Geologic Mapping Science Olympiad

Geologic maps

Geologic maps show the distribution of rocks at the Earth's surface. Usually, the rocks are divided into mappable units that can be easily recognized and traced across an area. The divisions (contacts between units) are based on color, texture, or rock composition. Geologic maps only show what is exposed at the surface of the Earth.

Geologic cross sections

Geologic cross sections are drawings of a vertical slice through the Earth.

Superposition

Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest at the top. Steno's law.

Original horizontality

Layers of sediment are originally deposited horizontally. The principle is important to the analysis of folded and tilted strata. Steno's law.

Lateral continuity

Layers of sediment initially extend laterally in all directions, in other words, they are laterally continuous. Steno's law.

Cross-cutting relationship

A rock or fault is younger than any rock or fault through which it cuts. Hutton's law.

Faunal succession

Sedimentary rock strata contain fossilized flora and fauna which succeed each other vertically in a specific order that can be identified over wide horizontal distances. Smith's law.

Now find the North Branch of the San Gabriel Fault. What are the colors, names, and abbreviations of the rock units on either side of the fault?

Qg = light blue = Surficial sediments/landslide

Qog = light grey = Surficial sediments

Qoa = beige = Surficial sediments

Qd = purple = Quartz Diorite

Find the Devonshire Fault on the cross-section labeled A-B. Is this fault a normal, reverse, or strike-slip fault?

Reverse fault

On cross-section A-A' find the fault below the Cheops Pyramid. This fault formed between the depositions of two rock units. Name these two rock units.

Bright Angel Formation and Dox Formation

Anticline

A fold in the form of an arch, with the rock strata convex upward and the older rock in the core

Brittle deformation

A permanent change in shape or volume, in which a material breaks or cracks

Compression

A stress that acts in a direction perpendicular to and toward a service

Craton

A region of continental crust that has remained tectonically stable for a long time

Dip

The angle between a tilted surface and a horizontal plane

Ductile deformation

A permanent but gradual change in shape or volume of the material, caused by flowing or bending. Also known as plastic deformation

Elastic deformation

A temporary change in shape or volume from which material rebounds after the deforming stress is removed

Fold

A bend or warp in layered rock

Geologic cross section

A diagram that shows geologic features that occur underground

Geologic map

A map that shows the locations, kinds, and orientations of rock units, as well as structural features such as faults and folds.

Isostasy

The flotational balance of the lithosphere on the asthenosphere. ie. the Appalachians (inactive mountain chain displaying topographic relief)

Normal fault

A fault in which the block of rock above the fault surface (hanging wall) moves downward relative to the block below (foot wall)

Orogen

An elongated region of crust that has been deformed and metamorphosed through a continental collision. ie. The Alps, Himalayas, and inactive Appalachians

Pressure

A particular kind of stress in which the forces acting on a body are the same in all directions

Reverse fault

A fault in which the block on top of the fault surface (hanging wall) moves up and over the block on the bottom (foot wall)

Shear

A stress that acts in a direction parallel to a surface

Strain

A change in shape or volume of rock in response to stress

Strike

The compass orientation of the line of intersection between horizontal plane and a planar feature, such as a rock layer or fault

Strike slip fault

A fault in which the direction of movement is mostly horizontal and parallel to the strike of the fault

Stress

A force acting on the surface, per-unit area, which may be greater in certain directions than others

Structural geology

The study of stress and strain, the processes that cause them, and the deformation and rock structures that result from them

Syncline

A fold in the form of a trough, with the rock strata concave upward and the younger rock in the core

Tension

A stress that acts in a direction perpendicular to and away from the surface

Thrust fault

A reverse fault with a shallow angle of dip (less than 45). Common in mountain chains along convergent plate boundaries. ie. the Canadian Rockies. This type of fault represents a disturbance of the original stratigraphic sequence

Topographic map

A map that shows the shape of the ground surface, as well as the location and elevation of surface features, usually by means of contour lines

Differential stress

When the force is greater in one direction then in the other

Uniform stress/confining stress

Also known as pressure, the forces on a body of rock are uniform in all directions

