Goals for this lab Learn the basics of rock identification Learn how to distinguish between 3 rock types Observe hand samples and infer how they might have been formed
*Solid aggregate of mineral grains, mineral crystals, or other rocks *Some exceptions **Obsidian is made of volcanic glass **Coal is made of plant fragments *The materials forming rocks come from the Earth’s mantle as magma, from space, from organisms, or from the breakdown of other rocks and minerals.
*Environmental changes and processes affect the rock forming materials and existing rocks *These changes and processes produce 3 distinct groups of rocks **IGNEOUS **SEDIMENTARY **METAMORPHIC
What is an igneous rock? Crystalline or glassy rocks formed from the cooling and solidification of molten magma (below Earth’s surface)/lava (on Earth’s surface) Compose the majority of the earth Can use the texture and mineralogy of these rocks to determine where in the Earth they formed
Igneous Rock Textures Where the rock forms in the Earth and how quickly it cools determines what kind of texture it will have
Cooling Rates and Igneous Textures The slower the crystals form, the larger they will be. Deep = Hot = Slow Cooling= Large Crystals = Phaneritic Shallow = Cooler = Fast Cooling = Small Crystals = Aphanitic Cooled slowly and then abruptly brought near surface and cooled quickly = both large and small crystals = Porphyritic Cooled extremely quickly = Glassy Cooled quickly and bubbles present = Vesicular Explosive welding of materials from volcanism = Pyroclastic/Fragmental
Igneous Rock Textures Glassy Vesicular Pyroclastic/Fragmental
Mineralogy cont. Felsic rocks dominated by K-feldspar, Na Plagioclase, quartz, and biotite usually light in color typical of continental crust (Granite and Rhyolite) Intermediate rocks dominated by plagiocase, amphibole, pyroxene, biotite, quartz intermediate color Andesite and diorite Mafic rocks Dominated by Ca-Plagioclase, pyroxene, olivine, amphibole Usually dark in color Typical of oceanic crusts (and the Moon, Mars, and Venus!) (Basalt, Gabbro) Ultramafic rocks Dominated by olivine, minor amounts of pyroxene and Ca-plagioclase Rarely seen on Earth’s surface Major constituent of Earth’s Mantle Peridotite
Sedimentary Rocks Formed by surface processes Sediments are formed from weathering and erosion Weathering = chemical and physical processes that break up rocks into fragments of various sizes Erosion = set of processes that loosen soil and rock and move them
Sedimentary Rocks Loose sediments form sedimentary rocks through the process of lithification Lithification = converts sediment into solid rock by Compaction = grains are squeezed together by weight of overlying sediment into a mass denser than original Cementation = minerals precipitate around deposited particles and bind them together
Sedimentary rocks Metamorphic rocks Plutons Desert Playa lake Delta Glacier The sedimentary stages of the rock cycle Weathering breaks down rocks. Erosion carries away particles. Transportation moves particles downhill. Deposition occurs when particles settle out or precipitate. Diagenesis lithifies the sediment to make sedimentary rocks. Burial occurs as layers of sediment accumulate.
Sedimentary Rock Texture Step 1 in identifying a Sedimentary Rock 3 types Detrital/Siliclastic – rock made of fragments of other rocks Biochemical/Bioclastic – composed of organically derived material Chemical – sedimentary rocks precipitated out of solution
Step 2 in Identifying Sedimentary Rocks Detrital Rocks
Oil and gas Organic Matter Coal Pressure Heat to 90° - 120° C Heat to 90° - 120° C Bioclastic Rocks
Metamorphism Metamorphism is the solid-state transformation of a protolith (parent or pre-existing rock) into texturally or mineralogically distinct new rock as the result of high temperature, high pressure, or both.
Metamorphism is Described by Texture, Index minerals, Grade, and Facies
Identifying Metamorphic Rocks Step 1 Determine the rock texture Foliated textures – rocks exhibit foliation…layering or parallel alignment of platy or flat mineral crystals (if the rock appears layered, it is foliated) due to pressure and recrystallization Nonfoliated textures – rocks exhibit no layering, yet they may exhibit stretched fossils or long, prismatic crystals that have grown parallel to the pressure field
Foliation Determined by the degree of Cleavage Schistosity Banding
Increasing intensity of metamorphism Increasing crystal size Increasing coarseness of foliation Low gradeIntermediate grade High grade
DiagenesisLow grade Intermediate grade High grade Slaty Rock Cleavage Phyllite Texture Schistosity (abundant micaceous minerals) Gneissic Banding (fewer micaceous minerals) Migmatite very flat foliation scaly glittery layer of visible platy minerals and/or linear alignment of long prismatic crystals alternating layers or lenses of light and dark medium to coarse grained minerals Banding Foliated rocks are classified by the degree of cleavage, schistosity, and banding. wavy or wrinkled foliation of fine grained minerals giving rock metallic luster
Progression of metamorphism Slate PhylliteSchist Start with a shale and then hit it with pressure and heat.
You end up with something that is really Gneiss!
Metamorphic Rock Textures (Unfoliated Textures) -Crystalline Texture – medium to coarse grained aggregate of intergrown, equigranular, visible crystals (example: Marble) -Microcrystalline Texture – fine grained aggregate of intergrown microscopic crystals (example: hornfels) -Sandy Texture – medium to coarse grained aggregate of fused, sand-sized grains that resemble sandstone (example: quartzite) -Glassy Texture – homogeneous texture with no visible grains or other structures and breaks along glossy surfaces (anthracite coal)
Identifying Metamorphic Rocks Step 2 Determine the rock’s mineralogical composition and/or other distinctive properties Other Distinctive Features to Note Stretched or Sheared Grains – deformed pebbles, fossils, mineral crystals, that have been stretched, shortened, or sheared Porphyroblastic Texture – arrangement of large crystals (PORPHYROBLASTS) set in a finer- grained groundmass (sort of sounds like porphyritic texture) Hydrothermal Veins – fractures filled by minerals that precipitated from hydrothermal fluids
With increasing metamorphic grade, mineral composition changes. Mineral suites define metamorphic facies.
Identifying Metamorphic Rocks Step 3 Use Five Step Chart for Metamorphic Rock Analysis to determine the name of the rock you are identifying Step 4 Based on the name of the identified metamorphic rock, name the rock it was before metamorphism (this is the metamorphic rocks “parent” rock or protolith)
Metamorphism of Igneous Rocks For most purposes, just put “meta” in front of the protolith name. Examples:metabasalt metarhyolite If a mafic or intermediate metamorphic rock is dominated by amphibole and feldspars: -Amphibolite