Re: how can lava form a plateau?

Some kinds of lava are very liquid and runny when they are erupted, and 
flow very easily.  Other kinds of lava are “sticky” and do not flow very 
well.  Liquid lava is called basalt, and basalt is the kind of lava that 
can form plateaus.  Sticky lavas build up to form cone-shaped volcanoes.  
A good example of a cone-shaped volcano made from sticky lavas is Mount 
Fuji in Japan.

A good example of liquid basalt lava is the kind of lava that erupts in 
Hawaii.  You may have seen pictures of rivers of molten lava in Hawaii.  
Liquid lava can flow for many miles before it finally cools and stops.  As 
it flows it spreads out, rather like water, instead of building up into a 
cone-shaped mountain.  Liquid lava can form a plateau when many eruptions 
take place, one after another over a very long time, perhaps thousands or 
even millions of years.  Each lava flow will flow until it cools and 
hardens, often forming a flat plain of rock. Hawaii has many of these 
large, flat lava plains.  The next eruption will flow over the cold rock 
of the previous eruption, adding another layer.  Over many eruptions the 
thickness of the stack of lava flows will increase, and a plateau may be 
the result.

There are many volcanoes like Hawaii that have liquid basalt lava. There 
is also a rare type of eruption called a “flood basalt” eruption.  A flood 
basalt eruption has the same type of liquid basalt lava as Hawaii, but is 
very much larger.  It can happen either on land or in the oceans, and it 
will erupt such huge amounts (floods) of basalt lava that the lava flows 
will cover vast areas of land or of the ocean floor.  Flood basalt 
eruptions take place, in fact, as many eruptions, one after another over a 
very long time in the same area.  Luckily for us flood basalt eruptions do 
not happen very often – the last one was about 17 million years ago in 
what is now the states of Oregon and Washington, and lasted, on and off, 
for about 2 million years.  A flood basalt eruption can build up a huge 
plateau.

The flood basalts in Oregon and Washington are called the Columbia River 
Volcanic Province because the Columbia River flows through them.  The lava 
flows cover an area of about 165,000 square kilometers (about 64,000 
square miles).  Many lava flows are stacked up one on top of the other, so 
that if you visit the area you can see the different lava flows as layers 
in the mountain sides. Have a look at:  http://volcano.und.nodak.edu/vwdocs/volc_images/north_america/crb.html
Some of the individual lava flows are huge, up to 20 to 30 meters (about 
60 to 100 feet) thick.  The Columbia River Volcanic Province has been 
eroded by rivers, rain, wind and ice for so long that it is now worn down 
and divided up into hills and valleys.

Sticky lavas do not flow like basalt.  Instead they flow very slowly and 
do not flow very far.  They tend to pile up close to the crater where they 
are erupted, forming a cone-shaped pile of lava and rubble instead of a 
flat plain.  There are many volcanoes of this kind, and when we think of 
volcanoes we often think of a cone-shaped mountain like Mount Fuji.  Have 
a look at: http://volcano.und.nodak.edu/vwdocs/volc_images/img_fuji.html