Eruptions in the postshield stage cap the volcano with a carapace of lava, containing low silica and high alkali contents, the reverse of the stage before it. Some Hawaiian volcanoes diverge from this, however. Lava is erupted as stocky, pasty aā flows along with a lot of cinder. Caldera development stops, and the rift zones become more inactive. The new lava flows increase the slope grade, as the aā never reaches the base of the volcano. These lavas commonly fill and overflow the caldera. Eruption rate gradually decreases over a period of about 250,000 years, eventually stopping altogether as the volcano becomes dormant.