Rocks and Rock Cycle Chapter 4

Rocks Magma – hot, molten rock from the Earth’s interior is the parent material for all rocks. Three Major Types of Rock Igneous Rock Sedimentary Rock Metamorphic Rock

Rock Cycle Any of the 3 major types of rock can be changed into another type. Various geological forces and processes cause the rock to change from one major type to another and back again.

Rock Cycle

Igneous Rocks Formed when magma cools and crystallizes. Most abundant rock on Earth Intrusive (Plutonic) – Igneous rocks that are formed below the Earth’s surface Slow rate of cooling, which results in the formation of large crystals (coarse-grained) Extrusive (Volcanic) – Igneous rocks that are formed at the Earth’s surface. Rapid cooling – results in formations of small crystals (fine-grained)

Classifying Igneous Rocks Igneous rocks are classified by texture and mineral composition. Texture – size, shape, and arrangement of interlocking crystals Fine-grained Coarse-grained Porphyritic Glassy

Fine-grained Texture Extrusive Igneous Rock crystals to small to be seen by the unaided eye. Vesicles are common in fine-grained igneous rocks. Vesicles are voids that are left by gas bubbles that are formed as the lava is solidified.

Coarse-grained Texture Intrusive Igneous Rocks Formed below the Earth’s surface Slow cooling allowing for large crystals to form Crystals roughly equal in size Can be seen by the unaided eye

Porphyritic Texture When magma that already contains some large crystals suddenly erupts at the surface, the remaining molten portion of the lava would cool quickly. Resulting Rock Large crystals imbedded in a matrix of smaller crystals

Glassy Texture Formed during volcanic eruptions, molten rock is ejected into the atmosphere, where it is cooled very quickly.

Composition of Igneous Rocks Igneous rocks are mainly composed of silicate minerals. Dark Silicates – are rich in iron and/or magnesium and are relatively low in silica. Olivine, pyroxene, amphibole, and biotite mica are common dark silicates. Light Silicates – contain greater amounts of potassium, sodium, and calcium Quartz, muscovite mica, and feldspars (most abundant mineral group)

Bowen’s Reaction Series

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Sedimentary Rocks Sedimentary rocks are formed by the process of erosion. The weathering debris are constantly being swept away from bedrock and carried away by water, ice, and wind (aka. Erosion). The debris is eventually deposited into lakes, river valleys, seas, and countless other places. As the sediments accumulate, the material near the bottom are compacted by the weight of the overlying layer. Over long periods of time, these sediments are cemented together by mineral matter deposited from water in the spaces between particles.

Sedimentary Rocks Lithification – refers the processes by which sediments are transformed into solid sedimentary rocks. Compaction – most common Cementation - minerals are carried in solution by water seeping through the pore spaces between particles. Over time, the cement precipitated onto the sediment grains, filling the open spaces, joining the particles together. Calcite, silica, and iron oxide are the most common cements

Sedimentary Rocks Sedimentary rocks provide geologists with evidence of Earth’s long history. Past environments Conglomerate – indicates high-energy environment, such as rushing stream, where only a coarse material can settle out. Black Shale or Coal – associated with low-energy, organic- rich environment such as a swamp or lagoon. Ripple marks – may indicate a beach or stream channel environment Mud cracks – form when wet mud or clay dries and shrinks, possibly signifying a tidal flat or desert basin. Fossils – traces of remains of prehistoric life, perhaps the most important inclusions found in sedimentary rocks. Fossils are important tools used in interpreting the geologic past.

Classifying Sedimentary Rocks Detrital Sedimentary Rocks – formed from weathering rocks, such as igneous rocks. Mainly formed by clay minerals and quartz Geologists us particle size to classify detrital sedimentary rocks. Chemical Sedimentary Rocks – formed when dissolved substances are precipitated back as solids. Formed by material that is carried in solution to lakes and seas. Limestone is the most abundant chemical sedimentary rock (composed of calcite (CaCO3)).

Conglomerate vs. Breccia

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Metamorphic Rocks This process is the transformation of one rock type into another caused by changes in temperature and pressure (stress). Contact Metamorphism – alteration of a rock, mainly by heat, that occurs near a heat source such as a magma chamber Regional Metamorphism - metamorphism across a broad area caused by the elevated temperatures and pressure of plate collision or deep burial

Metamorphic Rock Textures Foliated – caused by differential stress, the crystals will recrystallize with preferred orientation, perpendicular the direction of the compressional force. show bands due to pressure Nonfoliated – metamorphic rocks composed of only one mineral with no banding

Nonfoliated vs Foliated

Classifying Metamorphic Rocks