Goals of the lab • Observe the different textures of igneous rocks. • Observe the differences in colors of igneous rocks. • Based upon your observations of textures and colors, make inferences about the magma from which these rocks have been formed. • Identify the names of the igneous rocks that you have observed. • Given the name of an igneous rock, suggest how this rock has formed.
Main ideas • Textures of igneous rocks provide evidence about the cooling history and the presence of gases in the magma from which igneous rocks have formed. • Colors of igneous rocks provide evidence about the composition of the magma from which igneous rocks have formed. • Names of the igneous rocks are based on their observed textures and colors. • The names of the igneous rocks imply a history about how these rocks have formed. For example, has the rock been formed by a volcanic eruption at the Earth’s surface? Or has it been formed by the slow cooling of magma within the Earth’s interior.
Procedure • Form teams of students • Each team of students will be given a set of rocks. • Each member of the team is to create a chart that will organize information about each rock • First, you will observe and record the texture of each sample. • Next, you will observe and record the colors of each sample. • Next you will determine the density of the samples and record the information • Based on the texture, color, and density you will make inferences about the composition, cooling history, and presence of gases in the magma from which the rocks have formed. • You will identify the names of the samples of igneous rocks that you are studying. • Given the name of an igneous rock, you will describe the history of how this rock has formed.
Just to make it interesting… • One of the samples is not an igneous rock at all • See if you can figure which one!
Choices to be used while filling in chart: Texture (based on crystal size) Color Light, medium, dark Hue Density Record the # value, plus Light (density value is less than 2) Medium (value is between 2 and 3) Heavy (greater than 3) • Coarse-grained • Fine-grained • Glassy • Vesicular (holes) • Composition • Mafic • Felsic • Intermediate
Choices to be used while filling in chart: Gases History Indicate the following: Intrusive or extrusive Ocean or continental formation location How much time did it take the rock to form? • Are there holes that provide evidence of the presence of trapped gas at time of formation, or not? Rock Names • Select from given list
Textures • Coarse-Grained - minerals are large enough in size that you can see the individual crystals of minerals. • A coarse-grained texture indicates that there has been time for the minerals to grow into larger crystals and thus an inference from this evidence is that the magma has cooled slowly.
Texture • Fine-Grained- minerals are so small in size that you cannot see individual crystals. If you were to look at igneous rocks with this texture using a microscope, you would be able to see the small crystals. • A fine-grained texture indicates that crystals have not had the time to grow into a size that can be seen unless magnification is used and thus the inference is that the magma has cooled relatively rapidly.
Texture Glassy- • the igneous rock looks like glass (very shiny and smooth), there are no individual minerals that can be seen even using a microscope. • A glassy texture indicates thatthere was no time for crystals to grow and thus the inference is that the magma has cooled very quickly. Petrologists and volcanologists can hypothesize that the magma was literally ejected out of the earth as a volcano erupted and it almost instantaneously went from liquid magma to solid rock.
Texture • Vesicular- the igneous rock has holes (called vesicles). These vesicles are holes that were created by the escape of gas bubbles when the rock formed • A vesicular texture indicates that gases must have been present when the magma was erupting at the surface. Volcanologists can infer from this that the magma associated with this volcano must have contained dissolved gases that escaped, leaving behind the holes (or vesicles), in the rock.
How composition relates to origin • Did the igneous rock form from a magma that contains higher proportions of the elements iron and magnesium? These types of magma are associated with igneous rocks that form in the oceans. • Or, did it form from a magma that contains higher proportions of the elements potassium and sodium? These types of magma are associated with igneous rocks that form on the continents.
Color • Light-Colored – the igneous rocks are dominated by lighter-colors such as white, gray, and pink. • Light-colored igneous rocks indicate that the magma from which is was made contained higher proportions of the elements silicon, oxygen, potassium, and sodium. • Igneous rocks that are light-colored are called felsic in composition. Felsic igneous rocks are formed by magmatic activity that has occurred on the continents on land). • Dark-Colored – the igneous rocks are dominated by dark greens and black • Dark-colored igneous rocks indicate that the magma contained higher proportions of the elements iron, magnesium, and calcium. • Igneous rocks that are dark-colored are called mafic in composition. Mafic igneous rocks are formed by magmatic activity that has occurred beneath the oceans.
What if it is neither or a mix? • This is because there is a gradual variation in the composition of magmas from which igneous rocks are formed. Mafic (dark-colored) and felsic (light-colored) igneous rocks are examples of end member compositions. Intermediate composition igneous rocks, with their associated intermediate colors, are also commonly observed.
Igneous rock chart Use back of chart For more space
Rock Names • Match the name to your rock sample based on your observations • Basalt- fine grained , mafic, heavy to medium • Gabbro- coarse grained, mafic, medium to heavy • Granite- coarse grained, felsic, heavy to medium • Obsidian- glassy, mafic, medium • Pumice- vesicular (very small holes), felsic, light • Tuff- fine grained, felsic, light to medium • Scoria – vesicular (larger holes), mafic, medium to light
Applying what you have learned • By Newark, there is an outcrop of rock called the Iron Hill Gabbro. • Rock is 400-500 millions or years old • Based on its name, complete the following
Iron Hill Gabbro • Describe the type of magma and cooling history for this rock in your comp book/notes. • Is a gabbro commonly associated with oceans or continents? • What does the presence of the Iron Hill Gabbro indicate about its location?
History of Iron Hill Gabbro • For example, the Iron Hill Gabbro, a 400-500 million year old igneous rock found in the northwestern portion of New Castle County, Delaware, has a coarse-grained texture and is dark-colored. The coarse-grained texture suggests that the magma from which this rock formed cooled relatively slowly and the dark color is indicative of a mafic composition magma. Thus, petrologists can infer that this rock, now found on land, far from the middle of an ocean basin, must have been formed within the oceanic crust near a mid-ocean ridge spreading center. Delaware at that time was near an active plate boundary beneath about a mile to a mile and a half of ocean water!
Check in • What information can be inferred from the texture of an igneous rock? • What information can be inferred from the color of an igneous rock?