Columbia Plateau

18th October 2008 (Saturday)

Field trip to Columbia Plateau!

Background:

Columbia Plateau is part of the Columbia River Basalt, which is a stack of basaltic lava flows covering approximately 200,000 sq km in Washington, Idaho, and Oregon. This stack is, in places, up to 4km thick! As basalt is formed from lava during volcanic eruptions, these basults were formed during a large eruption 17 and 6 million years ago that engulf the whole area.

On top of these volcanic eruptions, after the basalt has formed, the whole area was cut by a number of deep, broad canyons (coulees), carved during repeated glacial outburst floods at the end of the last glacial period (last ice age), about 15,000 years ago. Evidence suggests that there were several drozen such floods over a period of approximately 2500 years, resulting in the many amazing landforms in this area!

For my field trip i went to the part that is at central Washington.

On the way to see the plateau we made 4 stops to see the rock types at these areas. With each stop (a few km away from each other) the rocks just get younger and younger.

Stop 1

The large white block of rock is the result of lahars from an eruption of some volcano a few billion years ago. These volcano can no longer be found as it is already eroded away.

Stop 2

Down the road, and we can see basalt from another eruption a few million years ago. (billions to millions..) As basalt is resistant to erosion, there are not eroded away like the other soil and rocks around them.

There are 2 different 'layers': at the top is just a huge mass of basalt, but at the bottom there is polygonal shaped basalt. The difference is due to the amount of water that was able to pass through the lava when it was cooling. So from where the polygonal shaped basalt starts forming, we know that water was not able to pass through to that depth and lava was able to cool without any interference from water.

Stop 3
Pillow Basalts
Actually they are not very obvious in this picture. But pillow basalts forms when basalt erupts underwater or flows into the sea. The cold water quenches the surface and the lava forms a distinctive pillow shape, through which the hot lava breaks to form another pillow.
So this means that this area used to be underwater few hundred thousand years ago!
Stop 4
Here the rocks are fairly young in geological years, This place also used to be underwater. Notice how there you can see 'horizontal lines' on the rock face, indicating the flow of the river. At the bottom there are rocks and sand, and this further prove that this place used to be a river as sediments are usually pushed to the river banks.

Views i tried to take from the van when travelling from stop to stop.
the river is running along the road.. could that be the river that created all the formation we see above? no idea! :p

We had lunch at the last stop and then we headed off to see the Columbia Plateau!

But as we continue to drive towards it, we seem to go into no man's land.

Caught a glimpse of another mountain at the rest point! Is this Mount Hood or Mount St Helens? i have no idea! but just like the fact that ice-capped mountains can be seen anywhere i go! (but they are still active volcanoes though :p)
Soon the impressive plateau came into view! Had to go across a bridge but the view was amazing! Continued to see different basalt landform..

On top of one of the plateau, there were some