Stream foam and fall leaves
Stream foam collecting around fall leaves

Foam isn’t just stuff that comes out of shaving cream cans, keeps your take-out coffee hot, or floats in the Great Pacific Garbage Patch.2 It is a ubiquitous natural phenomenon that is fascinating and can affect rainfall; has caused seabird die-offs; and influences global climate. Don’t sell foam short!

A day in the life of a watery foam

Foam-producing waves in Acadia National Park

Perhaps the most familiar foams are aqueous foams like those in the ocean or a rapidly flowing stream when air and water are mixed vigorously, such as when waves crash against a rocky shore, or when the wind whips the sea into white-capped waves and forms millions of bubbles ranging in size from a fraction of a millimeter to larger than a centimeter in diameter. Sea foams are “closed-cell” so the air contained in one bubble isn’t connected to the air in other bubbles.

Sea foam
Sea foam hurled into the air from breaking waves
Air (or other gasses3) and water are necessary parts of the aqueous foam recipe. However, they are not enough. If you shake up a bottle of tap water, foam will not form. An additional ingredient called a surfactant4 is needed to decrease the surface tension of the water. In tap water (or distilled water), the water molecules are so strongly attracted to each other that a stable bubble layer (foam) can’t form. In sea foam, fats and proteins – produced as seaweeds and other algae or organisms break down – decrease the attraction between water molecules and allow bubbles to form.
Tap water and seawater foam after vigorous shaking
Tap water and seawater after being vigorous shaken

Sea foam has been estimated to cover 1-4% of the world’s oceans at any time. This corresponds to a mind-staggering 1,390,000 to 5,560,000 square miles of foam which is on the order of the area of the United States.5 Unsurprisingly, this much ocean foam has consequences.

First, sea foam is highly reflective compared to seawater – roughly 5-10 times as much sunlight is reflected from sea foam than from foamless seawater. This can have a direct effect on local temperature, air, and water currents. For example, the less solar radiation absorbed, the lower the local temperature.

Second, sea foam doesn’t last forever. It may last for seconds to hours, depending upon conditions and the nature of the foam. When the bubbles making up the foam burst, they don’t simply sink back into the water which birthed them. The upper portions of the popped bubble become airborne particles, and the lower portions of the bubble in contact with the ocean bounce up and produce microscopic, watery beads that also become airborne. These particles contain salt and organic matter and, over the course of a day, may total 6 million tons of microscopic particles, or the equivalent of 400,000 blue whales each day.

These particles become the nuclei around which water droplets and ice crystals form in the generation of clouds and fog.6 These foam-generated particles constitute 30%-75% of all naturally produced aerosols worldwide and have a comparable percentage effect on cloud formation.

There is no FOMO for foam because it is intimately involved in our lives even if we aren’t aware!

Toxic foams

Unlike the Blob of Hollywood fame, sea foams won’t engulf and consume their victims.  Nevertheless, on the Olympic Peninsula in 2009, sea foam surfactants generated by blooms of toxic, microscopic algae killed thousands of sea birds by removing their waterproofing and leaving them susceptible to hypothermia. Fortunately, these events are very rare. However, windblown foam generated by the breakdown of toxic algal blooms, such as those causing red tides, has been reported to cause eye, skin, and respiratory irritation in people exposed to large amounts along the Gulf coast of Florida.

Fire and ice – stable foams

Sea foams are usually stable for less than a day. However, there are other natural foams that can last for days or even years.

Lava flows on Hawaii
Lava flows 6

Reticulite is a foamy, glassy, volcanic rock that occurs when dissolved gasses rapidly form bubbles in lava. This usually occurs only during lava “fountains” when a jet of lava gushes into the air propelled by the expansion of the dissolved gasses.

Reticulite, composed of volcanic glass, is fragile and very lightweight. Unlike the aqueous sea foam, reticulite is often an open-cell foam in which the walls of the reticulite are broken, leaving a glassy network. This is possible because once the reticulite cools and hardens  the structure of the foam is no longer maintained by a balance between the pressures inside and outside the bubbles, as is the case with sea foam.

