Geological Processes

The foundation of modern geology: Continental Drift: Earth's lithosphere (crust and upper mantle) is divided into tectonic plates that move slowly over the asthenosphere. This movement, measured in centimeters per year, drives most geological processes. Plate Boundaries: Where plates meet, three types of interactions occur: - Divergent: Plates move apart, creating mid-ocean ridges and rift valleys. Magma rises, forming new crust. - Convergent: Plates collide, one subducting beneath the other. Creates mountain ranges, volcanoes, and metamorphic zones. - Transform: Plates slide past each other, creating faults like the San Andreas. Mountain Building: Orogeny occurs when continents collide or when oceanic plates subduct beneath continents. This creates the world's mountain ranges and exposes deep rocks at the surface. Rockhounding Impact: Plate boundaries create diverse geological environments. Subduction zones produce volcanic rocks and associated minerals. Collision zones expose metamorphic rocks and pegmatites.

Formation of rocks from molten material: Magma Formation: Magma forms when rock melts, typically due to increased temperature (from depth or friction), decreased pressure (as rock rises), or addition of water (lowers melting point). Intrusive Igneous: Magma that cools slowly underground forms large crystals. Granite, gabbro, and pegmatites are examples. Pegmatites often contain large, well-formed crystals of rare minerals. Extrusive Igneous: Lava that erupts and cools quickly forms fine-grained or glassy rocks. Basalt, obsidian, and pumice are examples. Some volcanic rocks contain gem-quality crystals. Fractional Crystallization: As magma cools, different minerals crystallize at different temperatures. Early-formed crystals may settle, changing the remaining magma's composition. This creates diverse rock types from a single magma source. Volcanic Activity: Eruptions bring deep materials to the surface. Volcanic rocks often contain interesting minerals, and volcanic ash can preserve fossils.

Formation of rocks from accumulated materials: Weathering: Physical and chemical breakdown of rocks at the surface. Creates sediment particles of various sizes - from boulders to clay. Erosion and Transport: Water, wind, ice, and gravity move sediment. During transport, particles are sorted by size and rounded by abrasion. Deposition: Sediment settles when transport energy decreases. Different environments (rivers, lakes, oceans, deserts) create different sedimentary deposits. Lithification: Sediment becomes rock through compaction (weight of overlying material) and cementation (minerals precipitate between grains, binding them together). Types of Sedimentary Rocks: - Clastic: Formed from rock fragments (sandstone, shale, conglomerate) - Chemical: Formed from dissolved minerals (limestone, rock salt, gypsum) - Organic: Formed from biological materials (coal, some limestones) Fossil Formation: Sedimentary rocks preserve fossils. The best rockhounding for fossils occurs in sedimentary formations.

Transformation of existing rocks: Metamorphism: Changes in mineralogy, texture, and sometimes composition due to heat, pressure, and chemically active fluids. The original rock (protolith) is transformed but not melted. Regional Metamorphism: Large-scale metamorphism affecting vast areas, typically associated with mountain building. Creates schist, gneiss, and other foliated rocks. Often produces beautiful crystals. Contact Metamorphism: Occurs when hot magma intrudes cooler rock. Creates a "baked" zone (aureole) around the intrusion. Can produce interesting minerals like garnet, andalusite, and cordierite. Hydrothermal Metamorphism: Hot, mineral-rich water alters rocks and deposits new minerals. Creates many ore deposits and can produce beautiful crystals in veins and cavities. Metamorphic Grade: Intensity of metamorphism, from low (slate) to high (gneiss, migmatite). Higher grades often produce larger, better-formed crystals. Rockhounding Value: Metamorphic rocks often contain well-formed crystals, interesting textures, and sometimes gem-quality materials.

Processes that shape the Earth's surface: Weathering: Breakdown of rocks in place. Physical weathering (freeze-thaw, root growth) and chemical weathering (dissolution, oxidation) create soil and expose fresh rock surfaces. Mass Wasting: Downslope movement of rock and soil due to gravity. Landslides, rockfalls, and debris flows expose new rock and can concentrate heavy minerals. Stream Processes: Rivers erode, transport, and deposit sediment. They expose rock layers, create terraces, and concentrate heavy minerals in placer deposits (important for gold prospecting). Glacial Processes: Glaciers erode bedrock, transport material, and deposit it as they melt. Glacial deposits (till, outwash) can contain interesting rocks and minerals from distant sources. Wind Processes: In arid regions, wind erodes and deposits sand. Creates dunes and can polish and round rocks. Desert varnish (dark coating on rocks) is a wind-related process. Coastal Processes: Waves, tides, and currents shape coastlines. Beach deposits can contain interesting rocks and minerals.

How specific minerals form: Crystallization from Magma: As magma cools, minerals crystallize in a specific order (Bowen's reaction series). Early minerals (olivine, pyroxene) are mafic; later minerals (quartz, feldspar) are felsic. Precipitation from Solution: When water becomes supersaturated, minerals precipitate. This forms many sedimentary minerals and creates crystals in geodes and veins. Metamorphic Recrystallization: Existing minerals recrystallize into new forms under heat and pressure. Can create larger, better-formed crystals. Hydrothermal Deposition: Hot water dissolves minerals from one location and deposits them elsewhere as it cools. Creates many ore deposits and beautiful crystal formations. Biological Processes: Some minerals form through biological activity. Shells create limestone; some bacteria create iron minerals. Weathering Products: New minerals form as rocks weather. Clay minerals, iron oxides, and other secondary minerals are common weathering products.