Introduction to Rocks

1. Introduction to Rocks Solid material aggregates (minerals, glass, organic material) that make up our Earth

2. The Study of Rocks • Petrology: • General term for the study of rocks • Petrography: • Description and classification of rocks • Largely observational • Petrogenesis: • History and origins of rocks • Uses inductive and deductive reasoning, experimentation, and theoretical analyses

3. The Major Classes of Rocks • The Rock Cycle • Simplified model of how rocks form • Natural rocks are a continuum, gradations between classes

4. The Major Classes of Rocks • Igneous rocks: • Rocks formed by the solidification of molten rock material (liquid ± crystals ± gasses = magma) • Volcanic: at or near the earth’s surface • Plutonic: well below the earth’s surface

5. The Major Classes of Rocks • Sedimentary Rocks • Formed at surface conditions through • Accumulation of particulate materials (clastic) • Chemical or biochemical precipitation (chemical) • A combination of these processes

6. The Major Classes of Rocks • Metamorphic Rocks • Pre-existing rocks (igneous, sedimentary or metamorphic) modified mineralogically or texturally or both in response to • Heat • Pressure • Directed stresses • Chemically active fluids Without melting

7. Core = 16% (Fe-Ni) Mantle = 83% (meta/ig) Crust = < 1% Crust: Total volume is ~95% ig-met, ~5% sed rock Of exposed land surface 66% sedimentary 34% igneous and metamorphic (1/2 each) Rock Distribution in the Earth _ | | | Of total | Earth volume | _|

8. Distinction of the Major Classes of Rocks • Petrography and petrogenesis are interconnected! • Rock classification is based on processes of formation resulting in both: • Distinct textures and structures (microscopic and megascopic) • Distinct composition (mineral associations) • Rock formation is tied to specific tectonic settings and regimes (remember the Wilson cycle!)

9. Distinction of the Major Classes of Rocks Distinct textures and structures • Texture = physical character of rock (size, shape, orientation, distribution of grains; intergrain relationships) • Structure = arrangement of features of rock larger than grains (layers, holes, fractures, contact relationships) • Both microscopic and megascopic • Examples: • Horizontal layering, clastic (aggregate of clastic particles) texture: Sedimentary rocks • Cross cutting relationships, interlocking crystalline texture: Igneous rocks • Interlocking crystalline texture either eqi-granular/ isotropic or aligned/foliated; outcrop scale planar or lineated fabric: Metamorphic rocks

10. Distinction of the Major Classes of Rocks • Distinct composition (mineral associations) reflect conditions of formation • Temperature • Pressure • Chemical conditions

11. Distinction of the Major Classes of Rocks • All processes that produce rocks occur at specific sites within or at margins of tectonic plates • Rock-forming sites have specific conditions (temperature, pressure, chemical conditions) that control the resulting rock features (textures, structures, mineral assemblages) = petrotectonic assemblages • Features can be used to understand history and evolution of an individual rock, a region, or the entire Earth

12. Plate tectonic processes Fractionate (separate into discrete fractions of different compositions) earth materials Create rocks with different compositions Increase the diversity of rocks with time The Tectonic Rock Cycle