Much of the solar system is made up of basalt, and the state of Oregon in the United States is doubly so. Basalt is an igneous rock that is most common in the volcanic vents that create new oceanic crust, but is also present in many terrestrial volcanoes, especially the (relatively) lower, dome like shield volcanoes.
Basalt differs from other rocks formed from lavas in the fact that it has a higher percentage of iron, and a correspondingly smaller proportion of silicon. Because of this, it is much more fluid than other lavas. The big imposing scenic stratovolcanoes aren't made of basalts, but of andesite and other ignatious rocks that can be piled up into each other to form big sharp peaks into the sky. Basalts, on the other hand, flow smoothly, forming either a gently sloping peak, or something that is not even recognizable as a volcano. A good example of the latter is most of the state of Oregon, which was formed around twenty million years ago when floods of basaltic lava, some of them thousands of feet deep, erupted forth. Even as far as geological events go, this was a gigantic event, being the third largest outpouring of lava in hundreds of millions of years, the only comparable events being the Deccan Traps and Siberian Traps, both of which were linked to mass extinctions.
All of this is prelude to the Boring Basalts, which happened somewhat closer to recent times---only five million years ago, or so. By that time, the thick shield of basalt had sagged in what would be the Portland area and had been covered with sedimentary beds washed down by the Columbia and Willamette rivers. The focus of the areas vulcanism had shifted to building up the Cascade Mountain Range. Yet, for some reason, small floods of basalt shot up and appeared around the Portland area. They founded a number of small, dome like volcanoes. These include Mount Tabor, Rocky Butte, Kelly Butte, Powell Butte, Mount Sylvania. There are around three dozen of them in all, in about a thirty mile radius of Portland. Most of these small mountains are smaller than one thousand feet now, although they were probably somewhat taller when they first formed.
As I said, basalts usually spread out relatively thinly, but most of the mountains formed by the Boring events are fairly tall. There could be many reasons for this. My first guess was that basalts have differing compositions, with some having more silicon, and thus being more sticky, than others. It could be a possibility that whatever lava was down in the mantle just happened to be a little richer in silicon before it erupted upwards into the Portland area. Since the planet tends to have a lot of silicon in it, this seems like a likely story. The book Roadside Geology of Oregon says that it is due to the fact that when only a small amount of lava is erupted, the solid skin it forms on contact with the air is enough to keep the still molten lava below from spreading out.
I should also point out that while the Boring Basalts are "small" in terms of other volcanic events, and are certainly small in comparison with the almost astronomically significant Columbia River Basalts, it was probably a tremendous event at the time it took place. The effects of thousand foot tall, thousands of feet wide mountains of lava erupting into what would have been a densely populated ecosystem would be immense. It can only be imagined how it would have effected the flora and fauna in the area. I have also not read why the fields sprung into action at that point. They seem to have been extinct for a million years, but there is no way of knowing what form tectonic activity might next take in the Pacific Northwest. The mountains and hills of the Boring basalts are still there to witness, and people who have no interest in geology still enjoy climbing Mt. Tabor or Rockey Butte. The landforms were also reshaped dramatically by the Lake Missoula Floods, which made many of their slopes much steeper, and also created a network of kolks and channels around them.
As a final note, I would like to say that as much as I tried to describe something about the geology of these hills, it doesn't capture my true interest in them, which can really only be understood by those who have had a chance to see them rising above the fog early in the morning. Also, that they get their name from the town of Boring, although I am sure many people would think that these basalts, like many others, were very boring.