Upon Completion of This Lab You Will Be Expected To

GLS100 Lab - Identification and Interpretation of Igneous Rocks
Physical Geology– Dr. Lindley Hanson

Objectives:

Upon completion of this lab you will be expected to:

1.  Describe the economic significances of igneous rocks

2.  Identify and describe environments of igneous rock formation

3.  Define igneous rock textures and assign them to their environment of formation

4.  Identify the four broad classes of magma and the criteria used to define them

5.  List and explain the criteria used to classify rocks

6.  Identify igneous rocks

7.  Discuss the various eruptive styles of lava

8.  Infer eruptive hazard from existing rock

WHY ARE IGNEOUS ROCKS IMPORTANT?

“Civilization exists by geological consent, subject to
change without notice.”

-Will Durant, US historian (1885-1981)

Invaluable resources occur in igneous rocks. Important metallic minerals such as gold, silver, copper, nickel, lead, mercury, and molybdenum are mined from them, as well as non-metallic minerals such as quartz, mica, and feldspar. Diamonds blown up from the mantle are found in volcanic vents called diatremes. Pumice is used as a light-weight aggregate in concrete and plaster, and as an abrasive. Granite and gabbro are abundantly used as curbing, dimension stone, countertops, facing stone, stairs and paving.

Geologists study the location and composition of igneous rocks to reconstruct past volcanic arcs and to track the shifting positions of tectonic plates. But more importantly, volcanic deposits reveal the past and future behavior of active and seemingly dormant volcanoes. We know from their deposits that volcanoes such as Yellowstone, Tambora, Krakatau, and Toba are capable of producing global eruptions that will drastically alter society.

IGNEOUS ENVIRONMENTS
Igneous rock form by the cooling and solidification of molten rock material called magma. Magma cools in a variety of settings (fig.1) creating a wide array of textures. Rocks formed on the surface are called extrusive and those formed beneath the surface are intrusive, and within each of these categories are two more.

PRE-LAB EXERCISE 1: IGNEOUS ENVIRONMENTS

Define each of the four cooling environments on the left and match them with the diagram on the right. (You may have to refer to your text.)

IGNEOUS TEXTURES

Texture refers to the features that control the appearance of the rock, such as the size and shape of crystals, vesicles, fragments and any other components. Textures of igneous rocks (fig. 2) primarily reflect their cooling environment. However, composition and gas content also play a role. Glassy textures develop when magma cools too rapidly to form minerals. Gas-rich lava will froth and/or be propelled into the air.

Violently explosive eruptions are typical of felsic lavas, which are notoriously viscous and gas-rich. Such eruptions produce frothy bombs of pumice and deposits of fine glassy ash called tuff. Once all gas is released by an eruption, any remaining lava will cool around the vent forming a massive, felsic-glass (obsidian) plug.

Basaltic lavas on the other hand are less violent because of their greater fluidity and lower gas content. Extruding mafic lavas form frothy pieced of dark scoria and finely-crystalline, sheet-like lava flows of basalt.

Magmas that cool slowly crystallize, and crystal size reflects the length of time spent cooling. Rocks cooled a kilometer or more beneath the surface will have noticeably visible crystals (phaneritic texture), while those cooled in thick basaltic lava flow or narrow hypabyssal dike will typically develop fine crystals, visible only with a microscope (aphanitic). If magma starts crystallizing deep down and is subsequently carried upward into a shallower environment the resulting rock will exhibit two distinct crystal sizes (porphryritic), the larger more visible crystals are called phenocrysts. Texture can say a lot about a rock if you know how to read it.

PRE-LAB EXERCISE 2: IGNEOUS TEXTURES

Match the textures in the middle with the correct environment of formation described on the left. Then match the hand samples (right) shown in class with the texture and description.

NOTES:

MAGMA AND ROCK COMPOSITIONS

When rocks melt, magmas are created, and because there are many types of rocks there is a range of magmas. Basically, magmas are placed into compositional ranges (fig. 3) defined by their silica content (SiO2), which best indicates their source. For example, felsic magmas are produced from melting continental rocks, rich in silica and poor in iron and magnesium. Mafic magmas having less silica and more iron and magnesium are produced from melting oceanic crust or partial melting of mantle rocks. Intermediate magmas are largely created by mixing or differentiation, and compositionally lie between felsic and mafic magmas. Finally, ultramafic rocks, which are mantle-derived, have the lowest silica and greatest iron and magnesium content. As you can see in figure 3, igneous rock compositions lie along a broad continuum ranging from ultramafic (left) to felsic (right). Note also that the abundance of less important elements, such as calcium (Ca), sodium (NA), and potassium (K) also varies.

