The two most abundant elements of the earth's crust are oxygen and silicon.
Most crustal rocks are composed of silicon compounds.
Silica is silicon dioxide (SiO2). Silicates are compounds of silicon with oxygen and one or more metals.
Silicates are crystalline solids whose basic structural unit is the SiO44– tetrahedron.
The wide variety of silicate minerals is due to many different ways the SiO44– tetrahedron can combine with metal ions.
14-2. Minerals
Minerals are naturally occurring, crystalline, inorganic solids having fairly specific chemical compositions.
More than 2000 minerals are known, but most are rare.
Some physical properties of minerals are color, hardness, crystal form, and cleavage (tendency to split along certain planes).
Examples of six common minerals are:
Quartz (SiO2) crystals are often found in narrow deposits called veins.
Feldspars are silicate minerals having very similar properties and are the most abundant single constituent of rocks. Varieties include orthoclase (silicate of K and Al) and plagioclase (silicate of Na, Ca, and Al).
Mica varieties include white mica (silicate of H, K, and Al) and black mica (silicate of H, K, Al, Mg, and Fe).
Ferromagnesian minerals are silicates of Fe, Mg, and usually other metals such as K, Al, and Ca.
Clay minerals are silicates of Al, some with a little Mg, Fe, and K.
Calcite (CaCO3) is the chief mineral in limestone and marble.
14-3. Igneous Rocks
Igneous rocks, such as granite, basalt, and obsidian (natural glass), are rocks that have cooled from a molten state.
Fine-grained igneous rocks (examples: rhyolite, andesite, basalt) have cooled rapidly at or near the earth's surface. Coarse-grained ones (examples: granite, diorite, gabbro) have cooled slowly well below the earth's surface.
14-4. Sedimentary Rocks
Sedimentary rocks have consolidated from materials derived from the disintegration or solution of other rocks and deposited by water, wind, or glaciers.
Sedimentary rocks are divided into two categories:
Fragmental rocks
Chemical and biochemical precipitates
Three types of fragmental sedimentary rocks are:
Conglomerate (cemented gravel)
Sandstone (cemented sand grains)
Shale (consolidated mud or silt)
Examples of precipitated sedimentary rocks are:
Limestone (mostly the mineral calcite)
Chalk (loosely consolidated variety of limestone)
Chert (flint and jasper are varieties)
14-5. Metamorphic Rocks
Metamorphic rocks are formed from preexisting rocks that have been altered by heat and/or pressure.
Many metamorphic rocks display foliation, which is an arrangement of flat or elongated mineral grains in parallel layers that give the rocks a banded or layered appearance.
Examples of foliated metamorphic rocks are:
Slate (weakly metamorphosed shale)
Schist (severely metamorphosed shale or fine-grained igneous rock)
Gneiss (severely metamorphosed rocks, except pure limestone and pure quartz sandstone)
Examples of nonfoliated or weakly foliated metamorphic rocks are:
Marble (metamorphosed limestone)
Quartzite (metamorphosed sandstone)
14-6. Earthquakes
Most earthquakes are due to the sudden movement of rock along fracture surfaces called faults.
Earthquake magnitudes are expressed on the Richter scale.
Reliable earthquake predictions do not exist.
14-7. Structure of the Earth
The two main categories of earthquake waves are "body waves" (which move through the earth's interior) and "surface waves" (which travel along the earth's surface).
Types of body waves include:
Primary (P) waves, which are longitudinal or pressure waves and can travel through a liquid.
Secondary (S) waves, which are transverse waves and cannot travel through a liquid.
Types of surface waves include:
Love waves, which are transverse waves in which the earth's surface vibrates from side to side.
Rayleigh waves, which involve orbital motions that cause the earth's surface to move up and down.
P waves travel faster than S waves, and surface waves are the slowest.
The earth is composed of a series of layers:
The crust (the outer layer)
The mantle (the layer beneath the crust)
The core (the earth's innermost zone)
The internal structure of the earth has been determined with the help of P and S earthquake waves.
P and S waves are refracted as they travel within the earth.
P and S waves do not reach shadow zones on the other side of the earth from their origins. This is evidence for a central core.
S waves cannot travel through the core, suggesting that at least the outer core is liquid.
Faint traces of P waves in the shadow zones suggest that the earth's inner core is solid.
The lower boundary of the crust is known as the Mohorovicic discontinuity.
14-8. The Earth's Interior
The upper mantle appears to be composed mainly of ferromagnesian minerals.
Enormous pressures in the lower mantle have compressed minerals into crystal forms that are the most compact possible including very dense oxides of Si, Fe, and Mg.
The liquid outer core and the solid inner core are believed to be composed mainly of iron and nickel.
