Rocks: Mineral Mixtures Chapter 4

Understanding Rock Section 1 Rock = solid mixture of crystals of one or more minerals. Some types of rock, such as coal, are made of organic materials The Value of Rock Humans have a long history with rock Tools, weapons, buildings, roads, & monuments Rocks hold a record of what the Earth & other planets were like before recorded history Concrete in which rock is an important ingredient is one of the most common building materials used today Fossils let us know the history of life on the Earth. From studying fossils we both answer questions and spark thousands more

Section 1 The Rock Cycle The way rock forms determines what type of rock it is. There are 3 different types of rock: igneous, sedimentary, & metamorphic. Rock Cycle = the process by which one rock type changes into another. This cycle involves: Deposition Cooling Compaction & cementation Solidification Weathering Metamorphism Erosion Melting grains of sand & other sediment are eroded from the mountains & washed down a river to the sea. Over time, the sediment forms thick layers on the ocean floor. Eventually, the individual grains of sediment are pressed & cemented together, forming Sedimentary Rock When large pieces of the Earth’s crust collide, enormous stresses build up. When this happens, some of the rock is forced downward. At these lower levels the intense heat & pressure “cooks” & squeezes the sedimentary rock, changing it into metamorphic rock. Magma = hot liquid formed when rock partially or completely melts. If the metamorphic rock breaks off the wall of a magma chamber, it will become surrounded by magma. If the magma is hot enough, the metamorphic rock will melt completely, & the material that used to sand grains will become part of the magma. The original sand grains from step 1 have changed a lot, but they’re not done yet! Magma is usually less dense than the surrounding rock, so it tends to rise to higher levels of the Earth’s crust. Once there, it cools & solidifies, becoming igneous rock. Erosion of the overlying rock exposes the igneous rock at the Earth’s surface. The igneous rock the weathers & wears away into grains of sand & clay. Theses grains of sediment are then transported & deposited elsewhere, beginning the whole process again.

The Rock Cycle: Cont. Section 1 Define the three main types of rock: Igneous = forms from the cooling of magma. (melting, cooling, & solidification) Magma = hot liquid formed when rock partially or completely melts. Lava = magma flowing out onto the Earth’s surface. Sedimentary = forms when sediments are compacted & cemented together. (weathering, erosion, deposition, & compaction & cementation) Metamorphic = formed when the texture & composition is changed by heat or pressure deep underground (pressure & heat = metamorphism)

Section 1 Igneous is divided into 2 groups: dependent on how & where it forms Magma cooling Under the Earth’s surface forms intrusive igneous rock Lava cooling on the Earth's surface forms extrusive igneous rock

The Nitty-Gritty on Rock Classification Section 1 The Nitty-Gritty on Rock Classification Rocks are further divided into other groups Composition Composition = the minerals a rock is made of Texture Texture = determined by the sizes, shapes, & positions of the grains of which it is made Coarse-grained – larger grains in the rock Medium-grained – between fine & coarse rock Fine-grained – small grains such as silt or clay These divisions are based on differences in the way the rock forms

Igneous Rock Section 2 Igneous – comes from the Latin word for “fire” Magma cools into various types of igneous rock depending on the composition of the magma & the amount of time it takes the magma to cool & solidify Like all other rock, igneous rock is classified according to its composition & texture.

Origins of Igneous Rock Section 2 Origins of Igneous Rock There are three ways a rock can melt: Temperature – Temperatures increase as you travel deeper into the Earth. This causes minerals in rock to melt. Different minerals melt at different temperatures. Depending on how hot a rock is, some of the minerals melt while other minerals remain solid

Origins of Igneous Rock: Cont. Section 2 Origins of Igneous Rock: Cont. Pressure – Pressure increase as you travel deeper into the Earth. This forces minerals to stay solid, when otherwise they would melt from the intense heat. When hot rocks rise to shallow depths, the pressure is finally released & the minerals can melt. Composition – Sometimes fluids enter a rock that is close to its melting point. When these fluids combine with the rock, they can lower the melting point of the rock enough for it to melt & form magma.

Composition & Texture of Igneous Rock Section 2 Composition & Texture of Igneous Rock Composition Felsic composition – Light-colored rocks; lighter in weight; rich in silicon, aluminum, sodium, & potassium Mafic composition – dark-colored rock; heavier; rich in iron, magnesium, & calcium Texture Longer it takes for lava to cool more time mineral crystals have to grow & the coarser the texture of the resulting igneous rock Chart on page 88

Igneous Rock Formations Section 2 Igneous Rock Formations Intrusive Igneous Rock Intrusive = rock that results from when magma cools beneath the Earth’s surface Usually has a coarse-grained texture it is well insulated by the surrounding rock & thus cools very slowly Plutons (plutonic rock) – large, balloon-shaped intrusive formations that result when magma cools at great depths Intrusive igneous rock formations occur in many different shapes & sizes. Dikes, plutons, & batholiths cut through existing rock formations. Sills & laccoliths intrude between layers of existing rock formations. Batholiths are the largest igneous formations of all.

