Craters of the Moon National Monument in Idaho is such a bizarre place you can actually imagine you’re walking on the moon. It was created over a period of about 15,000 years by one volcanic eruption after another. Now, the entire area is covered with a thick layer of lava, and may very well be the most inhospitable place on earth.
About 15,000 years ago a volcano erupted and covered a huge area with black lava. Once the volcano had spilled tons of lava, the eruption stopped and all was quiet.
Fast forward a couple thousand years and the volcano sprang to life once again with a powerful eruption. Part of the lava from this flow landed on top of the first lava flow, while other parts ended up on new ground.
Every two thousand years or so, the volcano erupted yet again, spilling tons of volcanic rock on the ground. In some places, many layers of lava built up, while other areas have only one or two layers.
Now, Craters of the Moon is a vast area many miles across covered with black basalt lava flows. As you explore it, you can see exactly how the lava came out of the volcano.
- Cinders – When a volcano first erupts, there is a lot of built-up pressure, so it tends to explode violently sending tons of lava into the air. The lava cools and solidifies while still in the air, and comes to the ground as solid rock. Cinders tend to be fairly small pieces of volcanic rock with lots of holes in them from all the gas it held. It is a very lightweight stone because of all the air holes.
- Pahoehoe – Generally, the next stage of lava flow is called pahoehoe (pronounced pa-hoy-hoy). Pahoehoe is a Hawaiian word that means “ropey”, and it’s a very good description of how the lava looks. A pahoehoe flow is very hot and liquidy – basically a hot river of volcanic rock. Many times, the top layer of a pahoehoe flow will cool and solidify before the bottom layers. The bottom layers continue flowing on even though the crust can not, which makes tubes. Pahoehoe is often smooth and shiny.
- A’a – As the force of the flow decreases, it will turn into an a’a flow (pronounced ah-ah). An a’a flow is much cooler and slow-moving than pahoehoe, which leaves a rough surface. A’a is a Hawaiian word which means “tough on feet”, which is a very apt description.
- Block – The very last stage of a lava flow will create block lava. When the lava has cooled off to the point where it is just barely moving, it creates solid blocks of black basalt with hardly any air bubbles trapped inside. This lava is very dense and heavy because there is no air in it.