12. Tectonic landforms Introduction Introduction Major features of continents Major features of continents Major features of the oceans Major features of the oceans Relationships of major topographic features to plate tectonics Relationships of major topographic features to plate tectonics

Tectonic landforms Introduction Plot of elevations of earth’s surface shows two prominent levels, corresponding to average altitude of continents and average depth of ocean basins Plot of elevations of earth’s surface shows two prominent levels, corresponding to average altitude of continents and average depth of ocean basins About 2/3 of the earth’s surface consists of the floors of the oceans About 2/3 of the earth’s surface consists of the floors of the oceans Only a small fraction of the earth’s surface rises higher than average altitude of continent or lies below mean ocean depth Only a small fraction of the earth’s surface rises higher than average altitude of continent or lies below mean ocean depth

Major features of continents Four major features: Four major features: Cratons (composed of shields and stable platforms)Cratons (composed of shields and stable platforms) Subduction zones and folded linear mountain beltsSubduction zones and folded linear mountain belts Crustal spreadingCrustal spreading Transform faultsTransform faults

Major features of the oceans Submarine topography strongly controlled by crustal processes and structures. Submarine topography strongly controlled by crustal processes and structures. Among the most significant features are: Among the most significant features are: Ocean ridgesOcean ridges Rift zonesRift zones Abyssal floorsAbyssal floors Deep-sea trenchesDeep-sea trenches Volcanic arcsVolcanic arcs

Plate boundaries

Topography – plate tectonics Plate boundaries coincide with oceanic ridges, deep-sea trenches, and folded mountain belts. They are zones of earthquakes and volcanic activity. Plate boundaries coincide with oceanic ridges, deep-sea trenches, and folded mountain belts. They are zones of earthquakes and volcanic activity. Most plate boundaries are therefore not associated with coastlines between oceans and continents Most plate boundaries are therefore not associated with coastlines between oceans and continents

Tectonic plates…. Seven large crustal plates can be identified on the basis of their geophysical and topographic characteristics Seven large crustal plates can be identified on the basis of their geophysical and topographic characteristics Volcanic landforms concentrated along plate margins, especially where plate convergence occurs Volcanic landforms concentrated along plate margins, especially where plate convergence occurs Volcanism is also extensive along spreading centers, although it is much less obvious because it occurs along oceanic ridges below sea level. Volcanism is also extensive along spreading centers, although it is much less obvious because it occurs along oceanic ridges below sea level.

Volcanoes Built by the accumulation of their own eruptive products (as opposed to folding, uplift, etc…); conical hill built around a vent that connects with reservoirs of molten rock below surface of the Earth. Built by the accumulation of their own eruptive products (as opposed to folding, uplift, etc…); conical hill built around a vent that connects with reservoirs of molten rock below surface of the Earth. Driven by gas pressure (among other factors), molten rock forces its way upward and may ultimately break through zones of weaknesses in the Earth’s crust. Driven by gas pressure (among other factors), molten rock forces its way upward and may ultimately break through zones of weaknesses in the Earth’s crust. If so, eruption begins (non-explosive lava flows and/or explosive lava fragments) If so, eruption begins (non-explosive lava flows and/or explosive lava fragments)

Magma – lava: Magma – lava:

Volcanic landforms Four different types of volcanic landforms: Four different types of volcanic landforms: Lava plainsLava plains Volcano conesVolcano cones calderascalderas

Lava plains: Huge, coalescing and overlapping flows of lava producing relatively flat surfaces (plains) Lava plains: Huge, coalescing and overlapping flows of lava producing relatively flat surfaces (plains) Can cover a few hundred thousand km 2 in some regions (e.g southern Brazil, different areas of USA – in Oregon, Washington…)Can cover a few hundred thousand km 2 in some regions (e.g southern Brazil, different areas of USA – in Oregon, Washington…) Volcano cones: Shape varies type of volcano and nature of eruption that occurred. Volcano cones: Shape varies type of volcano and nature of eruption that occurred.

Calderas: Steep-sided circular depressions. Calderas: Steep-sided circular depressions. Size usually many times greater than size of original volcano vent Formed by collapse cones or magma chambers or result from destruction of volcanic cones by violent eruptions

Types of volcanoes Cinder cones Cinder cones Composite volcanoes Composite volcanoes Shield volcanoes Shield volcanoes Lava domes Lava domes

Hotspots, sea floor spreading, and shield volcanoes Chain of Hawaiian volcanoes created by motion of Pacific Plate over a hotspot – i.e. a plume of upwelling basaltic magma deep within the mantle (arising far down in the asthenosphere) Chain of Hawaiian volcanoes created by motion of Pacific Plate over a hotspot – i.e. a plume of upwelling basaltic magma deep within the mantle (arising far down in the asthenosphere) Each major pulse of the plume sets off a cycle of volcano formation. Motion of the oceanic lithosphere eventually “carries” the volcano away from the location of the deep plume (and it becomes extent) Each major pulse of the plume sets off a cycle of volcano formation. Motion of the oceanic lithosphere eventually “carries” the volcano away from the location of the deep plume (and it becomes extent)

Formation of a chain of volcanoes… Erosion processes dominate and ultimately, a volcano becomes a low island Erosion processes dominate and ultimately, a volcano becomes a low island Process is repeated and a chain of volcanoes eventually from (by plate moving over a hotspot) Process is repeated and a chain of volcanoes eventually from (by plate moving over a hotspot)

Iceland and sea floor spreading Some basaltic volcanoes occur along mid-oceanic ridges, where sea floor spreading is in progress. Some basaltic volcanoes occur along mid-oceanic ridges, where sea floor spreading is in progress. Outstanding example is Iceland, in North Atlantic Ocean. Iceland constructed entirely of basal. Outstanding example is Iceland, in North Atlantic Ocean. Iceland constructed entirely of basal. Mount Hekla, an active volcano on Iceland, is a shield volcano somewhat similar tot hose of Hawaii. Mount Hekla, an active volcano on Iceland, is a shield volcano somewhat similar tot hose of Hawaii.

Volcanoes: contemporary activity There are more than 500 “active” volcanoes in the world (those that have erupted at least once within recorded history). – 50 of which are in the US (Hawaii, Alaska, Washington state, Oregon, and California) There are more than 500 “active” volcanoes in the world (those that have erupted at least once within recorded history). – 50 of which are in the US (Hawaii, Alaska, Washington state, Oregon, and California)