The Earth Story

Walk across Pillow Lava, Sicily. Each of these bulbs erupted below the ocean surface and have since been uplifted above the waves.

Want to check out an active mid-ocean ridge? The University of Washington has a series of geophysical monitoring stations on the Juan de Fuca ridge off the Pacific Northwest US Coastline. In 2015 they were able to track a series of eruptions due to the presence of earthquakes and other seismic signals. Here you can travel along the ocean floor as they pick up the equipment on that ridge - lots of pillow basalts erupted underwater and a bunch of organisms living off the energy supplied by these eruptions.

Remnant of an early Earth

Earth’s mantle is a convecting solid. It is hot enough that the material in the mantle flows viscously on a geologic timescale; over millions of years it churns and mixes. How effective is it at mixing? Over the 4.5 billion year history of the Earth, the mantle should have overturned completely several hundred times.

Imagine taking a tube of Aquafresh (the tri-colored toothpaste), emptying it into a jar, and then stirring it a hundred times. Would you be able to find any part of the toothpaste’s original colors? This article tells the story of a group of scientists basically doing exactly that for the Earth.

When the solar system formed, it received elements that were recently made in supernovas. Some of these elements are still around (heavy atoms like the Rare Earth Elements make it possible for me to type this story) but some atoms don’t live that long. Radioactive elements are made in supernovas, but they gradually decay over time. Some radioactive elements like Uranium are still around today, but others decay faster, with much shorter half-lives.

Geologists call these isotopes “short-lived” even if their half-lives are millions of years because they’ve long since decayed away. There is no trace of those isotopes left in the Earth, except for the daughter isotopes – the atoms that the short-lived nuclei decayed into. The only way we know about these short-lived isotopes is to find places with extra daughter isotopes. If you find such a place, with more of a daughter isotope than the rest of the planet, you know it must have somehow formed when that isotope was alive.

For example, we find some meteorites with extra of the isotope Magnesium 26 (magnesium with 14 neutrons). 26-Mg is made by decay of a short-lived isotope of aluminum, 26-Al. That means those meteorites must have formed within the first few million years of the solar system, since the half-life of 26-Al is only 730,000 years. If a rock formed 10 million years after the solar system formed, there’d be almost no 26-Al left and that rock would get no extra 26-Mg.

Some short-lived isotopes can be used to tell really interesting geologic stories. For example, an isotope of the element Hafnium (mass 182) decays to Tungsten with a half-life of 8.9 million years. These elements are rare but scientists can measure them today using modern instruments, and they hold information about an important geologic story.

Hafnium and Tungsten behave different chemically. Hafnium fits into minerals with oxygen backbones, like those of our mantle and crust, while Tungsten goes into metals like those that made the core. If the Earth’s core formed after all the Hafnium 182 had decayed, there would be almost no Tungsten 182 in the mantle because it all would have gone into the core.

The Earth’s mantle has some Tungsten 182 in it and geologists use that amount to estimate that the core formed something like 50 million years after the planet formed. Beyond that measurement, if you found two rocks with different amounts of Tungsten 182, that would imply they must have formed before Earth’s core formed. However, the whole planet has churned like that tube of Aquafresh since then, so the whole planet should have about the same ratio of Tungsten 182 to other isotopes of Tungsten. It would be stunning to find a rock from Earth that differed in its Tungsten isotope ratio.

That, of course, is the measurement that this group, led by Dr. Hanika Rizo (now at the University of Quebec), pulled off. They found rocks that shouldn’t exist based on what we know about the mantle.

Baffin Island, in northeastern Canada, contains a flood basalt province. The rocks in this photo are pillow basalts from that island; they were erupted about 60 million years ago as the plume that is found today under Iceland began to erupt. This is one of two sites this research group found to contain an anomalous Tungsten isotope signal; the other was the Ontong Java Plateau, a gigantic platform in the Pacific Ocean (https://tmblr.co/Zyv2Js1oYgyfZ).

Both of these sites are large igneous provinces. They seem to reflect a pulse of hot mantle coming up from deep, reaching the crust, and melting. The fact that both of them have tungsten isotopes that differ from the rest of the planet means that something in the source of these rocks, something deep in the mantle, formed in the first 50 million years of this planet’s lifetime and somehow hasn’t been mixed back in since.

Geophysical models tell us that a normal, convecting mantle shouldn’t be able to keep parts isolated for 4.5 billion years. That’s like stirring the toothpaste and finding a part that clearly shows the three colors. If everything stirred, the individual colors would be long gone. The only way for this to happen is if some properly kept part of the mantle from being stirred.

Deep in the mantle there are two large areas that are distinct seismically – we call them LLSVP or “superplumes” (more here: https://tmblr.co/Zyv2Js1pESuuK). We don’t know exactly what those things are, but some geologists have proposed they could be sources for plumes. If they’re denser than the surrounding mantle, they could be made of stuff stuck near the core mantle boundary for billions of years and if part of it got too hot, it could rise up to the surface and melt.

