Chapter 5 Notes:
Modern geology is said to have begun in 1795. In that year, the Scottish geologist James Hutton described a new concept, called Uniformitarianism. Prior to this theory, scientists believed that all major earth features were created by a series of catastrophic events. Hutton explained it as: "the present is the key to the past."
The study of rock is an important part of understanding Earth processes.
A Rock can be generally defined as a group of minerals bound together in some way.
- Geologists have noticed three things about rock
- Erupting lava hardens into rock
- Sands and clays harden over time into rock
- Heat from volcanism can change rock into another form of rock
- Igneous Rock
forms by the cooling and hardening of hot molten rock from inside Earth. Molten rock is called Magma
- Sedimentary Rock
forms by the hardening and cementing of layers of sediments. The sediment may consist of rock fragments, plant and animal remains, or chemicals
- Metamorphic Rock
forms when rocks that already exist are changed by heat and pressure into new kinds of rocks
Igneous Rock
- Magmas with high silica are slow moving and typically light in color: called Felsic magma.
- Magmas with low silica are fast moving and typically dark in color: called Mafic magma.
- Reminder that when rocks form from cooling magma underground, they are said to be Plutonic
- When rocks form from cooling lava on the surface, they are called Volcanic
- Rocks are not only grouped by their mineral content but by their Texture as well
- Texture depends on size and shape of the mineral crystals that compose the rock
- Texture is also controlled by the amount of gases dissolved in the magma/lava mixture. High amounts of gases will produce a rough textured rock; Low amount of gas - smooth texture.
Porphyritic Textures
- Some igneous rocks have two different textures
- Usually large crystals are surrounded by fine crystals
- These are called Porphyry
Families of Igneous Rocks
- All members are formed by volcanic actions!
- Divided into three families: Coarse-grained, Fine-grained, and Glassy
Granite Family
- Forms from felsic magmas
- Consist of feldspar and quartz minerals
- Usually light in color
- Same chemical composition but different textures
- Common members are Granite, Rhyolite, and Obsidian & Pumice
Gabbro Family
- Forms from mafic magmas
- Consist mainly of feldspar and augite minerals
- Same chemical composition but different textures
- Usually dark in color and more dense than Granite family
- Common members are Gabbro, Basalt, and Basalt Glass
Let's Take a Good Look at These Samples!!!!
REFERENCE THE CHART ON PAGE 68!!!
Sedimentary Rocks
- Although most of earth's crust is made of igneous rocks, much of its surface is made of sedimentary rock!
- Three main kinds of sedimentary rock!
Clastic
- Sed. Rock formed from fragments of other rocks
- Examples are Shale, Sandstone, and Conglomerate
Chemical
- Sed. Rock formed from mineral grains that fall out of a solution by evaporation or by chemical action
- Examples are Rock Salt and some Limestone
Organic
- Sed. Rock formed from the remains of plants and animals
- Examples are Coal and some Limestone
Clastic Rocks Form
- Ocean water, lake water, and groundwater all contain natural cements in the form of dissolved minerals
- Natural cements include Silica, Calcite, and Iron Oxide
- Pressure of overlying sediments is sometimes enough to even make fine sediments such as clay or silt to stick and cement without cementing agents
- Clastic rocks are often Permiable (water can enter inside) and Porous (fine air pockets) because of the fine air spaces that form in the cementing process.
- Exception is Shale - a clay-formed rock which is made of such fine clay that water will not permeate.
Chemical Rocks Form
- Minerals like calcite chemically react with the water around them and cement together forming rocks like Limestone and Rock Salt
Sedimentary Rock Features
- Rock that shows features discussed in Chapter 16
- Examples are Cross-bedding, Shells, Ripple marks, and Mud cracks
- Additional features include Geodes - form when groundwater flows through cracks in limestone, dissolving the limestone and depositing quartz or calcite crystals in the open cavities.
SEE THE CHART ON PAGE 74!!!
Metamorphic Rocks
- Rocks formed from high pressure and heat applied to present rock forms
- Ex. - Limestone under heat and pressure will convert itself to Marble
- Common rocks in the Metamorphic class are rocks such as Marble, Slate, Gneiss, and Quartzite
- Limestone -> Marble
- Shale -> Slate -> Phyllite -> Schist
- Sandstone -> Quartzite
- Granite -> Gneiss
Regional Metamorphism
- Occurs when large areas of rock are under intense heat and pressure
- Occurs often during mountain building
Contact Metamorphism
- Occurs when hot magma forces its way into overlying rock
- The magma bakes the rock and converts it into a new form
- Much less rock is affected by this process over regional metamorphism
SEE THE CHART ON PAGE 78!!!
The Rock Cycle
- The igneous rocks of the world are often referred to as the primary or parent material of the crust of earth
- As they are attacked by weather and erosion, sediments occur and form
- These sediments of igneous rock "particles" will cement together to form a new sedimentary rock
- If these sedimentary rocks are exposed to excessive heat or pressure, they may metamorphose!
- The new metamorphic rock may then at some point melt to a liquid - forming, once cooled, into a brand new igneous rock!
- This is the rock cycle - it never ends or starts at a certain point - it continues forever!
SEE THE CHART ON PAGE 79!!!