Because all minerals have unique chemical compositions they require a specific abundance of elements to form. Therefore it seems reasonable that the minerals crystallizing from a melt will be controlled by its composition. The characteristic mineral assemblages related to each magmatic range are shown in figure 3. The crystalline rocks, composed of these are shown in the top two rows of figure 3. Note that the glassy rocks, which cool too fast to crystallize, are not shown. Their elemental composition will be the same as the lava they formed from, and are shown at the bottom of the classification chart in figure 4. In hand sample their general composition can be inferred from their color and texture.

CLASSIFIYING AND INTERPRETING IGNEOUS ROCKS

All rocks are classified on the basis of texture and composition. Using figures 3 and 4 classify the specimens listed in Table 1. Draw samples marked with an asterisk and label their identifying features. From the color and/or mineral composition determine the composition as either felsic, intermediate, mafic or ultramafic. From the texture determine whether the rock is intrusive or extrusive.

LAB EXERCISE 1: CLASSIFICATION

Directions

Texture:

Identify the texture. Use figure 2 for reference.

Composition:

Check whether the rock is ultramafic (UM), mafic (M), intermediate (I) or felsic (F). The general composition can be inferred from the color: UM=green; M=black; I = gray; felsic buff, tan, gray or maroon. Gray porphryric-aphanitic rocks are best identify from their phenocryst.

Identify any visible minerals: Figure 3 will tell you which minerals to expect in a rock of a certain color (composition). You need to determine which minerals are actually present in your sample. Only identify the minerals you can see, such as the phenocrysts in a porphyritic rock or grains in a phaneritic rock.

Illustration:

Draw the rocks marked by the asterisk and detail your observations.

Rock name:

Identify the rock, using figures 3 or 4. However, keep in mind that figure 3 is only for the crystalline rocks and does not include the glassy rocks.

Environment:

From the texture interpret whether the sample is intrusive (INT) or extrusive (EXT). If you can fine-tune your interpretation place a(n) H (hypabyssal), P (plutonic), Py (pyroclastic), or E (effusive) in the appropriate (INT or EXT) column.

NOTES:

Table 1. Data and interpretation.

ERUPTIVE STYLE OF VOCANOES

The volcanic explosivity index (VEI) classifies volcanoes by their explosiveness and volume of material ejected. Mega-colossal (VEI = 7-8) eruptions spew vast amounts of ash and sulfur dioxide into the stratosphere where it circles the globe causing conditions akin to a nuclear winter. Volcanoes listed high on the scale present a clear danger to society. For example, the eruption of Toba 71,000 years ago drove the human population close to extinction. Several geneticist believe we are all descended from the mere ~1000 breading pairs that survived the eruption. The sudden collapse of the advance Minoan Civilization on Crete is attributed to 1630 BC eruption of Santorini in the Aegean Sea. David Keyes (1999) in his controversial book Catastrophe: An Investigation into the Origins of the Modern World links the Dark Ages, the historic bubonic plague, the disintegration of the Byzantine Empire, and the spread of Islam to an eruption of Krakatau around 535 AD. More recently, the 1815 eruption of Tambora in Indonesia caused the “year without a summer” that produced global famine. In New England, the unusually cold summer of 1815 resulted crop failure forcing many families to leave the region and head West. Yellowstone in Montana has a violent prehistoric past, and all evidence indicates that it will erupt again. Although mega-colossal eruptions are few and far between, we cannot ignore the potential threat they impose. For this reason, many volcanoes that we deem dangerous from the analysis of their past deposits are being closely monitored.

LAB EXERCISE 2: EXPLOSIVITY INDEX

1. Notice that the names of the VEI=8 volcanoes are lakes. In fact, even Yellowstone contains a large lake. How do you explain this phenomenon?

2. Some of the samples used in the lab were taken from the areas listed on the right. For each sample write the rock name, composition, and VEI Index.

3. State and explain any correlations observed between composition, texture and VEI index.

1