Geologic Mapping Terms
Fold
A bend or warp in layered rock
Pluton
A body of intrusive igneous rock (called a plutonic rock) that is crystallized from magma slowly cooling below the surface of the Earth. One of seven intrusive igneous rock bodies, also including sills, laccolths, pipes, stocks, batholiths and dikes.
Strain
A change in shape or volume of rock in response to stress
What is the difference between Dike and Sill?
A dike is a sheet of rock that formed in a fracture in a pre-existing rock body. Dikes can be either magmatic or sedimentary in origin. A sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock or beds of volcanic lava.
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)
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)
Strike slip fault
A fault in which the direction of movement is mostly horizontal and parallel to the strike of the fault
Syncline
A fold in the form of a trough, with the rock strata concave downward and the younger rock in the core
Recumbent fold
A fold that is so strongly overturned that it is almost lying flat
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
Stress
A force acting on the surface, per-unit area, which may be greater in certain directions than others
Geologic cross section
A geologic cross section is a drawing of a vertical slice through the Earth and shows geologic features that occur underground.
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.
Azimuth
A horizontal angle measured from north or south. The angle between the projected vector and a reference vector on the reference plane.
Topographic map
A map that shows the surface features of an area.
Pressure
A particular kind of stress in which the forces acting on a body are the same in all directions
Ductile deformation
A permanent but gradual change in shape or volume of the material, caused by flowing or bending. Also known as plastic deformation
Brittle deformation
A permanent change in shape or volume, in which a material breaks or cracks
What is the difference between Pluton and Batholith?
A pluton is a relatively small intrusive body (a few to tens of km across) that seems to represent one fossilized magma chamber. A batholith is much larger (up to hundreds of km long and 100 km across) and consists of many plutons that are similar in composition and appearance. Batholiths indicate a long period of repeated igneous intrusions over a large area, such as might be expected along a subduction zone.
Craton
A region of continental crust that has remained tectonically stable for a long time
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.
Cross-cutting relationship
A rock or fault is younger than any rock or fault through which it cuts. Hutton's law.
Formation
A rock unit that can be distinguished from units above and below it on the basis of rock type and recognizable boundaries
Laccolith
A sheet intrusion (or concordant pluton) that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base. One of seven intrusive igneous rock bodies, also including sills, pipes, stocks, plutons, batholiths and dikes.
Stock
A stock is a remnant of the vent of a volcano or plutonic body with an areal extent less than 40 square miles (or 100 square kilometers). Far below the surface, a large magma chamber will slowly cool to form small plutons and large batholiths. One of seven intrusive igneous rock bodies, also including sills, laccolths, pipes, plutons, batholiths and dikes.
Shear
A stress that acts in a direction parallel to a surface
Tension
A stress that acts in a direction perpendicular to and away from the surface
Compression
A stress that acts in a perpendicular direction and leads to failure due to tension
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
Elastic deformation
A temporary change in shape or volume from which material rebounds after the deforming stress is removed
Catastrophism
A theory that correlates major catastrophic events to changes in the Earth surface.
Uniformitarianism
A theory that states that "the present is the key to the past".
Anticline
A type of fold that resembles a "rainbow" with the oldest rock layers in the middle.
Xenolith
A type of igneous rock clast that is often found in other igneous or sedimentary rock layers.
Nonconformity
A type of unconformity that separates igneous/metamorphic rocks from overlying sedimentary rock strata.
Disconformity
A type of unconformity that shows an erosional surface between two parallel sedimentary layers.
Angular Unconformity
A type of unconformity where underlying strata have been tilted relative to the overly sediment layers.
Orogen
An elongated region of crust that has been deformed and metamorphosed through a continental collision. ie. The Alps, Himalayas, and inactive Appalachians
Geologic contacts
Boundaries between rocks on a map of the area
Extrusive Igneous Rock Bodies
Flows and tuff.
Flows
Flows are solidified deposits of lava, resulting from a single or short-lived series of eruptive events. They are deposited on the surface close to their source, and are rarely very extensive. Over time, flows piled on top of one another can resemble the layering of sedimentary rocks, and are subject to similar principles of relative age dating. One of two types of extrusive igneous rock bodies, also including tuff.
Faults
Fractures or breaks in rocks on which movement has occurred
Microfracture
Fractures that are so tiny that they can only be viewed under a microscope
Geologic map
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.
