Rocks

What are Rocks? Rocks are a combination of minerals that are bonded together in some way. All rocks are made of minerals Monomineralic- contain one mineral Polymineralic- contain more than one mineral Rocks are classified into three groups by how they are formed

3 types of Rocks Metamorphic Rocks: are formed when old rocks change by heat and pressure. Sedimentary Rocks: Form from sediments (pieces of other rocks) that are cemented together. Igneous Rocks: Form from molten rock that cools and hardens. Rocks are classified by how they form

Igneous means “born of fire” The Rock Cycle Igneous Igneous means “born of fire”

Igneous means “born of fire” The Rock Cycle Igneous : Form from the cooling and hardening of Magma or lava. Igneous means “born of fire”

1. Igneous Rocks Form from liquid (molten) rock called Magma or Lava that cools and hardens (solidifies) All igneous rocks are identified on the basis of composition and texture The rate at which rocks cool from the liquid magma determines their texture. Faster it cools, the smaller the crystals Slower it cools, the larger the crystals

Classification of igneous rocks- 2 types Intrusive Form from magma Cooled and solidified slowly deep underground Extrusive Form from lava Cooled quickly at or near the surface of the earth.

Igneous rocks are also formed causing the magma to rise when volcanoes erupt, causing the magma to rise above the earth's surface.

Igneous Rocks Can be: Felsic: light colored rocks that are rich in elements such as aluminum, silicon. Lower density Mafic: dark colored rocks that are rich in iron and Mg. Higher density Coarse-grained texture: takes longer to cool, larger visible mineral crystals Fine-grained texture: cools quickly with small mineral crystals Glassy Texture: cools the quickest, no mineral crystals form Vesicular texture: Gas pockets (holes) (note) Texture refers to the size/appearance of the minerals

Igneous Rocks Coarse-Grained Fine-Grained Felsic Granite Rhyolite Mafic Gabbro Basalt

Minerals in Igneous rocks Plagioclase felspar- most common mineral found in igneous rocks Quartz- looks dark and glassy Potassium feldspar Muscovite- not that common Biotite- occurs in small amounts Pyroxene Olivine

Igneous Rocks Is this rock Felsic or Mafic? Obsidian What is Obsidian? Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form. Igneous Rocks Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form. Is this rock Felsic or Mafic? Is it fine-grained or coarse-grained? Is this rock Intrusive or Extrusive?

Glassy Texture                                                                     Glassy textured igneous rocks are non-crystalline meaning the rock contains no mineral grains. Glass results from cooling that is so fast that minerals do not have a chance to crystallize. This may happen when magma or lava comes into quick contact with much cooler materials near the Earth's surface. Pure volcanic glass is known as obsidian (see photo).

Vesicular Texture                                                                                This term refers to vesicles (holes, pores, or cavities) within the igneous rock. Vesicles are the result of gas expansion (bubbles), which often occurs during volcanic eruptions. Pumice and scoria are common types of vesicular rocks. The image to the left shows a basalt with vesicles, hence the name "vesicular basalt".

Fragmental Texture                                                                          The last textural term reserved for pyroclastic rocks, those blown out into the atmosphere during violent volcanic eruptions. These rocks are collectively termed fragmental. If you examine a fragmental volcanic rock closely you can see why. You will note that it is comprised of numerous grains or fragments that have been welded together by the heat of volcanic eruption. If you run your fingers over the rock it will often feel grainy like sandpaper or a sedimentary rock. You might also spot shards of glass embedded in the rock.

Types of igneous rocks Granite- most abundant igneous rock Can be made up of feldspar, quartz, mica, and hornblende

Obsidian Obsidian rocks are igneous rocks that form when lava cools quickly above ground. Obsidian is actually glass and not a mixture of minerals. The edges of this rock are very sharp.

Pumice Pumice rocks are igneous rocks which were formed when lava cooled quickly above ground. This rock is so light, that many pumice rocks will actually float in water. Pumice is actually a kind of glass and not a mixture of minerals.

Extrusive Insrusive