Something like the LLSVP being stuck in the mantle for >4 billion years is required by this measurement. Somehow, the Earth’s mantle took material in it 4.5 billion years ago, when the planet was in its infancy, and stored it until its release in a large igneous province a wink of an eye ago (geologically speaking). We can’t say exactly how the Earth did this yet, but knowing that these rocks exist will give us a fundamentally new insight into the mantle below.

-JBB

Image credit: Don Francis of McGill University. Press release: https://carnegiescience.edu/node/2031

Original paper: http://science.sciencemag.org/content/352/6287/809.full

Oregon Coast Range Pillow Basalts

The Coast Range in Oregon lies to the west of the more well-known Cascade Range, creating the other side of the Willamette Valley and providing spectacular views of the Cascades and the seashore on a clear day. However, unlike the Cascades, which are composed of andesites and rhyolites (silica-rich, viscous rocks characteristic of continental volcanic arcs), the Coast Range is composed of basalts and sandstones. How did these oceanic rocks end up forming mountains up to 1249 m (4097 ft) high?

The northwest coast of North America is defined by the subduction of smaller plates, including the Juan de Fuca, Explorer, and Gorda plates, beneath the behemoth North American continent. 400 km west of the Coast Range lies the divergent plate boundary between the Juan de Fuca plate and the Pacific plate, and 150 km offshore lies the Cascadia megathrust subduction zone, in which the Juan de Fuca plate sinks back down into the asthenosphere (the uppermost part of the mantle, typically found at 100 km deep).

However, not all rocks are doomed to melt. The oldest rocks in the Coast Range, the Siletz River volcanics, formed during the Paleocene and middle Eocene (60-45 million years ago). These Siletz River volcanics provide clear examples of pillow basalts, indicating that these rocks were formed underwater. Note the radial jointing in the second picture -- the inside of these round bubbles looks like a bomb blast because of even, outside-in cooling.

Mary’s Peak, the highest summit of the Coast Range, is actually an old hot spot volcano, formed from a weak point in the ocean floor similar to modern Hawaii. The island chains towered above the sea floor, moving east in a conveyor-belt like system. Over time, these rocks were slammed into the continent at a rate of 4 cm/year, and instead of subducting, accreted onto the side of the North American plate. With more accretion, these sea mounts were uplifted and are now the Coast Mountain Range. These oceanic basalts define the west coast of Oregon, as the Columbia River basalts define the east and the Cascade range defines the center.

AGB

Photo Credit: 1 - Loren Kerns - https://flic.kr/p/ebwoJD 2 - Amanda Barker - https://flic.kr/p/EH9vMf References: Bishop, Ellen Morris, "In Search of Ancient Oregon", 2003 http://www.oregongeology.org/sub/publications/ims/ims-028/unit07.htm

Pillow pile

Have you ever felt that the pillow on your bed has been too compressed and you need to add something thicker? Our reader Mark Tingay has provided the ultimate solution; a seemingly never-ending pile of pillows.

These are, of course, pillow basalts. We showed this example of pillow basalts while they were forming a few days ago (http://tmblr.co/Zyv2Js1vPTE_d); when lava erupts on the ocean floor it forms a crust and then occasionally that crust breaks, allowing another blob of lava to pour out. Pillow basalts are perhaps the most common igneous texture on the entire planet; they make up the entire ocean floor. When oceanic crust is made at a mid-ocean ridge, or when an eruption occurs at a hotspot like Hawaii, the lava forms piles of pillow basalts that can be several kilometers thick.

In the nation of Oman, one of the Earth’s most famous ophiolite complexes is exposed at the surface. An ophiolite is a chunk of the ocean crust that has been thrust up onto a continent, allowing it to be examined out of the water. These pillow basalts are part of a thick section of rock that lets us see the structure of the ocean crust and preserves a remnant of the processes that occur beneath mid-ocean ridges.

-JBB

Image credit: Mark Tingay (with permission) https://twitter.com/MarkTingay/status/650087340866600962/photo/1

Growing pillow

Pillow lavas are one of the most common rock structures on Earth. When low-viscosity, basaltic lavas erupt beneath the waters of the ocean, the surface that contacts water cools almost instantly to a solid crust. However, the interior of the pillow stays molten and continues flowing, putting pressure on the crust.

This pressure eventually stretches and cracks the solid crust, exposing new lava to water that will again chill almost instantly. This photograph of a growing pillow is about 1 meter across.

A full lava flow will grow a large pile of pillows; as one pillow begins to harden the molten rock will find a new crack and a new blob will push out, forming another pillow.

Virtually the entire Earth’s ocean crust is made of a kilometer-thick stack of erupted pillow lavas.

-JBB

Image credit: NSF/NOAA http://www.pmel.noaa.gov/eoi/laubasin/laubasin-multimedia.html

Lava balloons

During volcanic eruptions in the Canary Islands in 1999 and 2011, these strange balls of rock appeared off the coast. They floated to the surface, stayed there for a bit, gave off steam, and then sank again. This behavior earned them the nickname “lava balloons”.

