Geology Facts

U.S. Department of the InteriorU.S. Geological Survey

Contact: Kate Barton

Last Modification: 18 Oct 2000 (ebj)USGS Privacy Policy

U.

Earth’s Layers

• The Earth consists of a solid inner core (Fe and Ni), a liquid outer core (Fe and Ni), a plastic-like mantle (Si, O, Fe, Ni), and a thin rocky crust (Si, O).

The lithosphere is the crust and upper mantle

Ocean crust (made of basalt) is thinner, younger, and denser than continental crust (made of granite).

Convection currents move tectonic plates. Hot material

rises, cools, becomes more dense, and sinks

• Convergent boundaries are colliding plates which cause folded or thrust faulted mountains, subduction zones, (volcanic and trenches) and reverse faults (thrust faults)

Divergent boundaries are dividing plates and cause sea-floor spreading, mid-ocean ridges, rift valleys, and volcanoes. Normal faults are produced from this movement.

Transform boundaries slide past each other and strike slip faults and earthquakes are produced.

Earthquakes can result with any plate

movement

Hot spots are not related to plate

movement

A fault is a break or crack in the Earth’s

crust where movement has occurred

Volcanic activity is associated with

subduction, rifting, or seafloor spreading

An ocean plate will always sink under a continental

plate because it is more dense, creating a trench

Appalachian Mountains are folded

mountains

3 seismic stations are always needed to find the

epicenter of an earthquake

Earthquake Waves

• P waves travel the fastest and reach the seismic station first. P waves travel through solids and liquids. P waves slow down and bend when they hit the liquid outer core.

• S waves do not travel through liquids.• L waves, surface waves cause the most

damage

Most volcanoes and earthquakes occur in the “Ring of Fire.”

The San Andreas Fault is responsible for

the earthquakes in California

The North American Plate is traveling in a westward course

The Pacific Plate is traveling in a northwestern course

There are 3 kinds of volcanoes, shield,

composite (also known as strata), and cone

Magma is found underground and

lava is above ground

Minerals

• A mineral is found in nature, inorganic, solid, with a definite chemical composition and structure. (crystalline)

Mineral properties depend on their atomic structure

Ores are useful and profitable

• Ores of Common Metals

Contains: bauxite, bornite, chalcopyrite, chalcocite, pit ore, magnetite, hematite, specular hematite, galena, pyrolusite, garnierite, sphalerite, chromite, stibnite, and molybdenite.

Minerals are determined by their streak, cleavage,

fracture, hardness, color, luster, and special properties

• Streak- The color of the mineral in powder form. This test is done by rubbing the mineral across a white streak plate. Several minerals have a streak that is not the same color as the mineral itself. Most metallic luster minerals have a dark colored streak

Cleavage and Fracture• Cleavage or Fracture- If a mineral breaks along flat, smooth surfaces it

shows cleavage. Cleavage can be in one, two or three directions. Some examples are:

• Mica- cleavage in one direction, see the diagram below (upper right),• Feldspar- cleavage in two directions, see the diargam below (lower left),• Calcite, Galena and Halite- cleavage in three directions, see the diargam

below (lower right).

• If a mineral breaks along irregular rough surfaces it shows fracture. Quartz shows a special type of fracture called concoidal (shell-like) fracture.

Hardness• Hardness- The ability of one mineral to scratch another. The softer mineral gets scratched.

You test a mineral's hardness by scratching the unknown mineral with an object of known hardness. Moh's Scale of Hardness is used to rate the hardness of a mineral. The chart below shows the ten minerals that make up the hardness scale and some common materials with their hardness to test unknown minerals. One on the scale is the softest and ten is the hardest.

• To find the hardness of a mineral you first have to find out what it can scratch and what can

scratch it. For example, if a mineral can scratch glass but can't scratch a streak plate, the mineral has a hardness of 6 on the Moh's Hardness Scale.

Color

• Color- The most easily observed property, but usually the least useful. A mineral's color can be changed by the impurities that are found in the mineral.

Luster

• Luster- The way light is reflected from a newly exposed surface. Described as either metallic or nonmetallic. Some examples of minerals with metallic luster are pyrite, galena and magnetite. Some examples of minerals with nonmetallic luster are calcite, quartz and feldspar. Nonmetallic luster can also be described as glassy, pearly, waxy and earthy (dull).

Special Properties

• Acid test- This test is performed by dropping weak hydrochloric acid on the mineral. If it reacts (fizzes) then the mineral is calcite. This test will also help to identify the rocks limestone and marble, because calcite is the principal mineral in both.

