Lava domes are built by slow eruptions of highly viscous lava. Lava tubes are cave-like volcanic straights formed by the hardening of overlaying lava. Lava caves include but are not limited to lava tubes. Felsic magma or lava is higher in viscosity than mafic magma/lava. The lower of these groups is a series of volcanic rocks formed by lava flows. Flood basalt describes the formation in a series of lava basalt flows. The southern part of the city rests on hardened basalt lava flows. They are thought to have formed from eruption of icy lava. They also occur on the surface of lava or pyroclastic flows. The county includes a number of extinct volcanoes in the Boring Lava Field. Thundereggs are found in flows of rhyolite lava. Rainier is also capable of producing pyroclastic flows and expelling lava. Their tent was pitched within of the crater's edge, secured by lava blocks. Thick scoria beds were produced by the fragmentation of lava flows. Shastina has been built by mostly pyroxene andesite lava flows.