Elastic limit

The degree of stress beyond which the material is permanently deformed

Strain rate

The rate at which rock is forced to change its shape or volume

Brittle - ductile transition

The depth below which ductile properties predominate

Hydrolytic weakening

Trace amounts of water entering strong minerals such as quartz and olivine and significantly weakening them

Temperature

When exposed to this in a higher degree rock becomes less brittle and more ductile; this is seen deep inside the planet. Rock is like glass in this respect

Confining pressure

A high amount of this reduces the brittleness of rock because it hinders the formation of fractures. Near the earth's surface this is low and rock exhibits brittle behaviour, at great depth this is hot and rock tends to be ductile and deforms.

Rates of deformation

The rate at which stress is applied to a solid. If stress is applied quickly the rock may have a brittle manner but if the stress is applied over a long period of time the rock maybe behave in a ductile manner

Composition

Determines exact point at which it's brittle - ductile transition will occur

Overturned rock layers

When rock layers are tilted even further than the vertical orientation

Microfracture

Fractures that are so tiny that they can only be viewed under a microscope

Fault

A fractured rock along which movement has occurred along the fracture surface

Left lateral

A strike slip fault that appears to be moving to the left to an observer standing on either block.

Right lateral

A strike slip fault that appears to be moving to the right tune observer standing on either block. ie. The San Andreas Fault

Monocline

Simplest type of fold, a local steeping in otherwise uniformly dipping strata. ie. a tablecloth draped over a table

Axial plane

Imaginary plane dividing the fold in half, as asymmetrically as possible

Flanks or limbs

The two halves of the fold, on either side of the axial plane

Axis or hinge

The line where the axial plane intersects the fold. This may be simple and horizontal or tilted and plunge

Axial trace

The projection of the fold axis or an axial plane onto a horizontal plane (A map). A map needs to show a line representing the axial line and axial plane, a symbol to indicate the type of fold, an arrow to show the direction in which the axis is plunging and a number to show the plunge angle

Domes

The structure caused by the up warping of strata

Basins

The structure caused by the downwarping of strata

Asymmetrical fold

A fold with one limb dipping more steeply than the other

Overturned fold

A fold with one limb tilted so far over that it is upside down

Recumbent

A fold that is so strongly overturned that it is almost lying flat

Outcrop

Places where bedrock is exposed at the surface. Use for geologic mapping

Geologic contacts

Boundaries between rocks on a map of the area

Topographic profile

Using the height information from a topographic map to produce this type of profile

Relief

The difference between the lowest and highest elevations in an area

Contour lines

Lines of equal elevation used on topographic maps

Formation

A rock unit that can be distinguished from units above and below it on the basis of rock type and recognizable boundaries

disconformity

A type of unconformity that shows an erosional surface between two parallel sedimentary layers.

Law of Original Horizontality

The law stating that when sediments are deposited, they are originally deposited horizonatally.

Catastrophism

A theory that correlates major catastrophic events to changes in the Earth surface.

Index Fossil

These remains can be used to date rock units in terms of their relative geologic time periods.

Nonconformity

A type of unconformity that separates igneous/metamorphic rocks from overlying sedimentary rock strata.

Cenozoic

The "present" time period that is marked with the presence of Pangea and multiple ice ages.

Uniformitarianism

A theory that states that "the present is the key to the past".

Law of Superposition

The law stating that in horizontal layers, the oldest rocks are at the bottom and the youngest rocks are at the top.

Anticline

A type of fold that resembles a "rainbow" with the oldest rock layers in the middle.

Angular Unconformity

A type of unconformity that where underlying strata have been tilted relative to the overly sediment layers.

Mesozoic

The "middle" time period that is marked with the presence of dinosaurs.

Law of Lateral Continuity

The law stating that layers of rock will extend until they "pinch" out from lack of material.

Paleozic

The "ancient" time period that is marked with the presence of the super-continent Gondwana.

Syncline

A type of fold that resembles a "sink" with the youngest rock layers in the middle.

Xenolith

A type of igneous rock clast that is often found in other igneous or sedimentary rock layers.