• Magnet test- If there is a magnetic attraction between your mineral and a metal object, then the mineral has a high iron content. The mineral magnetite will attract metal objects.

Igneous Rocks

Igneous rocks are classified by their composition and texture

• Igneous rocks are produced by the cooling of magma or lava.

• Fast cooling = extrusive, small crystals. Slow cooling = intrusive, larger crystals

Extrusive small grains, glassy, air holes present (obsidian, pumice, basalt)

Intrusive large grained, granite

Metamorphic formed by heat and pressure

Metamorphic classified as foliated (banded) or non foliated

• Foliated rocks are slate,

schist, gneiss

• Non-foliated include

marble and quartzite

Limestone morphs into marble. Sandstone morphs into quartzite

Sedimentary

• Sedimentary rocks are formed from rock fragments, organic material, or chemical precipitation

• Sedimentary are classified as clastic, organic, and chemical

Limestone is formed both chemically and organically

• Sedimentary are found in flat layers or strata. Fossils are found in these layers

Clastic rocks are made of fragments –conglomerate, sandstone, shale

Destructive Geology

• Chemical weathering occurs mainly in warm, humid climates

Weathering can occur as either a

mechanical or chemical process

Erosion is the process of materials being transported by

moving water, ice, wind, or gravity

Streams and moving water are the

major agents of erosion

Deposition is the dropping or settling

out of sediments

High erosion= high relief areas,

high deposition= low relief areas

Large particles settle out first, sediment size from

largest to smallest- sand, silt, clay

As particle size increases, permeability increases (ability to hold water).

• Permeability is the ability of a material to drain

Porosity is the amount of pore space

in a rock or sediment

• Different grain sizes and packing arrangements result in different porosity values.  Top: individual pore spaces decrease in size with decreasing grain size. Bottom: porosity varies with packing (arrangement) of grains.

Soil evolution starts with the weathering of bedrock Organic material must be present in order to have soil. (humus)

Soil profiles consist of 3 horizons: O topsoil (humus) most evolved, A less humus, leaching of minerals from A, and B weathered rock

C These layers are on top of bedrock

Groundwater layers from the surface down would include zone of

aeration, water table, and zone of saturation

An aquifer is a layer of rock that

transports groundwater freely

• A spring is an area where the water table reaches land’s surface

The watersheds of Virginia are the Sounds of North Carolina, the Chesapeake Bay, and the Atlantic Ocean.

The Coastal Plain is the flattest area underlain by all the types of

sediments produced by the erosion of the Appalachian Mountains

Piedmont is underlain by igneous and metamorphic rocks produced by

ancient volcanoes. Separated by the fall line from the Coastal Plain.

Blue Ridge Mountains are oldest in the state

they were produced by converging plates.

Valley and Ridge long parallel ridges composed of folded and faulted rocks that occurred during the collision of Africa

and North America during the Paleozoic.

• Karst Topography and fossils are abundant

Karst topography has caves and sinkholes produced by

acidic groundwater dissolving limestone

Appalachian underlain by sedimentary rocks. Coal

resources are found here. Fossils present.

Virginia resources include limestone, coal

and gravel

Renewable resources can be replaced by nature at a rate at which

they are used. Includes vegetation, water, and soil

Nonrenewable resources are renewed very slowly or

not at all. Includes coal, oil, and minerals

The Earth’s water supply is

renewable but also finite

A fossil is the remains, impressions, or

just the evidence of life preserved in rock

Virginia’s fossils are mostly marine and are from

all eras in history

The Principle of Superposition states that the oldest rocks are

found on the bottom of strata and the youngest are found on top

The Principle of Cross-cutting relationships state that an igneous

intrusion is younger than the layers it cuts across

Fossils, Superposition, and Cross-cutting relationships

are used to determine relative ages

Relative ages are placing events in sequence without

assigning exact numerical ages

Absolute age gives numerical age to an event

Radioactive decay or half-life is used to

determine the absolute age of rocks Radioactive isotope Daughter isotope Half-life (Myrs) ApplicationsCarbon 14 Nitrogen 140.0057 Formerly living material less than 30,000 yoUranium 235 Lead 207713 Lunar samples, meteorites, pre-Cambrian rocksPotassium 40 Argon 40 1300 Ocean floor basalts, lava flows, some sediment.Uranium 238 Lead 2064510 Lunar samples, meteorites, pre-Cambrian rocks

Rubidium 87 Strontium 8747,000 Very old rocks, similar to Uranium

1.Uranium dating is used to find the ages of the oldest rocks. 2.Carbon-14 is used to find the ages of human artifacts.

The Earth is about 4.6 billion years old

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