GEL 100 CHAP 7
Specific time when an event occurred or a particular # of yrs that have elapsed since an event occurred.
Absolute Age
exactly when an event took place
Absolute ages indicate what?
An absolute age is a specific age in years, or a statement that an event happened a particular # of yrs in the past.
An absolute age is a specific age in years, or a statement that an event happened a particular # of yrs in the past.
In the photo on the left, the geologist examines where a relatively young basalt dike cuts across part of a relatively older granite batholith in northern Michigan. In the photo on the right, a fault offsets sedimentary layers. The dashed lines show how much one layer was displaced by fault movement. The sedimentary deposits formed prior to movement along the fault.
Applying the principle of cross-cutting relationships.
The vertical orientation of the beds in these sedimentary layers found in Quebec, Canada, require that the layers were tilted from an initial horizontal orientation after they were deposited.
Applying the principle of original horizontality.
The principle of superposition requires that the oldest visible sedimentary rocks along the Colorado River, in Utah, are exposed at the bottom of the canyon while the youngest rocks form the top rim of the canyon.
Applying the principle of superposition.
Granite underlies sedimentary rocks in both of these outcrop sketches, but the age relationships of granite and sedimentary rocks are different in each sample.
Determining which rock is older.
The diagram shows red sandstone and shale beds, inclined at an angle downward to the right, that are overlain by horizontal layers of sandstone, limestone, and shale. A band of basalt cuts across all of the sed. layers, forming a dike. Discolored and hardened sed. rock borders the basalt, implying that contact metamorphism occurred adjacent to the dike.
How do geologists interpret Earth's history? A field sketch illustrates rocks exposed along a sea cliff. Each rock reveals its own record of the processes that formed it. With careful study you can understand the order in which these processes took place and even when they happened.
In the bottom diagram, the granite cuts across the sedimentary strata along an intrusive contact and the sedimentary rock is metamorphosed near the contact with the granite. Therefore, the granite must be younger than the sedimentary rock.
In the bottom diagram, the granite cuts across the sedimentary strata along an intrusive contact and the sedimentary rock is metamorphosed near the contact with the granite. Therefore, the granite must be younger than the sedimentary rock.
In the top diagram, geologists can tell that the sediment was deposited on top of the granite because the lowest sedimentary bed contains pebbles eroded from granite. The principle of superposition applies in this case and tells us that the sedimentary rocks are younger than the granite.
In the top diagram, geologists can tell that the sediment was deposited on top of the granite because the lowest sedimentary bed contains pebbles eroded from granite. The principle of superposition applies in this case and tells us that the sedimentary rocks are younger than the granite.
Ordering of objects or features from oldest to youngest; establishing the age of one thing as older or younger than another.
Relative Age
the principle of superposition
This diagram represents what?
1. the order in which the events occurred 2. when they occurred Or in other words, determine either the relative age or absolute age, or both, of each rock type.
To describe the geologic history of the events preserved in a rock record, you need to establish what 2 things?
To establish relative age, items or events are placed in order of what happened first, what happened next, and what happened last.
To establish relative age, items or events are placed in order of what happened first, what happened next, and what happened last.
fracture across which rocks are displaced as a result of tectonic forces.
What is a fault?
Geologic features that cut across rocks (e.g., faults, igneous intrusions) must have formed after the rocks that they cut through.
What is the principle of cross-cutting relationships?
Sedimentary beds are horizontal or near horizontal when deposited.
What is the principle of original horizontality?
Where rocks are found in layers, one above the other, the lowest rock formed first, with each successively higher layer being younger than the one below.
What is the principle of superposition?
Relative ages
________________ establish the sequence of a series of events w/o establishing exactly when each event occurred.