Reticulite formed from lava
Reticulite bubbles are about 1mm in diameter in this sample

At the other end of the temperature spectrum are foamy ice “cakes,” which swirl in the eddies of some winter streams. As with sea foam, turbulence is required to mix the air and water, often in a waterfall. The slow, rotating water of the downstream eddy allows the foam to coalesce into the rounded cakes of frozen foam. Under the right conditions, these foams may persist for days.

Frozen foam ice cakes in a stream
Frozen foam ice cakes in a stream

Biological foams

A variety of insects, such as the eastern tent caterpillar, produce foamy, protective coatings around their eggs. My favorite foam-producing insect is the froghopper, also known as the spittlebug. The nymphs produce a foamy nest that surrounds their bodies and may provide protection against predation and dehydration.8 The foam consists primarily of plant sap that the nymph has consumed and secreted. The nymph then actively pumps air into the liquid to produce bubbles.
Spittlebug making a foam nest
Spittlebug making a foam nest
Spittlebug foam nest
Spittlebug foam

Ordinarily, the nymph breathes by extending its abdomen through the foam into the surrounding air. However, if necessary, perhaps during a downpour, the nymph can breathe oxygen from the bubbles.

Beer. Or How to get a head

Beer head foam
Homebrewed beer - one of Forest’s and my first batch

No discussion of foam would be complete without mentioning beer. Beer’s head is an aqueous foam, like sea foam, but also has features in common with lava’s reticulite. Sea, stream, and spittlebug foams are made when a liquid is actively mixed with air. Beer and reticulite bubbles are formed from gasses already dissolved in the fluid – carbon dioxide in beer and water vapor, carbon dioxide, and other gasses in lava. When the beer bottle is opened or when the lava fountains into the air, the supersaturated, dissolved gasses spontaneously form bubbles. Mixing isn’t required.

I am foam. Hear me roar!

Without foam, we’d all be mired in FOMO – fear of missing out on a healthy ecosystem that supports life. Foam plays an important role in weather patterns, cloud formation, and rain cycles. We are just beginning to appreciate that just as foam protects spittlebug nymphs, a foamy nest protects us all.


1 Ignoring foam and its remarkable nature could lead to a case of FOMO – Fear Of Missing Out.

2 See

3 For example, carbon dioxide is the gas in foams of carbonated beverages.

4 “Surfactant” is a portmanteau, or combination, of surface-active agent, that is, SURFace-ACTive AgeNT.

5 The area of the United States, including Alaska and Hawaii, is 3,796,742 square miles.

6 See my essay, Crystalline Entities – Snow, Hoarfrost, and Rime Ice,

7 These lava flows are not the lava fountains that form reticulite.

8 The precise function of the foam is debated and likely has more than one benefit for the nymph.




Special thanks to Zoe Weil’s editorial expertise


5 5 votes
Article Rating
Notify of
Newest Most Voted
Inline Feedbacks
View all comments
2 months ago

Love this one!
we once had 10 foot high foam on the Kinnikinic River in Wisconsin and two thrill seeking kayakers lost their lives in it. It was created by fall leaves decomposing in the river! Thanks for this… loved the Acadia video.

Zoe Weil
Zoe Weil
2 months ago

Knowing all this I no longer have foam-oh 😉

2 months ago

This is wonderful, Edwin! Such an unusual topic done with great imagination and amazing photos. I could watch the Acadia foam film forever.

Barbara Acosta
Barbara Acosta
2 months ago

I love stuff like this that gives one a view into a world most of us rarely perceive. What is the foam i often see floating down streams? I imagine it must be some type of bio foam made from tannins or other tree detritus?

MP Champeau
MP Champeau
2 months ago

I will never again experience FOMO on foam — oh no! I always learn a lot reading “Nature Unveiled” but/and I found the photos and films particularly fantastic in this edition. I wish I’d had a science teacher at any point in my life who showed us any one of those photos or clips and asked us to identify and explain what it was. Truth and beauty. Who knew spittle bugs were so cute? Thank you for sharing your knowledge, accessible writing style and beautiful visuals with those of us who never knew the first thing about foam, and had no idea what we were missing out on.

Last edited 2 months ago by MP Champeau
2 months ago

I always thought of foam as fun and frivolous. Who knew it could be so important and diverse?? Great piece, Edwin! Of course this means that next time we have a beer together we have to ditch the can and use a glass!

Shopping Cart
Would love your thoughts, please comment.x