Lava balloons form originally as pillow basalts, one of the most common eruptive styles on Earth (http://tmblr.co/Zyv2Js1efh_DW). Lava that erupts beneath the waters of the ocean cools rapidly to a solid on the edges where the molten rock directly touches the water but stays molten on the inside, forming a blob-like shape called a pillow.

Lava erupting on the ocean floor also has gas trapped in it – molten rock carries water and a little bit of carbon dioxide up from the mantle. As pressure drops on the magma, that gas begins to escape, forming bubbles in the magma. If the magma stays under some pressure – as happens at the bottom of the ocean, some of the gas stays in the magma and only a small amount forms bubbles.

Volcanic islands like those in the Canaries erupt lava on their slopes at much lower pressures than occur on the floor of the ocean. Because of that lower pressure, more dissolved gases in the magma escape to form bubbles. Those bubbles reduce the density of the pillows forming on the sides of the volcano. If there are enough bubbles to reduce the average density of the pillow to less than the density of water, the pillow will break off and float up to the surface like…a lava balloon.

Once the balloons reach the surface, they cool off and the dissolved gases escape to the atmosphere. Water can then flow in to those now-open bubbles, filling them in, increasing the density, and causing them to sink.

Volcanic balloons are expected in places like the Canary Islands where pillow basalts form close to sea level and where the original magmas also have large amounts of water already dissolved in them – a property inherent to a couple volcanic island provinces like the Canaries.

-JBB

Image credits: Stavros Meletlidis (distributed via http://imaggeo.egu.eu/view/1990/) Center of Volcanology of the Azores University (CVUA)/Smithsonian http://bit.ly/1JI4t2F

Read more: http://bit.ly/1AtYWV9

Sleepy?

Geologists use the term “pillow basalts” for a specific feature formed in the ocean. Looking at this lovely example of one, I’m hoping you can figure out the origin of the term (sometimes we’re not all that creative).

Pillow basalts form when lava erupts underwater. The water cools the surface of the lava flow rapidly, forming a crust, but the hot lava inside tries to keep flowing forward and downslope. It cracks the crust and surges forward, only to run into more water and cool. Lava flowing under-water thus forms these elongate, somewhat rounded bulbs on the ocean floor that we call pillow basalts.

Pillows have formed probably for more than 4 billion years – basically as long as there has been standing water. They can be recognized in the geologic record by their shape and their texture – they will have fine-grained edges, cooled rapidly by the water, surrounding more coarse-grained interiors that stayed hot slightly longer. Oh, and sometimes recent pillow basalts, like these in the Galapagos, might have the occasional white mussel shell nearby.

-JBB

Image credit: NOAA https://flic.kr/p/fHZi9v

Read more: http://www.nps.gov/goga/forteachers/pillow-basalt-faq.htm

The Brahma Schist The Vishnu Schist unit that I’ve covered a couple times now is a group of metamorphosed sedimentary rocks found deep in the Grand Canyon that were metamorphosed when an island arc ran into the North American continent 1.7 billion years ago. When these types of continental collisions occur, there tends to be a lot of different types of rock involved. Sedimentary rocks like those at the Earth’s surface are involved, magma bodies are involved, but often there are other rocks in the collision that tell the story of the plates involved. These photos show another of those units from deep in the Grand Canyon: the Brahma Schist. This schist is a rock type called an amphibolite. It is made up mostly of the minerals plagioclase and hornblende, with various amounts of biotite as well. That mineral assemblage is characteristic of what happens when basaltic rocks are metamorphosed. Basalts are very common throughout the solar system; they make up most of the Earth’s ocean crust and so they are often caught in collisions between continents and island arcs. In most places where the Brahma schist is exposed, the metamorphism has destroyed any textural remnant of the original rocks, but the 2nd photo here managed to catch a really cool one. These spheroid-objects are actually remnant pillow basalts produced when basaltic lavas erupted on the ocean floor. This schist spells out a big part of the process for how the basement in the grand Canyon formed; a continent and an island arc came together, uplifting mountains and metamorphosing rocks like these, pieces of the old ocean floor that got caught up in the process. -JBB Image credits: Tisha Irwin (with permission, taken on sample on GC National Park Rim Trail) https://www.flickr.com/photos/tishairwin/14307794598 Visit her page: http://www.photonsandplutons.com/ Also used: Ilg et al., GSA Bulletin, 1996 http://gsabulletin.gsapubs.org/content/108/9/1149.short Previous articles: https://www.facebook.com/photo.php?fbid=71718732167564 https://www.facebook.com/photo.php?fbid=717596974968016 https://www.facebook.com/photo.php?fbid=718487278212319

Glassy Rim Texture The Image below shows a "glassy" rim on a pillow lava. The glass is a type of basaltic obsidian (felsic obsidian is also common). The glass forms when hot lava comes into contact with a cold surrounding, instantly crystallising the outer most layer of lava. The glass is not commonly preserved in the geological record as the elements that make it degrade to clay minerals. The brown and flaky material visible in the image below is an example of this break down. -LL Image- Glassy Rim of a Pillow Lava at Acci Castello, Sicily by Leah Lynham