Temperatures inside the earth are believed to range from 375 °C at the top of the mantle to at least 3000 °C at the core's center and possibly much more.
Most of earth's interior heat is due to radioactivity.
14-9. Geomagnetism
In 1600, Sir William Gilbert proposed that earth behaves like a giant magnet.
The earth's magnetic field is believed to arise from coupled fluid motions and electric currents in the liquid iron of the core.
14-10. Weathering
Erosion includes all the processes by which rocks are worn down and the debris carried away.
Weathering is the gradual disintegration of exposed rocks.
Chemical weathering refers to the chemical decomposition of rock.
Mechanical weathering refers to the physical disintegration of rock.
Soil formation is an important result of weathering.
14-11. Stream Erosion
Running water is the main agent of erosion.
In a young landscape, a stream carves a narrow V-shaped valley.
With time, continued stream erosion results in a flood plain with a flat floor.
In a mature landscape, a treelike drainage pattern of secondary streams develops.
In a old landscape, flood plains are broad and further erosion is slow.
Stream erosion would ultimately reduce the land surface to a flat plain if no other processes such as geologic uplift took place.
14-12. Glaciers
A moving mass of ice formed by accumulated snow is called a glacier.
Valley glaciers are glaciers lying in mountain valleys. Valleys carved by valley glaciers have U-shaped cross sections.
Continental glaciers or ice caps are domal masses of ice that cover large areas of the earth's surface.
Rock fragments imbedded in the bottom of a glacier cut and scrape the underlying rock, resulting in glacial erosion.
14-13. Groundwater
All the water that penetrates the earth's surface is called groundwater.
When spaces within the soil and any underlying porous rocks are filled with water, the ground is said to be saturated.
The water table is the upper surface of the saturated zone.
A spring is formed where groundwater from the saturated zone comes to the surface.
An aquifer is a body of porous rock through which groundwater moves.
14-14. Sedimentation
The eroded material transported by the agents of erosion is eventually deposited to form sediments.
The most widespread sediments collect near continental margins.
There are four common sites of deposition:
Flood-deposited debris in stream gravel banks and sandbars
The flood plains of meandering rivers
Alluvial fans, which are deposits of sediments where streams emerge from steep mountain valleys and flow onto plains
Deltas, which are deposits of sediments where a stream enters a lake or sea
Moraines are piles of debris that accumulate around the ends and along the sides of glaciers. This material is called till.
The most important agents of deposition are ocean currents because of the large volume of sediment they carry and deposit.
Examples of groundwater deposition include:
Deposition of minerals in veins within rock
Cave deposits including stalactites (hanging from cave ceilings) and stalagmites (rising from cave floors)
Deposits around hot springs and geysers
Lithification is the process by which sediments become rock.
Lithification includes:
Compaction, in which the sediment grains are squeezed together under the pressure of overlying deposits
Concentration, in which the sediment grains are bound together by chemical changes brought about by circulating groundwater
14-15. Volcanoes
The processes of vulcanism (the movements of molten rock) and diastrophism or tectonism (the movements of the solid materials of the earth's crust) act in opposition to the processes that would level the earth's surface.
A volcano is an opening in the earth's crust through which molten rock (called magma while underground, lava above ground) pours forth.
A volcano usually has a depression, or a crater, at its summit.
The two main factors that determine whether an eruption will be quiet or explosive are:
Viscosity (resistance to flow) of the magma
Amount of dissolved gases, such as water vapor (the most prominent), carbon dioxide, nitrogen, hydrogen, and various sulfur compounds in the magma
Magmas rich in silica (the most viscous) and dissolved gases result in explosive eruptions, while magmas with modest gas and silica contents result in quiet eruptions.
Lava hardens into one or another type of volcanic rock including:
Basalt (the most common)
Rhyolite (the most silica rich)
Pumice (light and porous)
Most active volcanoes occur around the borders of the Pacific Ocean, on some of the Pacific Islands, in Iceland, and in East Africa.
14-16. Intrusive Rocks
Plutons are intrusive bodies formed by the solidification of magma under the earth's surface.
Because plutons cool slowly, the resulting intrusive igneous rocks tend to be coarse-grained. An example is granite.
A dike is a wall of intrusive igneous rock that cuts across existing rock layers.
A sill is a pluton formation that lies between and parallel to existing rock strata.
A laccolith is a pluton formation that forms a mushroom-shaped intrusion that pushes up overlying rock strata.
A batholith is a very large pluton that can cover hundreds of thousands of square km. Batholiths are always associated with mountain ranges, past or present.
14-17. The Rock Cycle
Rocks can change from one kind to another in a variety of ways.
