Extrusive Landform | Characteristics | Formation | Impact on the Landscape |
LAVA PLATEAU | Extensive layers of basaltic lava (flood basalts) | Basaltic Lava (runny, flows over large distances), usually from a fissure | Columbia Plateau Covers 130,000km2 of Washington and Oregon. It is flat and extensive and it is composed of hundreds of separate basalt flows. In places it reaches thicknesses of 2000m. It has been cut through by the Columbia River. |
ACID CONE (Lava Dome) | Steep, convex sides to the cone. Mound shaped accumulation of rock. | Associated with destructive Margins - viscous, andesitic lava (high silica), cools / solidifes quickly. Doesn't flow far from the source. Domes often plug vents, trapping gases and leading to explosive eruptions. | Lava Domes at South Sisters and Mount St Helens in the Cascade Range - these features rise steeply from the surrounding land. |
SHIELD VOLCANO | Gentle sloping cone, consisting of layers of basaltic lava. | Basaltic, runny lava (low silica), flows long distances from the central vent. Takes long time to cool and as it cools, it forms gently dipping sheets. | Belknap Volcano, Oregon - extensive feature - 5 miles in diameter with gentle slopes. |
COMPOSITE CONE | Steep sided, symmetrical cones, often found at subduction zones, Layered features, consisting of both lava and ash. | Built up of alternating layers of lava flows, ash, cinders, blocks. Has a conduit system through which magma rises to the surface at the central vent in the crater at the summit. Lava may flow from fissures on the flank forming secondary cones. (As magma solidfies in the pipe system, it strengthens the cone). | Mount St Helens - is 2,549m high. Following the 1980 eruption a large crater formed and their are new lava domes which have formed inside. Fans of material can be found around the cone which were created by pyroclastic flows |
CINDER CONE | Slightly concave sides to the cone, largest material found closest to summit - materials getting finer further away (see formation) | Formed when pyroclastic materials are discharged. Largest material falls near the summit forming steep, but stable sides. Further down gentle slopes at the base of the cone (finer materials are carried further). | Black Butte, (Northern California) (another famous example of a Cinder Cone - is Paricutin which is in Mexico, created by an eruption in 1943) |
CALDERA | Large, steep walled, basin shaped depression. | Magma builds up in magma chamber and explosive eruptions empties the magma chamber. Unsupported roof of the chamber collapses in forming a depression. May fill fill with water, creating a lake. | Crater Lake, Oregon Caldera, now deepest lake in North America - 1,932 feet. Was a 3,660m mountain - Mount Mazama. |
