Igneous Rocks (IR) a.k.a Volcanic Rocks Ms. Tasneem

Rock cycle

Igneous Rock Formation

How do Intrusive IRs Form?

Igneous Intrusions Intrusion = liquid rock formed under the surface of the earth

Igneous Intrusions-Batholiths  Batholith (from Greek bathos, depth + lithos, rock) a large inclusion of intrusive (also called plutonic) igneous rock that forms from cooled magma deep in the earth's crust

Yosemite National Park

El Capitan

Igneous Intrusion – Stock & Sill  Stock = Smaller Batholith with surface exposures around 40 km 2  Sill = parallel to “country rock” beds and usually horizontal in orientation Intrudes between layers of sedimentary rocks

Igneous Intrusion – Laccolith & Dike Laccolith = dome shaped intrusion between two layers of sedimentary rock Dike = any geologic body that cuts across other rock formations

Intrusive Rock Formation  Intrusive (Plutonic) = crystallized from slowly cooling magma intruded within the Earth’s crust Granite Gabbro

How would you group these paperclips?

Igneous Rock Classification Color = (Proxy for silica content) Size = (Proxy for grain size)

Composition of IR 1. How are minerals related to rocks? 2. How would knowing the different types of crystals help you describe the characteristics of igneous rocks or distinguish among the three general types of rocks?

Composition of IR = MINERALS!

Compositional Classification Granite Quartz Orthoclase Biotite Plagioclase

Compositional Classification Granite Granodiorite Quartz Amphibole Plagioclase

Compositional Classification Granite Granodiorite Diorite Plagioclase Amphibole

Compositional Classification Granite Granodiorite Diorite Gabbro Plagioclase Pyroxene

Compositional Classification Granite Granodiorite Diorite Gabbro Peridotite Pyroxene Olivine

Size: Grain sizeColor: Silica Content  Coarse Grained = large crystals  Fine Grained = Small crystals  Mafic = IR rich in dark colored elements (Mg & Fe) = low silica  Felsic = IR rich in silica

Common Minerals

Rock Cycle

How do Extrusive IRs form?  Volcanic (extrusive) igneous rocks  Cool fast, small crystals  form when molten rock (magma) in the earth’s interior rises to the surface through pipes or fractures in the crust.

 Extrusive = or erupted as pyroclastic material, i.e., fragmented pieces of magma ejected and cooled in the air. Pumice Scoria Ash How do Extrusive IRs form? Obsidian

Classification of Igneous rocks  Texture  size of crystals most important Cooling rates –Surface, fast  Small crystals –Below surface slow  Larger crystals Which formed at surface? Both are the same chemistry RhyoliteGranite a)b)

Chemical Composition  Color Indicative of chemistry and temperatures of formation –Dark colors  High temperatures (1000 to 1200 o C)  Low silica content –Light colors  Lower temperatures  High silica content GabbroGranite Which is the high silica rock? Both formed below the surface a)b)

How did this form? Intrusive or Extrusive?

Formation of IR

The Legend of Finn McCool

What really happened…

More of Ms. Tasneem’s photos

Six common Igneous Rocks Volcanic Rocks Silica Content Minerals Present (in order of abundance) Plutonic Rocks Basaltlow pyroxene, olivine, feldspar, & amphibole Gabbro Andesiteintermediate feldspar, amphibole, pyroxene, biotite mica Diorite Rhyolitehigh feldspar, quartz, muscovite mica, & amphibole Granite Solidifying Temperature500 C 1000 C Lighter Color Increasing Grain Size

5) Imagine that all minerals found in rocks were the same color. What information would you no longer be able to infer? a. textureb. cooling rate c. composition

Analysis Time  Given: Bag rocks Identify the rocks