Physical Geology

The rock cycle and igneous rocks

Rock types: igneous, sedimentary, metamorphic

Isostacy, uplift, burial, erosion

Rock Cycle

Igneous Rock

Bowen's Reaction series

Igneous textures - phaneritic, aphanitic, porphyritic

Now that we know something about what minerals are, it is time to begin looking at the different ways that they form and combine into rocks.

How are minerals made into rocks? How do rocks change?

There are three fundamentally different kinds of rocks. Although each of the three kinds of rocks has a characteristic range of compositions, the three types are based on process (how the rocks are made) rather than composition (what they are made out of).

Igneous (from the L. ingnis - fire)

Igneous rocks are rocks that have crystallized (solidified) directly from a molten state. This is the ultimate origin of all the rock in the earth's crust. Their most general characteristic is that they are made-up of crystals of minerals, although often the crystals are too small to be observed with the naked eye.

Sedimentary (from the L. sedimentum- to settle, from sedere- to sit)

These are rocks that form from the settling into layers of particles of mineral and rock eroded from pre-existing rock. These particles are called sediments and they are carried by wind and water to some kind of a basin where they are deposited into layers or strata (sing. stratum). The layering of sedimentary rocks is expressed on a variety of scales, from mm to meters, and is the most distinctive feature of sedimentary rocks.

Metamorphic (from the G. meta- to change, and morphos- form, shape)

Metamorphic rocks form from pre-existing rocks that have had their texture and mineral composition changed by either heat or pressure. These changes occur without melting, when the rock is still in a solid or semi-solid state. Metamorphic rocks often show combinations of crystals and layering. Metamorphic rocks are also often very deformed, that is they show evidence of being compressed

and folded.

Now, some other basic terminology:

Isostacy - a very important concept for understanding the movement of rock in the crust.

The floating balance between uplift and burial. Because the lithosphere is essentially floating on the aesthenosphere, the higher you pile rock, the deeper it sinks.

Likewise, as you erode away rock and make the crust thinner, the crust will rise as it seeks a new isostatic equilibrium. In this way, deeply buried rock can be slowly moved toward the surface of the earth.

Weathering

The process of breaking down rocks, both mechanically and chemically, mostly due to the processes of erosion.

Burial (or subsidence)

The process of moving rock downward into the earth, either by piling more rock on top of it, or by causing the rock beneath it to sink. Burial removes rock from the process of erosion and subjects it to increasing temperature and pressure.

Uplift

The process of moving rock upward, either by pushing it on top of other rock (which then becomes buried), or by lifting the rock beneath it. Uplift moves rock into the realm of erosion.

The Rock Cycle

Because the Earth is a dynamic, active planet, rock in the crust and mantle is continually being changed from one type into another - in effect being recycled. Recycling is accomplished through the interaction of plate tectonics, uplift, burial and erosion.

Igneous Rocks

All rock has its ultimate origin as igneous rock. Igneous rock begins as molten magma (within the Earth) or lava (erupted onto the surface). Magma begins as molten rock derived from the mantle of the Earth. This magma is a soup of elements rich in silicon, oxygen, iron, aluminum, and others. Molten rock, solid crystals, and gases are present in most magmas. As magma works its way to the surface over time, it reacts with itself and with the rock it is moving through to form and reform a variety of silicate minerals.

Depending on when and where in the Earth the magma finally crystallizes, different types of igneous rock are produced.

Canadian geologist N. L. Bowen determined the sequence of minerals that would form in a melt of mantle composition if it solidified slowly at progressively lower temperatures. Bowen's reaction series shows the order in which minerals crystallize from cooling magma and then react with the magma to form minerals lower on the diagram. In simplest terms, Bowen's reaction series shows that those minerals with the highest melting temperatures crystallize from the cooling magma before those with lower melting points

Notice that there are two simultaneous sequences. One runs from the simple silicate olivine through increasingly complex silicates - pyroxene and amphibole chains to biotite mica sheets.

Likewise, for the 3D silicate feldspars there is a continuous change from Ca-rich to Na-rich plagioclase mineral.

Sodium-rich plagioclase (crystallize later at low temperatures) is associated with silicic rocks whereas calcium-rich plagioclase ( crystallize first at high temperatures) is associated with mafic rocks

In general, magmas have a tendency to become more enriched in silica as they crystallize at lower and lower temperatures. Also, the longer they spend in the crust, the more they are able to absorb more silica, particularly if they are moving through continental crust.

Igneous Rock Textures

Where and how long it takes a magma to crystallize not only affects the mineral composition of the rock formed, but also the texture of the rock.

Texture is defined as the shapes and sizes of the individual mineral grains in a rock.

Textures result from different cooling histories. Phaneritic or coarse-grained texture happens when the rock cools at depth very slowly, allowing time for individual mineral crystals to grow large enough to see with the naked eye.

Aphanitic texture is fine-grained such that the mineral crystals are microscopic due to relatively rapic cooling near the earth's surface.

Porphyritic texture is a combination of the two that results when the magma has two distinct episodes of cooling, one deep and one shallow.

The range of compositions on the chart run generally from dark colored minerals on the right that are characteristic of basaltic magmas, to mixed dark and light minerals characteristic of andesitic magmas, to light colored minerals characteristic of rhyolitic magmas.