Earth Materials continued Mineral Naturally Occurring Inorganic Solid Chemical Composition Crystalline Shape

Rock a coherent, naturally occurring solid, consisting of an aggregate of one or more minerals, or a mass of natural glass or organic matter.

Basic Rock Classifications Igneous Sedimentary Metamorphic

Rock Cycle And Pressure Cementation Erosion/ Weathering Sedimentary Rocks Erosion/ Weathering Sediment Heat Erosion/ Weathering Erosion Pressure Heat and Pressure Metamorphic Rocks Igneous Rocks Heat Cooling Magma

? Define - Freeze ?

Igneous Rocks

Igneous Rocks Two types of igneous rocks -a rock that forms when hot molten rock (magma or lava) cools and freezes solid Two types of igneous rocks Plutonic – cools underground Volcanic – cools above ground Plutonic rocks cool very slowly because they are insulated by being buried several miles deep within the Earth. Because of this slow cooling, mineral grains have the opportunity to grow large enough to see without the aid of a magnifying glass. This course-grained texture is called “phaneritic”. [fan-er-it-ik]. Volcanic rocks cool near or at the surface of the Earth and therefore do not have mineral grains large enough to see, although sometimes there are some larger grains mixed into these fine-grained rocks. This fine-grained texture is called “aphanitic”, [af-a-nit-ik]. If there are some larger grains mixed in then the term “Porphyritic” is added.

Factors controlling Melting 1. Temperature: Melt at [800oC and 1200oC] Source of heat [radioactive decay]

Geothermal Gradient – the rate at which temperature increases with depth in the Earth 30OC/km

Norman Bowen

Bowen’s Reaction Series Illustrates the order in which minerals crystalize from a magma Liquid The important concept to remember about this chart is: - dark minerals tend to solidify first. Close to 1200 degrees C. - light colored minerals tend to solidify last. Close to 800 degrees C. - quartz is the last mineral to become solid. - as solid rocks melt, the order is reversed. Quartz melts first and dark minerals melt last. Solid

Iron and Magnesium rich Dark colored minerals Al, Ca, and Sodium rich Light colored minerals

Factors controlling Melting 2. Pressure: as pressure increases, melting temperature increases as pressure decreases, melting temperature decreases Lowering pressure causes melting in divergent margins.

Relationship between P, T, and Melt Less Pressure The blue and purple curves represent the melting temperature of dry basalt and granite and how it changes with depth. More Pressure

Divergent Plate Boundary The melting of rock at divergent plate boundaries is accelerated do to thinning of the crust which means there is less weight, or decompression.

Factors controlling Melting 3. Water content: As water content increases, melting temperature decreases

Now, compare the curve of dry granite to wet granite. The presence of water during melting has a significant influence on melting temperature.

Water increases melting at convergent plate boundaries Ocean water is introduced into the melting process as the subducting oceanic plate is pulled down under the continental plate.

Where do igneous rocks occur? Most igneous rocks are found near plate boundaries, hot spots, and ancient regions of igneous activity.

Types of Molten Material 1. Magma – molten material below the Earth's surface 2. Lava – molten material above the Earth's surface

Magma Composition Changes Partial melting Assimilation Magma Mixing Fractional Crystallization Molten oceanic crust and mantle material are low in silica (mafic) content compared to continental crust. This means it has a low viscosity (it’s runny) and would create non-explosive volcanoes if it reaches to Earth’s surface. As this mafic melt rises though the continental crust it can become silica rich (silisic) and therefore make an explosive volcano.

Partial Melting some minerals melt at a lower temperature than others. As crust begins to melt, the light colored minerals (silisic, high in silica) will melt first. This early melt can flow away and what remains behind is mafic rich rock.

Assimilation – As mafic magma rises though silicic continental crust it’s chemistry changes to reflect the addition of silica. It becomes more viscous (sticky). Magma Mixing – Magma from to different sources can meet up and mix together which will change the overall silica concentration. Fractional Crystallization – As a body of begins to cool dark (mafic) minerals will line the chamber. The silica concentration in the remaining magma will increase.

Magma Viscosity Viscosity – resistance to flow honey – high viscosity water – low viscosity Viscosity increases as silica content increases.

Magma Chemistry Molten material can consist of liquid rock, mineral grains and gases (H2O, CO2, SO2). Silicon and oxygen (SiO2) make up the majority of magma 45% SiO2 - "low" silica content 75% SiO2 - "high" silica content An igneous rock such as basalt is considered low in silica even though it has up to 45% silica in it. As it solidifies from a magma the silica is all incorporated into minerals. There is no silica left over to form the mineral quartz. In high silica igneous rock quartz is formed.

Endmember Magma Chemistry Mafic – low SiO2 / high iron, magnesium Silicic – high SiO2 / high aluminum, calcium, sodium

Igneous Rock Types Intrusive (plutonic) rock – cools and solidifies below the Earth's surface. Extrusive (volcanic) rock – cools and solidifies above the Earth's surface.

Textures of Igneous Rocks Texture – overall appearance, related to size, shape, and arrangement of minerals. Texture is related to cooling history of an igneous rock, not its chemistry.

Textures of Igneous Rocks 1. Aphanitic (fine grained) Lava Earth’s surface 2. Phaneritic (course grained) Magma

All the terms on the left describe grain size which reflects the cooling rate. The terms across the top describe changing chemistry (silica content)

Aphanitic Texture – Rapid Cooling Small Crystals

Phaneritic Texture – Slow Cooling Large Crystals

Silicic Chemical Composition Phaneritic Texture – Slow Cooling Large Crystals Aphanitic Texture – Rapid Cooling Small Crystals Granite Rhyolite

Intermediate Chemical Composition Phaneritic Texture – Slow Cooling Large Crystals Aphanitic Texture – Rapid Cooling Small Crystals Diorite Andesite

Mafic Chemical Composition Phaneritic Texture – Slow Cooling Large Crystals Aphanitic Texture – Rapid Cooling Small Crystals Gabbro Basalt

PHANERITIC texture It means that the size of all grains in the rock are large enough to be distinguished with the unaided eye

APHANITIC texture It means that not all grains in the rock are large enough to be distinguished with the unaided eye. Most of the rock is background mass.

Pegmatites

Ingersoll Mine Ingersoll mine, Pennington Co., South Dakota, United States One of the many adits at the old Ingersoll pegmatite. Private and Very dangerous. No Trespass.

Glassy Texture - Very Rapid Cooling - No Crystal Structure

Pyroclastic Texture Ash Fall Ash Flow

Plutonic Structures

Plutonic Structures

Image courtesy of geoinfo.nmt.edu