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:

Geologists usually think of rocks in a second important way, however. Please memorize this 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:

The material that makes up any rock might have a complex history.

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:

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.

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:

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:

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

How do magmas form? Three factors influence melting point:

In detail:


Here's a paradox: Even though peridotite is typically what melts to make magma, hardly any magma ever solidifies into peridotite. WTF?

NOTE: 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?

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.

Process differences in igneous rocks:

Texture: Depending on how quickly they cool, igneous rocks can show two basic textures:

Fine points of texture:

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 MEMORIZE 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.

Key concepts and vocabulary: