Chapter 5 Rocks Classifying Rocks - Sarah Hall

What is a rock? A rock is: – A solid ( not necessarily hard) – Naturally occurring (except brick and concrete) –are composed of one or more minerals –Can be organic like coal, or peat

What characteristics are used to identify rocks? When studying a rock sample, geologists observe the rock’s –Color –Texture –Determine its mineral composition

Figure 3: Sample Answer Texture is described with the terms based on: Grain size –Fine-grained –Coarse-grained Grain shape –Smooth –Jagged Grain pattern – banded –nonbanded

Figure 3 Texture helps geologists classify rocks. Forming Operational Definitions – Looking at the rocks below, describe the characteristics of a rock that help you define what a rock’s “grain” is.

What are the three major groups of rocks? Igneous rock Sedimentary rock –Forms below the surface Metamorphic rock –Forms when an existing rock is changed by heat, pressure, or chemical reactions –Most metamorphic rock forms deep underground

Igneous rock Forms by the cooling of molten rock –Intrusive forms from magma below the surface –Extrusive forms from lava above the surface

Granite Light-colored, coarse- grained, no pattern Mostly quartz, feldspar, mica, and hornblende Often used for buildings and monuments

Basalt Dark-colored, fine- grained, extrusive Formed where lava erupted onto surface Most widespread igneous rocks Found locally in the Palisades along west shore of Hudson River, Connecticut River valley

Obsidian Natural volcanic glass Forms when lava cools very quickly Usually dark, but small pieces may be clear Fractures along curved (conchoidal) surface Used as spear and arrow points, knives

Sedimentary rock 3 types –Clastic – particles are pressed and cemented together –Organic – remains of plants and animals are pressed and cemented together –Chemical – particles precipitate out of solution

Clastic rocks–made of cemented sediments—are classified by their grain sizes.

Non-clastic rocks form by chemical precipitation (settling out from a solution.) Limestone is made from calcite, chert from quartz, and halite is rock salt.

Biologic sedimentary rocks come from the remains of organic matter. The most important of these is coal. Anthracite coal results from the greatest pressure and releases the most energy when burned. Other varieties are bituminous and lignite. “Petrified” (permineralized) wood is another organic rock.

Metamorphic rock Forms when an existing rock is changed by heat, pressure or chemical reactions Two textures –Foliated – banded –Non- foliated – not banded

“Foliated” rocks contain much mica and other rocks that produce layering or banding Gneisses and schists are common in New York City and Westchester. l

Non-foliated metamorphic rocks include marble, which comes from limestone, and quartzite, which comes from sandstone

What tests do geologists use to identify minerals in rocks? Scratch Test (Mohs) Use acid to determine whether the rock contains the mineral calcite Magnet test

Rock Cycle