So, we left last Thursday for Bend, Oregon and a whirlwind tour of luna-like volcanic features scattered around this area of the country. Outfitted with rockhammer, flashlights, detailed topographic, geological & road maps and on/off road motorcycles to get around the backcountry, we were equipped for some serious sightseeing!
First stop is the Lava Lands visitor center, HQ of the Newberry National Volcanic Monument. Here is the first luna-like feature on our list, Lava Butte. Lava Butte is a volcanic cindercone, the remnants of a pyroclastic (firefountain) eruption event *and* often (and in this case) also a subsequent basaltic magma eruption from a rupture typically in the base of the cindercone, the one in Lava Butte which can be seen in the USGS inset as a gash at the bottom side of the cone, with the lava flow field below that.Cindercones are one-shot affairs; after the eruptive phase which gives them their cone-like form and the basal rupture and subsequent magma flow, they die, never to erupt again.
The eruptive (pyropclastic) material cindercones belched forth on the Moon was not subject to the deformative effects of passing through a significant atmosphere, so some of the ejecta formed into glass spherules instead, like the famous 'glass beads' found during several Apollo missions. Other ejecta takes the form of gas-bubble-filled (vesiculated) basalt, also called scoria, shown in closeup in the montage above.
This rock is created on the Moon by a gasification process related to the vaccuum it erupts into on the surface as well as the rising magma entraining CO2 and carbon monoxide among other possible gases enroute to the surface and then released; on earth the most common gas causing vesicules in lava is steam from groundwater.
Cindercones are also less steep on the moon, often not much more than flat patches of dark debris and not much 'cone' to them at all ('firefountains' are simply flat dark patches on the surface with a very small & shallow or no visible depression at all associated with them.)
The larger varieties are cone shaped, however, but with slopes less steep than those on earth thanks to the much milder lunar gravity, allowing the ejecta to fly further afield thus creating a broader, shallower feature.
A little background on lunar dome-like strutures might be helpful at this point...Once it was thought that all lunar magmas were of very thin consistency and thus, combined with the low lunar gravity, incapable of building the acccumulative structures like shield volcanos and cinder cones--so these features were then considered rather a mystery.
Now, however, it has been surmised that lunar magmas have some variety to them after all; some are much more viscous (thick) than others and are capable of accumulation into mounds, cones and dome-like features. The idea is that the flood basalt flows are of the extremely thin variety, like motor oil, spreading far and wide and flat. But some of the later eruptive events that occurred, around the perifery of the basins, and within the confines of crater floors, and even in the highlands, are magmas of a thicker variety and capable of building visible relief. So the lumps we see are now thought to be several different kinds of formations and due to different *or* varying intensity of the same mechanisms.
So now the types of lunar lumps and dark spots can be classified. Lunar 'cones' are large cinder cones with flattened slopes which simply erupted long enough and volumously enough to eventually build a topographic structure. Dark spots lacking elevation are fire fountains, the same process that created cinder cones but of such short duration that not enough material was ejected to actually form a elevated structure. Low relief craters with associated pyroclastic ejecta rings are considerd lunar maars.
Then there are the classic lunar domes, larger blister-like formations with rounded profiles. One type has a central vent--these are considered to be shield volcanos. The other type of dome has no vent opening and these are thought to be surface features associated with upwellings of magma columns that distorted but failed to rupture the surface and erupt. Since all these types of features occur in fields or groups, they are often mixed in with each other and can be confusing to tell which is which--for example, is this lump a cindercone or a shield volcano? A hint is the size--cindercones are smaller than shield volcanos.
Let's get back on the road! Just across the road from Lava Butte is another lava tube cave, Lava River cave. This lava tube feature is much older and larger than Ape Cave of the last fieldtrip but also shorter in length. This is thanks to it's greater age, 100,000 years vs. Ape cave's 2000 years, which gave the Earth more time to fill the lower reachers of the original tube with sediment washed in by the weather; originally this tube was likely much longer & extensive than Ape Cave lava tube but today it is just over a mile in length. But *vast*!!
About 200 yards of the end of this tube is collapsed into a shallow, linear gulch, giving a hint of the nature of similiar features (sinuous rilles) on the Moon...all we have actually found (and can see with our telescopes) is what are thought to be the collapsed remnants of lava tube structures, now appearing as shallow valleys. It is assumed that intact, roofed portions of the rilles do still exist on the Moon today--but that will be for future explorers to groundtruth of course.
To note are the simularities between both these tubes, Lava River cave and Ape cave. Both display the same 'machine bored' look and the same upper and lower 'stacked' passages. This indicates that the processes which create these are not unique but typical. There are many other lava tubes in this entire region, some large enough to drive trucks into! The feature that took us the most searching to find was Hole-in-the-Ground, a volcanic maar, that was out in the woods and hard to discern from ground level--but once we finally reached the rim, wow! Pretty spectacular!
