concept that geologic processes operating at present are the same processes that operated in the past. Relationship to process and rate. "The Present is key to the past" modern geologist prefer the term "actualism"
• Surfaces separating two different rock types or rocks of different ages. Surfaces between 2 layers
• Bodies of rock of considerable thickness w/ recognizable characteristics that make each distinguishable from adjacent rock units.
• Beds of sediment deposit in horizontal layers
• Younger sedimentary rocks are on top of the older
• An original sedimentary layer extends laterally until it tapers or thins at its edges.
Cross Cutting Relationships
• A disrupted pattern is older than the cause of disruption. Younger rocks/features cut across fabrics or layering in older rocks/features.
• Fragments included in a host rock are older than the host rock. Parts of the country rocks included within intrusive bodies are older than the intrusion
-a contact that represents a significant gap in the geologic record, with the rock unit above being considerably younger than the unit below
• unconformity between parallel sedimentary layers, can be difficult to detect, need to examine fossil record.
• unconformity between sedimentary layers with different bedding orientations (usually inclined beds beneath near- horizontal layers above).
• Contact in which younger strata overlie an erosion surface on tilted or folded layered rock.
• unconformity between sedimentary layers sitting on top of older metamorphic or igneous rocks.
• Contact in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.
usually means determining time equivalency of rock units.
• Being able to trace physically the course of a rock unit
Similarity of Rock Types
• Must be careful using similar lithology. Fossil assemblages can be used to correlate formations of similar age.
• Not very reliable
• assuming similar sequences of rocks formed at same time - inaccurate if common rocks are involved
• Faunal Succession
fossils change in predictable manner over geologic time. Correlation by fossils
• Index Fossil
: fossil from a very short lived geographically widespread species known to exist during a specific period of time.
• Fossil Assemblages
: groups of fossils from species that coexisted can be diagnostic of a particular geological time.
• Determining the age of a rock through its radioactive element
(loss of particle consisting of 2 protons and 2 neutrons)
loss of an electron from a neutron to produce a proton
capture of an electron by a proton to produce a neutron
spontaneous nuclear change of isotopes with unstable nuclei
amount of time required for half of existing parent isotope to decay to daughter product, is constant for a given radioactive isotope.
movement in which bedrock, rock debris, or soil moves down slope in bulk, or as a mass, because of the pull of gravity.
general term for the slow to very rapid descent of rock or soil
• Consolidated material
• Unconsolidated material
Soil(debris, earth, or mud)
o Debris-implies coarse grained fragments predominate in the soil
o Earth-predominantly fine grained
o Mud- has a high content of water clay and silt
implies that the descending mass is moving downslope as a viscous fluid
descending mass remains relatively intact, moving along one or more well-defined surfaces
o Transitional slide
material slides along a plane approximately parallel to the slope of the surface.
o Rotational Slide(slump
): involves movement along a curved surface, the upper part moving downward while the lower part moves outward.
occurs when material free falls or bounces down a cliff
the driving force for mass wasting
• Shear Force
role in block sliding; component of gravitational force parallel to slope in downslope direction
• Shear Strength
role in debris/earth flows: resistance to downslope movement through friction between soil particles. Controlled by factors such as the cohesiveness of the material, friction between particles, pore pressure of water
• Normal Force
component of gravity acting perpendicular to slope; increases friction.
• Very slow, downslope movement of soil. Due primarily to freeze/thaw cycles.
takes place when motion is taking place within a moving mass of unconsolidated or weakly consolidated material.
• Earth Flows
finer grained material is predominant; flow of soils downslope as a viscous fluid. Can be slow or rapid.
• Solifluction & Permafrost
flow of water-saturated soil over frozen subsoil(permafrost)
• Debris Flow
flow involving soil in which coarse material is predominant. Less water than mudflow
• Mud Flow
flowing mixture of soil and water, usually moving down a channel. Fine grained material is predominant
• Debris Avalanche
very rapidly moving turbulent mass of debris air and water.
Rockfalls & Rockslides
coherent blocks of bedrock falling or sliding downslope
Debris Slides & Falls
-unconsolidated materials falling or sliding downslope