Rocks
Dr. Pamela Gore's lecture notes on sedimentary rocks
Dr. Susan DeBari's lecture notes on sedimentary rocks I and II
Geology 41 at Duke University - Sedimentary Rocks With Examples of Textures and Sedimentary Structures
Google - Search for Sedimentary Rocks

Metamorphic Rocks - Lecture Notes

Metamorphism is defined as a set of processes involving

and chemical fluids

in which rocks undergo a change in mineralogy, texture, or both.  A contact metamorphism occurs when rocks get close to molten rock (but does not undergo melting).  This metamorphism can be found surrounding most igneous rock formations.  When a large region of rocks get exposed to high heat and pressure, it is a regional metamorphism.  This happens when rocks are buried to a great depth.

Some regional metamorphism is associated with colliding plate boundaries.

Metamorphic Textures can be either foliated or non-foliated.
Foliated metamorphic rocks contain mineral grains that have been flattened.  The grains form an elongated, rod-like appearance.  There are several types of foliation: schistosity, slaty cleavage, gneissic banding.  Examples: Slate, Phyllite, Schist, Gneiss (click on graph below for pictures)
Non-foliated metamorphic rocks contain mineral grains that are equidimensional.  Examples: Marble, Quartzite

A low grade metamorphic rock is generated in a relatively low temperature and pressure conditions.  A high grade metamorphic rock is generated in high temperatures and pressures.  The grade of metamorphism is similar to the metamorphic facies.  [Note:  Generically, a facies is an assemblage of mineral or rock features reflecting the environment in which the rock was formed.]  (see pg. 166 in Tarbuck and Lutgens)

Example Progression from Low to High Grade Metamorphism

Example Progression from Low to High Grade Metamorphism

Geology Satire, A story read by Dr. Scott (audio, 3.7 Mb)

Metamorphic Rocks - Related Web Links

Dr. Andy Frank's Physical Geology Metamorphic Rocks
Dr. Pamela Gore's lecture notes on metamorphic rocks
Dr. Susan DeBari's lecture notes on metamorphic rocks
Geology 41 at Duke University - Igneous Activity and Metamorphic Rocks (Part I and Part II)
Google - Search for Metamorphic Rocks

Review Quizzes
Section 4

Review Quiz Section 4
Tarbuck and Lutgens, Essentials of Geology, Self-Quiz (Select Chapter Igneous Rocks, Metamorphic Rocks, or Sedimentary Rocks)
Igneous Rocks
- Dr. Andy Frank's Practice Exam
Sedimentary Rocks - Dr. Andy Frank's Practice Exam
Metamorphic Rocks - Dr. Andy Frank's Practice Exam
Igneous Rocks and Processes - North Dakota State University Self-Test, Geology 120
Metamorphic Rocks - North Dakota State University Self-Test, Geology 120
Sedimentary Rocks - North Dakota State University Self-Test, Geology 120