Lava FlowsRelease date:2020-10-19

The most recent lava flows formed the newperiod lava plateau, the surface of which has well-preserved flow structures and geomorphologicalfeatures. The lava flows making up the flow fieldcan be divided into two kinds: pahoehoe and aa.Pahoehoe accounts for about two-thirds of thearea, and aa about one third. Pahoehoe and aaoften form parts of the same lava flow, usually withaa lava forming the edge, and pahoehoe formingthe main body of the flow. The formation of aa isusually dependent upon a lower temperature, anda higher degree of crystalization, making the flowhigher in viscosity than pahoehoe.

The new period lava plateau preserves severalvolcanic flow units, each unit having a thicknessof 0.5-5m. The number of flow units erupted fromthe Laoheishan Volcano may be up to 17, whilefrom Huoshaoshan Volcano may be up to 7. Flowunits reduce in volume and area upwards, so thateach unit is separated by a step which is between1-2m high level. Observed from afar, units appearlike terraced fields.

The old period lavas form a mesa-type topography,because they have been eroded and dissected,and it is difficult to discern the original forms offlow units. However, one obvious feature of theseold period plateaus is the dominance of blockyterrain, and at a place known as Stone Pool, nearthe West Longmenshan Volcano, is an extremelyimpressive large expanse of classical block lavasurface. Table 2-7 lists important relics of the newand old period lava fields.

(1)Pahoehoe Lava

Pahoehoe is the form of the most fluid type oflava flow, characterized by a tube-fed delivery,a relatively smooth surface that may be coveredin festoons and wrinkles (ropy surface), and ahummocky appearance. The domes and ridges,known technically as lava rises and tumuli, thatform the hummocky surface terrain, were formedby inflation of the surface of the lava flow byinjection of fluid lava beneath a solidifying crust.

Irregular uplift of the flow surface left interveningdepressions, known as lava rise pits. Wudalianchi'spahoehoe is therefore identified as inflated orhummocky pahoehoe. In fact, the lavas show awealth of surface forms, including trunk-like lava,reptiles-like lava, rope-like lava, wavy lava, bread-like lava, raft-like lava, lava river, lava fissures, lavafalls, and so on. The typical and most commontypes of surface forms are:

Reptiles-like lava (shilong): This is the pattern of thesurface of the lava flow along the central thread ofthe new period flow. It is seen extending from thepoint of exit of the main lava stream from the craterof Laoheishan. At the edges of the pahoehoe flowsare small toe-like branches, the shaped like reptilesafter solidification.

Bread-like lava: The Laoheishan lava plateaudisplays some surfaces that have small surfacebulges, known locally as lava bread. Such bulgeshave diameters of 0.5 to 1m. Bulges may beballoon-like and were clearly viscous extrusions ofliquid lava through the crusted surface of the flow.Bulges are circular in plan and have a shell 2 to5cm thick. Inside they may be hollow.

Rope-like lava: During the cooling of the lava,continued flow beneath an elastic skin wrinkles,twists, and curls the surface into sequences of arcuate festoons, looking like coils of rope laid onthe ground. The direction of the coils indicates thelocal direction of the lava flow. The diameter ofthe coils may be 5-60 cm. Rope-like patterns mayalso be observed in the lava forming the floor ofcaves.

Wavy Lava: This lava surface displays sequencesof small waves, each with amplitude of severalcentimeters to 1m. The ridge line of the wave isgenerally perpendicular to the direction of flow ofthe lava. As well as being present in the body ofthe flow, wavy surface patterns may also be seenat the edge of the flow.

Raft-like lava: This surface pattern is mainlydistributed around the eastern side of theLaoheishan volcano. Rafts are made of crustalslabs and a single raft may be 4m wide and 20mlong. They were crustal slabs floating on a river ofdenser, fluid lava, and are aligned down the flowin neat rows.

Lava falls: These are mainly found at the edge ofthe pahoehoe lava, and are formed where surfacelava outflows tumble into a pit or down a scarp. Falls range from 2-4m in height, and have a widthof 0.5-4m. They are common in the lava to the eastand north of Laoheishan Volcano.

Crack hill and ridge: These features are tumuli andlava rises. They are inflation features which displaya surface dissected by deep cracks or clefts. Risesmay be round or oval shaped, a few meters to tensof meters in diameter. Known locally as Crack Hill,their geological name is tumulus. Sometimes thereare long inflation ridges, locally called crack ridges,the length being up to 1-2km. Occasionally fluidhas oozed through the clefts, and these are knownas "squeeze-outs".

(2)Aa Lava (Stone Sea)

Aa is a common type of lava, identified because itis covered by a clinkery surface. It forms in moreviscous flow, probably due to higher crystallizationof the lava, and because the escape of gas from thelava has been more difficult, surfaces are coveredwith tiny spines. The fragmentary surface of aalava flows may form due to cooling or a change interrain, which causes the flow rate to slow. Thus,as more mobile lava is fed into the flow at the vent,continuing forward movement lower down theslope causes the semi-consolidated crust to breakand fold. Pahoehoe surfaces may be transformedinto aa surface at the front-end of flows. The flowsfrom the Laoheishan and Huoshaoshan volcanoesdisplay large areas of aa lava, and these giving theappearance of a rough sea.

(3)Block Lava

Block lava flows are relatively common in theeastern part of the nominated property. They have the appearance of piles of massive blocks of lava,each block more than 1m across. The type of lavaflow that produced this blocky flow surface wasvery viscous and slow moving, causing the thickcrust to fracture and be thrust over the surface ofthe flow. Block lava flows are mainly to be seennorthwest of Molabushan Volcano, surroundingthe Longmenshan Volcano, and southwest ofBijiashan Volcano, Laoheishan Volcano, andHuoshaoshan Volcano. The largest example ofblock lava, covering an area of 20km2, is located700m west of the cone of the West LongmenshanVolcano, and is known as Stone Village. It is veryspectacular and resembles a black-brown stonevillage if seen from a distance - hence its popularname of Longmen Stone Village.

The weathered and partly buried lava blocksof very old blocky flows are to be seen in fieldson the east side of the First, Second, Third, andForth Lakes, and beside the Yaoquan River, inthe Shilong River valley. These blocks range from0.5-3m across, and some individuals can reach10m. They have a scattered distribution, and seenfrom a distance they resemble some black cattle,lying in the green grass.


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