The perpetually-interesting site Oddee hosted a series of satellite images of the Earth today, including this one from April of last year. Somehow I missed it then...

The image, originally from NASA's Earth Observatory (one of the finest websites I know of for those interested in Earth science), shows a collection of volcanoes in the western Arabian Peninsula. A large version of the image (unlabeled) is here.

The most spectacular thing about this image is the color contrast between the volcanoes on the left versus the volcanoes on the right. This spectacular contrast is indicative of the rock types involved in each volcano. On the left, felsic lava was erupting, which cooled into the extrusive rock rhyolite. On the right, mafic lava was erupting, which cooled into the extrusive rock basalt. Mafic igneous rocks like basalt have a higher proportion of the elements iron, magnesium, and calcium as compared to elements like silicon, potassium, and sodium. Felsic igneous rocks are, in a sense, distillates of mafic source rocks: they are made of minerals that are more easily melted.

Also worth noting is the way the basalt overlaps the rhyolite between Jabal Bayda' and Jabal Abyad tells us that the rhyolite came first, and the basalt came second, an example of relative dating. And these insights can be gleaned from space... or more accurately, from our computer screens, depicting an image from space. That's pretty incredible, when you think about it.

FYI, here's what NASA's William Stefanov wrote as the caption for this exceptional image:

The western half of the Arabian Peninsula contains not only large expanses of sand and gravel, but extensive lava fields known as haraat (harrat for a named field). One such field is the 14,000-square-kilometer Harrat Khaybar, located approximately 137 kilometers to the northeast of the city of Al Madinah (Medina). The volcanic field was formed by eruptions along a 100-kilometer, north-south vent system over the past 5 million years. The most recent recorded eruption took place between 600-700 AD.

Harrat Khaybar contains a wide range of volcanic rock types and spectacular landforms, several of which are represented in this astronaut photograph. Jabal ("mountain" in Arabic) al Qidr is built from several generations of dark, fluid basalt lava flows. Jabal Abyad, in the center of the image, was formed from a more viscous, silica-rich lava classified as a rhyolite. While the 322-meter high Jabal al Qidr exhibits the textbook cone shape of a stratovolcano, Jabal Abyad is a lava dome; a rounded mass of thicker, more solidified lava flows. To the west (image top center) is the impressive Jabal Bayda'. This symmetric structure is a tuff cone, formed by eruption of lava in the presence of water. The combination produces wet, sticky pyroclastic deposits that can build a steep cone structure, particularly if the deposits consolidate quickly.

White deposits visible in the crater of Jabal Bayda' and two other locations to the south are sand and silt that accumulate in shallow, protected depressions. The tuff cones in the Harrat Khaybar suggest that the local climate was much wetter during some periods of volcanic activity. Today, however, the regional climate is hyperarid - little to no yearly precipitation - leading to an almost total lack of vegetation.

Labels: basalt, blogs, igneous, middle east, satellite imagery, volcano, websites