Landform on a basalt region has usually ...a) rounded top b) flat top c) conical top d) none of these
Answers
Answer:
summit vent.
The low viscosity of the magma allows the lava to travel down slope on a gentle slope, but as it cools and its viscosity increases, its thickness builds up on the lower slopes giving a somewhat steeper lower slope.
Most shield volcanoes have a roughly circular or oval shape in map view.
Very little pyroclastic material is found within a shield volcano, except near the eruptive vents, where small amounts of pyroclastic material accumulate as a result of fire fountaining events.
Shield volcanoes thus form by relatively non-explosive eruptions of low viscosity basaltic magma.
Vents for most shield volcanoes are central vents, which are circular vents near the summit. Hawaiian shield volcanoes also have flank vents, which radiate from the summit and take the form of en-echelon fractures or fissures, called rift zones, from which lava flows are emitted. This gives Hawaiian shield volcanoes like Kilauea and Mauna Loa their characteristic oval shape in map view.
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Stratovolcanoes (also called Composite Volcanoes)
Have steeper slopes than shield volcanoes, with slopes of 6 to 10o low on the flanks to 30o near the top.
The steep slope near the summit is due partly to thick, short viscous lava flows that do not travel far down slope from the vent.
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The gentler slopes near the base are due to accumulations of material eroded from the volcano and to the accumulation of pyroclastic material.
Stratovolcanoes show inter-layering of lava flows and pyroclastic material, which is why they are sometimes called composite volcanoes. Pyroclastic material can make up over 50% of the volume of a stratovolcano.
Lavas and pyroclastics are usually andesitic to rhyolitic in composition.
Due to the higher viscosity of magmas erupted from these volcanoes, they are usually more explosive than shield volcanoes.
Stratovolcanoes sometimes have a crater at the summit that is formed by explosive ejection of material from a central vent. Sometimes the craters have been filled in by lava flows or lava domes, sometimes they are filled with glacial ice, and less commonly they are filled with water.
Long periods of repose (times of inactivity) lasting for hundreds to thousands of years, make this type of volcano particularly dangerous, since many times they have shown no historic activity, and people are reluctant to heed warnings about possible eruptions.
Cinder Cones(also called Tephra Cones)
Cinder cones are small volume cones consisting predominantly of tephra that result from strombolian eruptions. They usually consist of basaltic to andesitic material.
They are actually fall deposits that are built surrounding the eruptive vent.
Slopes of the cones are controlled by the angle of repose (angle of stable slope for loose unconsolidated material) and are usually between about 25 and 35o.
They show an internal layered structure due to varying intensities of the explosions that deposit different sizes of pyroclastics.
On young cones, a depression at the top of the cone, called a crater, is evident, and represents the area above the vent from which material was explosively ejected. Craters are usually eroded away on older cones.
If lava flows are emitted from tephra cones, they are usually emitted from vents on the flank or near the base of the cone during the later stages of eruption.
Cinder and tephra cones usually occur around summit vents and flank vents of stratovolcanoes.
An excellent example of cinder cone is Parícutin Volcano in Mexico. This volcano was born in a farmers corn field in 1943 and erupted for the next 9 years. Lava flows erupted from the base of the cone eventually covered two towns.
Cinder cones often occur in groups, where tens to hundreds of cones are found in one area.
Maars
Maars result from phreatic or phreatomagmatic activity, wherein magma heats up groundwater, pressure builds as the water to turns to steam, and then the water and preexisting rock (and some new magma if the eruption is phreatomagmatic) are blasted out of the ground to form a tephra cone with gentle slopes.
Parts of the crater walls eventually collapse back into the crater, the vent is filled with loose material, and, if the crater still is deeper than the water table, the crater fills with water to form a lake, the lake level coinciding with the water table.
Volcanic Domes (also called Lava Domes)
Volcanic Domes result from the extrusion of highly viscous, gas poor andesitic and rhyolitic lava. Since the viscosity is so high, the lava does not flow away from the vent, but instead piles up over the vent.
Blocks of nearly solid lava break off the outer surface of the dome and roll down its flanks to form a breccia around the margins of domes.
The surface of volcanic domes are generally very rough, with numerous spines that have been pushed
Explanation:
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Answer:
Landform on a basalt region has usually
flat top is the answer(sub-geography)
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