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Questions and Answers
What are primary structures primarily formed from?
What are primary structures primarily formed from?
Layering is considered a primary structure.
Layering is considered a primary structure.
True
What do primary structures help indicate in geology?
What do primary structures help indicate in geology?
Environmental conditions at the time of sedimentation.
If A and B are younging in the same direction, then A is a _____ bed than B.
If A and B are younging in the same direction, then A is a _____ bed than B.
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Match the concepts with their descriptions:
Match the concepts with their descriptions:
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What type of structure is formed by the collapse of wet sediments due to tilting or agitation?
What type of structure is formed by the collapse of wet sediments due to tilting or agitation?
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Mud cracks are formed by the wetting of clay and silt sediments.
Mud cracks are formed by the wetting of clay and silt sediments.
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What are structures formed after deposition but before lithification called?
What are structures formed after deposition but before lithification called?
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The type of joints formed through the cooling of volcanic rocks is called __________ joints.
The type of joints formed through the cooling of volcanic rocks is called __________ joints.
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Match the following terms with their definitions:
Match the following terms with their definitions:
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What are raindrop impressions primarily formed by?
What are raindrop impressions primarily formed by?
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Ropy lava tops are associated with primary structures formed at the tops of beds.
Ropy lava tops are associated with primary structures formed at the tops of beds.
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What are groove casts?
What are groove casts?
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The structures formed due to the interaction of low density silt and denser sand are known as ______.
The structures formed due to the interaction of low density silt and denser sand are known as ______.
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Match the following types of bedding structures with their descriptions:
Match the following types of bedding structures with their descriptions:
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Which of the following structures is specifically formed at the bottom of beds?
Which of the following structures is specifically formed at the bottom of beds?
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Palaeocurrent direction can be indicated by structures like groove casts.
Palaeocurrent direction can be indicated by structures like groove casts.
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The base of a pillow lava conforms to the shape of the ______ below it.
The base of a pillow lava conforms to the shape of the ______ below it.
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What types of vesicles can be found in pillow lavas?
What types of vesicles can be found in pillow lavas?
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Which structure is formed due to water escaping upwards through sedimentary layers?
Which structure is formed due to water escaping upwards through sedimentary layers?
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What type of unconformity involves a significant break in deposition with no angular discordance?
What type of unconformity involves a significant break in deposition with no angular discordance?
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Angular unconformities occur between horizontally bedded rock units with no structural deformation.
Angular unconformities occur between horizontally bedded rock units with no structural deformation.
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What principle states that inclusions within a host rock are always older than the host?
What principle states that inclusions within a host rock are always older than the host?
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Sedimentary rocks are originally deposited in __________ layers.
Sedimentary rocks are originally deposited in __________ layers.
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Match the types of unconformities with their definitions:
Match the types of unconformities with their definitions:
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Which principle suggests that sedimentary strata extend laterally within a basin?
Which principle suggests that sedimentary strata extend laterally within a basin?
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Fluvial channels represent a characteristic feature of nonconformities.
Fluvial channels represent a characteristic feature of nonconformities.
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What type of unconformity occurs when sedimentary structures overlie sharp contacts with igneous or metamorphic rock?
What type of unconformity occurs when sedimentary structures overlie sharp contacts with igneous or metamorphic rock?
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Study Notes
Fundamental Structures
- Primary structures are sedimentary structures present before deformation. They are helpful for identifying the original state of rocks and determining the "up" direction.
- Primary structures are either sedimentary or igneous. Secondary structures result from deformation.
- Layering (bedding and lava flows) is the most common type of primary structure.
- Primary structures indicate environmental conditions during sedimentation, the deposition basin's geometry, and the original orientation of rocks.
Younging
- If two beds (A and B) are younging in the same direction, A is younger than B. A normal stratigraphic sequence results.
- If A and B are younging in opposite directions, they are likely the same bed repeated by folding, not a standard stratigraphic sequence.
Primary Structures Formed at the Tops of Beds
- Examples include rain prints, ripple marks, and ropy lava tops.
Primary Structures Formed at the Bottoms of Beds
- Examples include scour-and-fill, load casts, and groove/flute casts.
Primary Structures Formed Within Beds
- Examples include convolute bedding, graded bedding, and cross-bedding.
Cross-Beds
- Cross-bedding is formed by sand dunes.
- The angle of repose in cross-bedding is related to wind direction.
Columnar Joints
- Columnar joints are hexagonal shapes in rocks.
- These shapes result from cooling and contraction of volcanic rock.
Slumps
- Slumps form by the collapse of wet sediment during tilting or agitation.
- Slumps are common in tilted cross-bedded strata.
- Slumps are penecontemporaneous (formed after deposition but before lithification).
Mud Cracks
- Mud cracks are formed from the desiccation of clays/silts.
Unconformities
- Unconformities separate rock layers.
- Normal conformity has no breaks in deposition or erosion.
- Unconformities include angular, disconformity, and nonconformity.
- Angular unconformity: rock layers are tilted with respect to each other
- Disconformity: an erosional surface exists between parallel rock layers
- Nonconformity: sedimentary layers lie above igneous or metamorphic rocks
Principle of Inclusion
- Inclusions within a rock are older than the surrounding rock.
Principle of Original Horizontality
- Sedimentary rocks are originally deposited horizontally.
- Exceptions include cross-bedding and dipping sedimentary strata which are shaped by tilting and/or folding.
Deformation
- Deformation is changes in rock form or size.
- Deformation is often related to plate margins.
- Deformation involves force and stress:
- Force is what tends to start or change motion.
- Stress is force applied over an area.
- Different types of stress create various effects on rock (compressional, tensional, shear stress).
Stress
- Uniform stress affects equally in all directions
- Differential stress has different stresses in multiple directions.
- Compressional stress shortens rock bodies
- Tensional stress lengthens rock bodies
- Shear stress produces slippage.
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Description
Test your knowledge on fundamental structures in geology, focusing on sedimentary and igneous primary structures. Understand the significance of layering, younging directions, and various examples of primary structures. This quiz will help you identify the original states of rocks and their environmental conditions.
