Associate Program Material

History of Rock Worksheet

Write a 500- to 750-word explanation regarding the role of plate tectonics in the origin of igneous rocks.

| ORIGIN OF IGNEOUS ROCKS AND THE ROLE OF PLATE TECTONICS |
|Igneous rocks form in three main places: where lithospheric plates pull apart at mid-ocean ridges, where plates come together at |
|seduction zones and where continental crust is pushed together, making it thicker and allowing it to heat to melting. There are two|
|ideas about igneous rocks that are geologically important. The first idea is that igneous rocks evolve - they change from one kind |
|of rock into another. The second idea is that rocks are not randomly distributed across the earth. Specific kinds of rocks are |
|always found in specific places for specific reasons, all tied into plate tectonic processes. Igneous rocks begin as hot, fluid |
|material, and the word "igneous" comes from the Latin for fire. This material may have been lava erupted at the Earth's surface, or|
|magma (un-erupted lava) at shallow depths, or magma in deep bodies (plutons). People commonly think of lava and magma as a liquid, |
|like molten metal, but geologists find that magma is usually a mush — a liquid carrying a load of mineral crystals. Magma |
|crystallizes into a collection of minerals, and some crystallize sooner than others. Not just that, but when they crystallize, they|
|leave the remaining liquid with a changed chemical composition. When a body of magma, as it cools, evolves, and as it moves through|
|the crust, interacting with other rocks, it evolves further. This makes igneous petrology a very complex field. Tell the three |
|types of igneous rocks apart by their texture, starting with the size of the mineral grains. Extrusive rocks cool quickly (over |
|periods of seconds to months) and have invisible or very small grains, or an aphanitic texture. Intrusive rocks cool more slowly |
|(over thousands of years) and have small to medium-sized grains. Plutonic rocks cool over millions of years, deep underground, and |
|can have grains as large as pebbles — even a meter across. Both intrusive and plutonic rocks have phaneritic texture. Igneous rocks|
|are classified by the minerals they contain. The main minerals in igneous rocks are hard, primary ones: feldspar, quartz, |
|amphiboles and pyroxenes (together called "dark minerals" by geologists), and olivine along with the softer mineral mica. The two |
|best-known igneous rock types are basalt and granite, which differ in composition. Basalt is the dark, fine-grained stuff of many |
|lava flows and magma intrusions. Its dark minerals are rich in magnesium and iron, hence basalt is called a mafic rock. So basalt |
|is mafic and either extrusive or intrusive. Granite is the light, coarse-grained rock formed at depth and exposed after deep |
|erosion. They can be found in the deep sea floor (the oceanic crust) is made of basaltic rocks, with ultramafic rocks underneath. |
|Basalts are also erupted above the Earth's great subduction zones, either in volcanic island arcs or along the edges of continents.|
|However, continental magmas tend to be less basaltic and more granitic. The continents are the exclusive home of granitic rocks. |
|Nearly everywhere on the continents, no matter what rocks are on the surface, you can drill down and reach granitoid eventually. In|
|general, granitic rocks are less dense than basaltic rocks, and thus the continents actually float higher than the oceanic crust on|
|top of the ultramafic rocks of the Earth's mantle. The behavior and histories of granitic rock bodies are among geology's deepest |
|and most intricate mysteries. |

Write a 500- to 750-word explanation regarding the role of plate tectonics in the origin of metamorphic rocks.

| ORIGIN OF METAMORPHIC ROCKS AND PLATE TECTONICS |
|Metamorphic rocks form when rocks change after undergoing extreme pressure or temperature increase. These temperature changes must |
|be hot enough to reorganize matter within the rock but not hot enough to melt it. Hot magma pushes itself to the surface at both |
|divergent plate boundaries and convergent plate boundaries. This magma comes in contact with rocks as it rises to the surface. The |
|magma is hot, heating the rocks around it. As the rocks heat, they change and become metamorphic rocks. This process is called |
|contact metamorphism. During metamorphism of rocks, most structural and textural features in the original rock—such as |
|stratification, graded bedding, vesicles, and porphyritic textures are destroyed. New minerals replaced those originally in the |
|rock to create a new rock texture. These are metamorphic rocks, a major group of rocks that results largely from the constant |
|motion of tectonic plates. Metamorphic rocks can be formed from igneous, sedimentary, or even previously metamorphosed rocks. |
|Regional metamorphism occurs at convergent plate boundaries, due to intense pressure. So when two plates collide, the Earth's crust|
|folds and faults, the intense pressure changes large areas of the Earth's crust into metamorphic rock. Mountain ranges are |
|typically metamorphic rock, due to plate tectonic processes. Metamorphism occurs while the rock remains solid, if it reaches a |
|point where the rock melts; it then becomes an igneous rock. Other rocks recrystallized and developed large mineral grains, and the|
|constituent minerals of many have strong fabrics with planar orientations called foliation. These are the hallmarks of |
|recrystallization in the solid state, a process we call metamorphism. The result is a new rock type with a distinctive texture and |
|fabric and, in some cases, new mineral compositions. The minerals in the metamorphic rocks did not crystallize from magma, but they|
|are stable only at high temperatures and pressures found deep in the crust. Light-colored dikes and sills of igneous rock cut the |
|metamorphic rocks. During metamorphism, new platy mineral grains grow in the direction of least stress, producing a planar texture|
|called foliation. Rocks with only one mineral (such as limestone) or those that recrystallize in the absence of deforming stresses |
|do not develop strong foliation but instead develop a granular texture. Mylonite develops where shearing along a fracture forms |
|small grains by ductile destruction of larger grains. The major types of foliated metamorphic rocks include slate, schist, gneiss, |
|and mylonite; important non-foliated (or granular) rocks include quartzite, marble, hornfels, greenstone, and granulite. They are |
|distinguished by their textures and secondarily by their compositions. |