Unit: 3 Earth’s Materials Earth Systems 3209 Unit: 3 Earth’s Materials Reference: Chapters 2, 3, 6, 7; Appendix A & B

Igneous Rocks (Overview) Unit 3: Topic 3.1 Igneous Rocks (Overview) Focus on . . . describing the terminology associated with Igneous rocks. Relating composition, parent rock, texture, and environment to the classification of igneous rocks.

Difference Between a Rock and a Mineral A rock is a consolidated mixture of one or more minerals. Ex. Granite and Basalt. In some cases a pure mineral may also be classified as a rock. Mineral A mineral must satisfy the following conditions; 1) Occur in nature 4) Definite chemical composition 2) Inorganic 5) Definite molecular structure 3) Solid

Rock Forming Minerals The rock cycle suggest that ALL rock types originally started as igneous rock (solidified magma). Igneous rocks form from Eight minerals called the rock forming minerals. 1) Olivine 5) Plagioclase Feldspar 2) Pyroxene 6) Orthoclase Feldspar 3) Amphibole 7) Muscovite Mica 4) Biotite Mica 8) Quartz

Magma and Lava Molten rock in the Earth is called magma. Magma is buoyant, rises to surface, & sometimes breaks through and flows onto Earth’s surface. When magma reaches Earth’s surface it is called lava.

Magma and Lava As magma cools, atoms arrange in an orderly crystal structure, this is known as Crystallization. Note: the different crystal Sizes. Coarse-grained igneous rock

Magma and Lava Crystal size will depend on; 1) Rate of cooling and 2) Cooling environment Coarse (larger) crystals - slow cooling rate and form inside of Earth (Plutonic) Fine (smaller) crystals - fast cooling rate and form on Earth’s surface (Volcanic)

Classifying Igneous Rocks An igneous rock is formed when magma or lava cools and solidifies. Igneous rocks are classified according to; Texture -- Describes the appearance of an igneous rock, based on the size, shape and arrangement of interlocking crystals. -- Depends on; how fast or slow the magma/lava cools. Mineral Composition -- Is the mineral makeup of an igneous rock based on the chemical composition of the parent magma. Text Reference pages 61 - 65

Texture depends on Cooling Environment: Volcanic (Extrusive) Environment: Igneous rocks that form at the surface. Fine crystals form on or near Earth’s surface where lava cools quickly in the matter of minutes to hours. Therefore, the quicker the molten rock cools, the smaller the crystals. Lava Cools Quickly Fine Texture

Texture depends on Cooling Environment: Plutonic (Intrusive) Environment Igneous rocks that form beneath Earth’s surface. Large crystals form deep within Earth where magmas may take up to tens of thousands of years to cool and crystallize. Therefore, the slower the molten rock cools, the larger the crystals. Magma Cools Slowly Coarse Texture

Texture depends on Cooling Environment: Plutonic rocks To see them, they must be uplifted to surface and softer surrounding rock eroded away. Magma Body

Texture depends on Cooling Environment: Plutonic rocks To see them, they must be uplifted to surface and softer surrounding rock eroded away.

Texture depends on Cooling Environment: Plutonic igneous rock example: El Capitan Yosemite National Park, California

Composition depends on Parent Material: Depends on the chemical makeup of the parent magma. Often classified as either Mafic or Felsic composition. Mafic Composition: are generally dark in colour since they are comprised of mostly dark-coloured minerals. Examples of dark-coloured minerals include: olivine; hornblende; and pyroxene. Felsic Composition: are generally light in colour since they are comprised of mostly light coloured minerals. Examples of light-coloured minerals include: quartz; orthoclase feldspar; and muscovite mica.

Composition depends on Parent Material: Magma cools and crystallize solidifies forms silicate minerals Two major silicate mineral groups: DARK silicates LIGHT silicates

Composition depends on Parent Material: Silicate group Rich in Poor in Examples DARK Fe and/or Mg Si olivine pyroxene amphibole biotite LIGHT Si, K, Na, Ca Fe and/or Mg quartz muscovite feldspars orthoclase plagioclase

Composition depends on Parent Material: Igneous rocks compositions mafic felsic magnesium + ferrum High in Mg and Fe. Dark and Dense Found in Ocean Crust feldspar + silica (quartz) High in Si. Lighter and Less Dense Found in Continental Crust

Example 1: What is the origin of fine-grained igneous rock? (A) lava that cooled quickly on Earth’s surface (B) lava that cooled slowly on Earth’s surface (C) silt that settled quickly in ocean water (D) silt that settled slowly in ocean water Which describes the cooling rate and crystal size of magma that cools deep beneath Earth’s surface? Cooling Rate Crystal Size (A) fast large (B) small (C) slow (D)

Example 2: Which rock type will form at location “Z” in the diagram below? (A) chemical sedimentary (B) clastic sedimentary (C) extrusive igneous intrusive igneous

Your Turn . . . Take the time and complete the following questions . . . (Solutions to follow) Question: Briefly describe how a geologist would use texture and mineral composition to determine the type of igneous rock.

Solutions . . . Question: Igneous rocks can be classified according to its texture and composition. Texture refers to the size, shape, and arrangement of minerals within an igneous rock and is determined by the environment and the rate at which the molten rock cools. The faster the cooling, the smaller the crystals. This can form a glassy or fine (aphanitic) texture. The slower the cooling rate, the larger the crystals. This can form a coarse (phaneritic) texture. Composition refers to the mineral make up of the igneous rock. Minerals in igneous rocks are generally dark in color or light in color. Igneous rocks are classified as mafic, intermediate and felsic. Each of these three classifications are based on the mineral composition and are reflected by the relative color of the igneous rock.

Summary . . . Overview of Points covered: Igneous Rocks; Form from Magma or Lava. Form in Plutonic (Intrusive) or Volcanic (Extrusive) Environments. Are generally Dark (Mafic) or Light (Felsic) in color.