Igneous RockInside the Restless Earth Chapter 2, Section 2
6th Grade Mrs. Boguslaw
Igneous Rock• Always begins as magma
• Magma can form when rock is heated, when pressure is released, or when rock changes composition
• Because magma is made of many different melted minerals, some minerals become solid before others do
Composition and Texture of Igneous Rock
• The longer it takes for magma or lava to cool, the more time mineral crystals have to grow
• Longer time —> bigger crystals —> coarser texture
• Less time —> smaller (or no) crystals —> finer texture
Intrusive Igneous Rock
• Rock formed from the cooling and solidification of magma UNDER the Earth’s surface
• Usually cools slowly, so has a coarse-grained texture with large crystals
Intrusive Igneous Rocks• Masses are named for sizes and shapes
• Batholith: largest
• Stock: smaller than batholiths
• Dike: sheetlike, cut across other previous rock units
• Sills: parallel to previous rock units
Extrusive Igneous Rock
• Rock that forms as a result of volcanic activity at or near the Earth’s surface, cools ABOVE the surface of the Earth
• Cools quickly, so usually fine grained with small or no crystals
Sedimentary RockInside the Restless Earth Chapter 2, Section 3
6th Grade Mrs. Boguslaw
Sedimentary Rock
• Forms at or near the Earth’s surface
• Forms without the extreme heat and pressure needed for igneous and metamorphic rock
• Most noticeable feature is strata - layers of rock
Origins of Sedimentary Rock• Wind, water, ice, sunlight, and gravity all cause rock to
weather into fragments called sediment
• Through erosion, this sediment is moved from one place to another
• During deposition, the sediment is deposited in layers
• As new layers of sediment are deposited, old layers are covered and compacted. Dissolved minerals form a natural cement that binds the sediment into sedimentary rock
Composition of Sedimentary Rock
• Classified by the way it forms
• Clastic sedimentary rock: made of fragments of rock cemented together by a mineral like calcite or quartz
• Chemical sedimentary rock: forms from solutions of dissolved minerals and water; rainwater dissolves rock and then it re-crystalizes
• Organic sedimentary rock: forms from the remains of once-living plants and animals; includes limestones and coal
Sedimentary Rock Structures
• Most important feature is stratification
• Stratification: the process in which sedimentary rocks are arranged in layers
• Strata are different depending on kind, size, and color of sediment
• Sedimentary rock can sometimes also show the motion of wind or water, called ripple marks
• Mud cracks can show areas that once had water that has dried up, like ancient lakes, streams, or shoreline
Metamorphic RockInside the Restless Earth Chapter 2, Section 4
6th Grade Mrs. Boguslaw
Origins of Metamorphic Rock
• The texture or mineral composition of a rock can change when its surroundings change
• If the temperature or pressure of the new environment is different from the one in which the rock formed, it will undergo metamorphism
Origins of Metamorphic Rock
• Contact Metamorphism
• Heated by nearby magma
• Does not melt the rock
Origins of Metamorphic Rock
• Regional Metamorphism
• Occurs when pressure builds up deep below other rock formations or when large pieces of the Earth’s crust collide
• Increased temperature and pressure causes rock to become deformed and chemically changed
Composition of Metamorphic Rock
• Metamorphism occurs when temperature and pressure inside the Earth’s crust change
• Minerals that were present when the rock first formed might not be stable in the new temperature and pressure conditions, so they change into more stable minerals
• Index minerals are minerals that form only at certain temperatures and pressures. They help us estimate the temperature, pressure, and depth at which a rock undergoes metamorphism
Textures of Metamorphic Rock
• Either foliated or non foliated
• Foliated Metamorphic Rock
• Mineral grains are arranged in planes or bands
• As heat and pressure increases, metamorphism continues
Textures of Metamorphic Rock
• Nonfoliated Metamorphic Rock
• Mineral grains are NOT arranged in planes or bands
• Commonly made out of one or only a few minerals
• Crystals change in size and composition during metamorphism
Metamorphic Rock Structures
• Deformation: a change in the shape of a rock caused by a force placed on it
• Folds or bends in rock are structures that indicate that a rock has been deformed