Igneous Rock Formations: Cont. Section 2 Igneous Rock Formations: Cont. Extrusive Igneous Rock Extrusive = Igneous rock that forms on the Earth’s surface. Most volcanic rock is extrusive. Contains either very small crystals or none at all because it cools quickly 3 examples of Extrusive Igneous Rocks are Pumice; Obsidian; Basalt Fissures – long cracks in the Earth’s surface form which lava sometimes flows Lava plateau – when a large amount of lava flows out a fissure and covers a vast area

Sedimentary Rock Section 3 Wind, water, ice, sunlight, & gravity all cause rock to weather into fragments. Through the process of erosion, rock fragments, called sediment, are transported from one place to another. Eventually the sediment is deposited in layers. Sedimentary rock then forms as sediments become compacted & cemented together.

Origins of Sedimentary Rock Section 3 Origins of Sedimentary Rock As sediment is deposited, the layers eventually become compressed. Dissolved minerals separate out of water to form a natural glue that binds the sediments together into sedimentary rock. Sedimentary rock forms at or near the Earth’s surface, without the heat & pressure involved in the formation of igneous & metamorphic rocks. physical features of sedimentary rock tell a part of history. The most notable feature is the strata. Strata = the layers found in sedimentary rock. The physical features of sedimentary rock tell part of its history.

Composition of Sedimentary Rock Section 3 Composition of Sedimentary Rock Two main categories of sedimentary rock: Clastic Sedimentary Rock Formed from rock or mineral fragments Chemical Sedimentary Rock Formed from solutions of minerals & water. (when minerals crystallize out of a solution, such as sea water, to become rock) The remains of animals can become part of sedimentary rock as well. They form fossils. Sedimentary rock is classified by the way it forms

Sedimentary Rock Structures Section 3 Sedimentary Rock Structures Rock features can tell you how the rock was formed Stratification = layering Strata differ from one another depending on the kind, size & color of their sediment. The rate of deposition can affect the thickness of the layers This rock can record the motion of wind & waves on lakes, seas, rivers, & sand dunes. When water & wind transport & deposit sediments, some unique patterns develop. Ripple marks are made by flowing water & are preserved when the sediments become sedimentary rock. When the bottom of a lake is made of fine-grained sediments like clay, the sediment cracks as it dries, forming geometric patterns. When new sediments fill the cracks & turn to rock, mud cracks are preserved. Strata are not always parallel like the layers in a cake. Some strata are slanted. Wind caused these slanted deposits, called cross-beds, but water can also cause them.

Metamorphic Rock Section 4 Metamorphic – “changed shape” All rock can change into metamorphic rock Origins of Metamorphic Rock Texture or mineral composition of a rock can change when its surroundings change Most metamorphic change is caused by increased pressure that takes place at depths greater that 2 km. Temperature at which metamorphism occurs ranges from 50oC to 1,000oC

Origins of Metamorphic Rock: Cont. Section 4 Origins of Metamorphic Rock: Cont. Contact Metamorphism Only happens to igneous intrusive rock When magma flows through the crust, its heat flows into the surrounding rock & “cooks” it. The greatest changes occur where the magma comes into direct contact with the rock & the effect lessens the further you travel away from that contact. The heat & fluids in the magma changes the minerals present into other minerals

Origins of Metamorphic Rock: Cont. Section 4 Origins of Metamorphic Rock: Cont. Regional Metamorphism This kind of metamorphic rock is underneath most continental rock formations. Enormous pressure builds up in rock that is deeply buried under other rock formations or when pieces of the Earth’s crust collide. The pressure & increased temperature that exist under these conditions cause rock to become deformed & chemically changed.

Composition of Metamorphic Rock Section 4 Composition of Metamorphic Rock Metamorphism can occur in many different environments. Temperature & pressure conditions, as well as the composition of the rock determine which metamorphic minerals will form. When scientists observe metamorphic minerals in a rock, they can estimate the temperature & depth (pressure) at which recently exposed rock under went metamorphism.

Texture of Metamorphic Rock Section 4 Texture of Metamorphic Rock Just like all other rock metamorphic rock is classified by composition & texture. Metamorphic rock has one of two textures: Foliate = consists of minerals that are aligned & look almost like pages in a book Nonfoliate = does not appear to have any regular pattern

Texture of Metamorphic Rock: Cont. Section 4 Texture of Metamorphic Rock: Cont. Foliated Metamorphic Rock Contains mineral grains that are aligned by pressure Metamorphic rocks can change again if their environment changes Nonfoliated Metamorphic Rock Commonly made up of only one, or just a few, minerals but none of the crystals are aligned.