A maar is formed by a huge explosion of gas filled lava; on earth caused by steam the rising magma entrains as it hits the groundwater and on the Moon likely by CO2 and/or CO. Entrained gas is what makes volcanic eruptions violent-- as upon reaching the lower pressure of the surface--or vaccuum on the Moon, the bubbles of gas within the magma burst and do so very violently!
This is exactly the mechanism that accounted for the violence of the Mt. St. Helens eruption of 1980. Maars are indicated on the lunar surface by largish impact-like craters that look just like Hole-in-the-Ground, and surrounded a region of the typical dark basaltic pyroclastic ejecta that flew into the atmosphere (such as it is on the Moon ;) ) when they blew up.
The most famous place where one can view lunar maars is on the floor of crater Alphonsus where we all know them as 'dark-halo-craters'.About 60 miles further down the backroads, through and north of Christmas valley, OR, lies a lonely sanican which makes the site of "Crack-in-the-ground" a terrestrial graben. This feature is analogous to the type of lunar rille, the graben, formed as a result of crustal fracture caused by either impact stresses or subsidence of basins.
This would be a *very* small feature on the Moon where the grabens we can observe are much more massive than this little one; many miles long and sometimes miles wide as well. A well known example of this type of graben would be the little rille in the floor of crater Fracastorius I've been talking about in another thread. It's a subsidence type fracture, just as this one is. In fact the mechaisms and causes of both these fractures are very similiar which is what makes the feature germain to our exploration.
Crack-in-the-Ground was also formed when the local basin(now called the Columbia flood basalt province) was created when the lava flooded plain eventually subsided (sunk into the crust under it's own weight) and ripped apart fractures like Crack-in-the-Ground as it did so. Walking down the center of this graben one can see how exactly the walls would mesh perfectly again if a giant could somehow squeeze it all back together. Strolling down a graben, how cool is that? ;)
The following day we go to Newberry Volcano, the namesake of this Volcanic monument. This is a ruined shield volcano, quite a large one as well at about ten miles in diameter at it's base. Shield volcanos are common structures on rocky planets, found on not only the Moon, but more famously (and hugely) on Mars as well. They are formed when multiple eruptive events consisting of more smoothly erupting, less gas filled magmas over time accumulate & build a mound of lava around the main vent-- in a Roman 'shield' shape which is where the name comes from.
Shield volcanos are basalt erupting volcanos, basalt being the generally more benign and least viscous (thick) of magmas thus forming a less steep volcano than we here on Earth are used to thinking of when we hear the word 'volcano'. Contrast the basaltic shield volcano to the towering, steep sided composite type volcanos like towering Mt. Hood. Composite volcanos, not found on the Moon, typically erupt and build themselves from a different sort of magma, much more thick than plain basalt, such as andesite and especially rhyolite magmas, which allow them to build much steeper formations as they aren't as runny as the basalt magmas.
There are more than a few shield volcanos on the lunar surface, just like this one. There are shield volcanos in Iceland that are the same dimentions and have the same slope as the lunar domes, so the formative process is thought to be the same for this type of volcano, here on the Earth or on the Moon. Newberry collapsed and formed a caldera, now occupied by two beautiful lakes surounded by campgrounds, as a result of emptying it's underground magma reservoir during it's last big eruption, thus allowing the solidified lava above it to collpse into the now empty magma chamber of it's own weight. A later and smaller eruptive event gave rise to the center island, actually a cinder cone now covered with forest.
Features like Newberry once was before it's collapse into a caldera can be found in some of the more famous dome areas of the Moon, such as the Hortensius area pictured in the montage above. Shield volcanoes form in clusters along with cinder cones and other volcanic features; this area in central Oregon is literally littered with them.
Another aspect of the Newberry caldera that showcases a feature also observable on the lunar surface is a lobate basaltic lava flow.
This one is composed of scoria, basalt filled with gas bubbles, but mostly of obsidian, a volcanic glass, sans gas bubbles, which is similiar to the volcanic glass created in lunar eruptions. The flows form similiar shapes, the hot but gradually cooling flows of magma spreading out like syrup on a table top and eventually freezing in the shapes we see them in today, on the Moon, on Mars and here on Earth.
So we come to the end of the trip into the volcanic hinterlands of the high, dry desert of central Oregon. A day and a half of travel suffices to show us the terrestrial equivalents of lunar maars, shield volcanos, lava flow fronts, grabens, sinuous rilles and cindercones. We walked and looked at the same features as the Apollo astronauts in training did to prepare them for the sights they were expected to encounter on their walks, rides and orbits upon & around the Moon's surface almost 40 years ago.
The spot pictured below (with my travelling companion enjoying the view and our gallant mounts waiting patiently ;) ) at the summit of Paulina peak, which at 8000feet is the highest remaining point on the ruined Newberry shield, was where astronauts Armstrong and Aldrin stood and took in the same sights we did; but for them soon to be totally and forever eclipsed in their minds by the fantastic wonders of the lunar landscapes they would soon visit. Well, we can too! Just not quite as close up!
