Geol 106 Ch 17
above, below; below
A sill produces a zone of baking immediately __ and ___ below it previously existing sedimentary rocks. A lava flow, in contrast, bakes only those rocks _ it.
radioactivity
It was not until the discovery of ____ near the end of the 19th century that absolute ages could be accurately applied to the relative geologic time scale.
sedimentary rock
Steno observed that when streams flood, they spread out across their floodplains and deposit layers of sediments that bury organisms dwelling on the floodplain. Subsequent floods produce new layers of sediments that are deposited, or superposed, over previous deposits. when lithified, these layers of sediment become ___
dual scale
Today, the geologic time scale is really a ____- a relative scaled based on rock sequences with radiometric dates expressed as years before the present.
bedding plane
Unless the erosional surface separating the older from the younger parallel beds is well defined or distinct, the disconformity frequently resembles an ordinary ____. Hence, many disconformities are difficult to recognize and must be identified on the basis of fossil assemblages.
older
a buried lava flow is -__ than the rocks above it
principle of cross-cutting relationships
a principle holding that an igneous intrusion or fault is younger than the rocks it intrudes or cuts across
principle of superposition
a principle holding that in vertical sequence of undeformed sedimentary rocks, the relative ages of the rocks can be determined by their position in the sequence-oldest at the bottom followed by successively younger layers
the principle of inclusion
a principle holding that inclusions or fragments in a rock unit are older than the rock unit itself; for example, granite inlcusions in sandstone are older than the sandstone
the principle of fossil succession, or the principle of faunal and floral succession
a principle of holding that fossils, and especially groups or assemblages of fossils, succeed one another through time in regular and predictable order
younger
a sill, resulting from laer igneous intrusion is ____ than all of the beds below it and _____ than the immediately overlying bed as well
disconformity
a surface of erosion or nondeposition separating younger from older rocks, both of which are parallel with one another
principle of orignal horizontality
according to this principle, sediments are deposited in horizontal or nearly horizontal layers because sedimentary particles settle from water under the influence of gravity ; therefore, a sequence of sedimentary rock layers that is steeply inclined from the horizontal must have been tilted after deposition and lithification
absolute dating
also called numerical dating; provides specific dates for rock unit or events expressed in years before the present
nonconformity
an erosional surface cut into metamorphic or igneous rocks is covered by sedimentary rocks. This type of unconformity closely resembles an intrusive igneous contact with sedimentary rocks
angular uncomformity; angular
an erosional surface on tilted or folded strata over which younger strata were deposited. The strata below the unconformable surface generally dip more steeply than those above, producing an ___ relationship
unconformity
break in the geologic record represented by an erosional surface separating younger rocks from older rocks; represent times of nondeposition, erosion or botoh
radiometric dating
dates are calculated from the natural decay rates of various radioactive elements present in trace amounts in some rocks
correlation
demonstration of the physical continuity of rock units or biostratigraphic units, or demonstration of time equivalence as in time-stratigraphic correlation
contact metamorphism
geologists observe whether the sedimentary rocks in contact with igneous rocks show signs of baking or alteration by heat which is called ____. A sedimentary rock that shows such effects must be older than the igneous rock with which it is in contact.
relative dating
placing geologic events in a sequential order as determined from their position in the geologic record
one event preceded another
relative dating dos not tell us how long ago a particular event took place, only that
conformable strata
sequences of rocks in which deposition was more or less continuous
principal of lateral cotinuity
states that sediment extends laterally in all directions until it thins and pinches out or terminates against the edge of the depositional basin
principle of superposition
the basis for relative-age determination of strata and their contained fossils
principle of uniformitarianism
the concept that the same processes seen today have been operated throughout geologic time
hiatus
the interval of geologic time not represented by strata
radiometric dating
the most common method of obtaining absolute ages
disconformity, angular unconformity, nonconformity
three types of unconformities