Intrusive Igneous Rock Bodies
Igneous rock formed from magma forced into older rocks at depths within the Earth's crust, which then slowly solidifies below the Earth's surface, though it may later be exposed by erosion. Igneous formations include sills, laccoliths, pipes, stocks, plutons, batholiths, and dikes.
Axial plane
Imaginary plane dividing the fold in half, as symmetrically as possible
Batholith
Large, regional-scale intrusive body or agglomeration of intrusive bodies (esp. plutons) that have no visible bottom. One of seven intrusive igneous rock bodies, also including sills, laccolths, pipes, stocks, plutons, and dikes.
Law of Original Horizontality
Layers of sediment are originally deposited horizontally. The principle is important to the analysis of folded and tilted strata. Steno's law.
Contour lines
Lines of equal elevation used on topographic maps
Outcrop
Places where bedrock is exposed at the surface. Use for geologic mapping
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.
Dike
Sheet-like intrusion that cuts across layers of bedrock. One of seven intrusive igneous rock bodies, also including sills, laccolths, pipes, stocks, plutons, and batholiths.
Sill
Sheet-like intrusion that forces it's way between and is parallel to preexisting layers of bedrock. One of seven intrusive igneous rock bodies, also including laccolths, pipes, stocks, plutons, batholiths and dikes.
Monocline
Simplest type of fold, a bend in rock strata that are otherwise uniformly dipping or horizontal.
Attitude
Strike and dip refer to the orientation or attitude of a geologic feature.
Paleozic
The "ancient" time period that is marked with the presence of the super-continent Gondwana.
Mesozoic
The "middle" time period that is marked with the presence of dinosaurs.
Cenozoic
The "present" time period that is marked with the presence of Pangea and multiple ice ages.
Offset
The amount of movement or displacement that has occurred on a fault
Dip
The angle between a tilted surface and a horizontal plane
Strike
The compass orientation of the line of intersection between horizontal plane and a planar feature, such as a rock layer or fault
Relief
The difference between the lowest and highest elevations in an area
Isostasy
The flotational balance of the lithosphere on the asthenosphere. ie. the Appalachians (inactive mountain chain displaying topographic relief)
Magnetic Declination
The horizontal angle between the true geographic North Pole and the magnetic north pole, as figured from a specific point on the Earth.
Plate tectonics
The large scale process that is the ultimate cause of most of the deformation seen in rocks on Earth
Law of Superposition
The law stating that in sedimentary horizontal layers, the oldest rocks are at the bottom and the youngest rocks are at the top in a time sequence. Steno's law.
Law of Lateral Continuity
The law stating that layers of rock will extend until they "pinch" out from lack of material.
Axis
The line where the axial plane intersects the fold. This may be simple and horizontal or tilted and plunge
Hinge
The line where the axial plane intersects the fold. This may be simple and horizontal or tilted and plunge, same as axis.
Footwall
The lower wall of an inclined fault.
Saddle
The saddle between two hills or mountains is the area around the highest point of the (optimal) mountain pass between the two massifs
Basins
The structure caused by the downwarping of strata
Domes
The structure caused by the up warping of strata
Structural geology
The study of stress and strain, the processes that cause them, and the deformation and rock structures that result from them
Limbs
The two halves of the fold, on either side of the axial plane
Hanging wall
The upper or overhanging wall of an inclined vein, fault, or other geologic structure —opposed to footwall.
Plunge
The vertical angle between the inclined linear feature and an imaginary horizontal plane
Index Fossil
These remains can be used to date rock units in terms of their relative geologic time periods.
Hydrolytic weakening
Trace amounts of water entering strong minerals such as quartz and olivine and significantly weakening them
Trend
Trend is the direction of the line formed by the intersection of the planar feature with the ground surface; trend is the same as strike only if the ground surface is parallel to the horizontal plane. The compass bearing in the direction of the plunge of the linear geological feature
Tuff
Tuff is used to refer to extensive deposits of volcanic ash. They can be very thick near volcanic vents, and become thinner farther from their source. Some tuff deposits have been found thousands of kilometers away from their sources.
Pipe
Volcanic pipes are subterranean geological structures formed by the violent, supersonic eruption of deep-origin volcanoes. One of seven intrusive igneous rock bodies, also including sills, laccolths, stocks, plutons, batholiths and dikes.
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
Differential stress
When the force is greater in one direction then in the other
