Geology

Pillow Basalt FAQ

Where does pillow basalt form?

Pillow basalt is a volcanic igneous rock that forms when lava of basaltic compositionis erupted underwater. The rapid cooling of the lava by cold water on all sides forms the pillow-shaped bodies, which can then break open and extrude more of the hot lava from inside. The rapid cooling also creates pillows that are composed of volcanic glass on the outside and that have very tiny, almost invisible crystals on the interior. Pillow basalt typically forms at volcanoes at mid-ocean ridges or at oceanic hot-spot volcanoes, such as those that formed the Hawaiian Islands. Basalt forms the crust of all the ocean basins and is therefore the most common rock in the Earth's crust.

What gives basalt its color?

Basalt has a lower percentage of silica and a higher percentage of iron and magnesium than other volcanic rocks. These characteristics give it a very dark, almost black color. Hot, mineral-rich seawater flowed through much of the basalt that is part of the Franciscan Complex, changing some of its minerals into chlorite and other green minerals. This altered basalt is called greenstone.

What makes basalt sometimes appear speckled?

Sometimes basalt in the Franciscan Complex will have small white or pink speckles or even small round holes in it. These are the result of gas bubbles that were captured in the lava when it cooled. Sometimes the gas bubbles are empty, but other time they get filled with quartz or calcite that precipitated out of mineral-rich water circulating through the basalt, producing the speckles. Sometimes the gas bubbles in Franciscan basalt get filled with a bright orange semi-precious form of quartz called carnelian. If basalt is erupted too far under the ocean, the pressure from all the overlying water will not allow the gas bubbles to form. This is the case for most Franciscan basalt, but the basalt at Point Bonita is an exception.

Graywacke Sandstone FAQ

Where is graywacke sandstone deposited?

Graywacke sandstone is a sedimentary rock that is made up mostly of sand-size grains that were rapidly deposited very near the source rock from which they were weathered. Graywacke is deposited in deep ocean water near volcanic mountain ranges, where underwater landslides and density currents called turbidites quickly transport sediment short distances into a subduction zone or ocean trench. This type of sandstone contains fewer grains made of quartz and more made of feldspars, volcanic rock fragments, as well as silt and clay than most sandstone. It is therefore also known as "dirty sandstone." The volcanic rock fragments give graywacke a greenish-gray color.

What makes the beds in graywacke?

Graywacke sandstone deposits display flat-lying beds, each composed of sedimentary particles of different sizes. The sandstone beds can be from inches to many feet thick and are often separated by thin, dark shale beds. Each sandstone bed was formed during a single turbidite or submarine landslide event and was deposited over a short period of time from hours to days. The thin shale beds formed between turbidite events, when mud particles slowly settled to the sea floor, and may represent thousands of years. Turbidites display graded bedding, that is, the grain size decreases upwards in the bed. During a turbidite event, the larger and heavier grains settle out first. As the energy in the landslide event decreases, finer and finer particles settle out to the sea floor.

Are there fossils in graywacke?

Graywacke sandstone occasionally contains fossil mollusks which sometimes can be used to tell when the rock was deposited. Sandstone deposits of the Franciscan Complex contain clams and ammonites from the Jurassic and Cretaceous periods. These provide ages for when the Franciscan oceanic rocks got close enough to North America for continental graywacke sediments to be deposited onto them. The shale layers between the graywacke beds may contain microfossils that also can be used to date the rocks and to determine the depth of water in which they were deposited. Sometimes trace fossils are also visible in graywacke. Trace fossils are the marks and tracks of animals that burrowed, fed, crawled and lived in the sediments. Trace fossils can provide information on how deep the water was and how much oxygen was present when the sediments were deposited.

Chert FAQ

Where is chert deposited?

Chert is a sedimentary rock rich in silica. Franciscan chert is formed from the tiny silica shells (0.5-1 mm) of marine plankton called Radiolaria. Radiolarian chert forms where two conditions are met. First, a deep, open ocean setting is required where there is little continental mud or carbonate sediment to dilute the "rain" of dead radiolarian shells settling to the seafloor. Second, the upper ocean waters need to be relatively rich in nutrients in order for abundant Radiolaria to thrive.

How do geologists get the Radiolaria out of the chert?

The beautiful and intricate Radiolaria tests can be extracted from the chert by crushing it into small pieces and then putting it in a solution of hydrofluoric acid. This is the same acid used to etch glass. The acid dissolves away the less durable rock matrix, leaving the exquisite three-dimensional shells exposed. The tests are then photographed using a scanning electron microscope.

What can the Radiolaria tell us?

Radiolaria are still living in the oceans today. Different species live in tropical oceans versus temperate or cold ocean water. By comparing the types of Radiolaria in local chert with modern forms, we know that Franciscan chert contains some tropical and subtropical forms. Based on this observation, it appears that the sediments forming the local chert were deposited far to the south of their current location around San Francisco. Bay Area chert possibly came from the north equatorial upwelling zone, at the latitude of present-day southern Mexico.

Scientists also study the various species of Radiolaria that are present in chert deposited at different times in the past. Through these studies, they have developed an evolutionary sequence for different species in the rocks. This evolutionary sequence, or biostratigraphy, is then linked to radiometric dates obtained from associated volcanic rocks. Then geologists can determine the age of the chert. The radiolarian species in the Franciscan chert in the Marin Headlands lived and died to form the rocks during the period from about 200 million to 100 million years ago.

What makes the chert bedded?

The prominent bedding seen in Franciscan chert leads to the name ribbon chert. The hard, silica-rich chert beds are separated by thin beds of soft, clay-rich shale. These dramatically alternating beds are the result of a process called diagenetic enhancement. When the chert-forming sediments were laid down, some levels had slightly more silica than others. When the sediments were transformed into rock in a process called diagenesis, the silica in the less silica-rich zones migrated into the more silica-rich zones, increasing the silica contrast between the levels and enhancing the bedding to form ribbon chert.

Why did some levels of chert-forming sediment have more silica than others?

There are two theories for the original differences in silica levels that led to the formation of chert beds. The first theory is that silica levels were controlled by changes in the oceanographic process called upwelling, which brings nutrient-rich water to the ocean surface and allows the Radiolaria to thrive. Some evidence indicates that upwelling in the world's oceans is cyclic and strongly controlled by the Earth's orbital cycles. The second theory is that the silica-rich beds were deposited by small underwater landslides that sorted the fine clays out from the heavier radiolarian shells. There is some microscopic evidence of graded bedding that supports this possibility. Both processes, cyclical upwelling and underwater landslides, could have played a role in the formation of the Franciscan ribbon chert.

Why is chert different colors?

Most local chert is red and less commonly green, but it may be a range of colors. The color reflects the amount of oxygen present in the sediment when it became rock. If oxygen is plentiful in the sediment, it oxidizes small amounts of iron present and the chert is red. If oxygen is scarce, the iron is reduced and the chert is green or black.

Why are the chert beds so bent and folded?

Many Franciscan chert beds are highly folded and contorted, but within a short distance they often appear to be unfolded. Some scientists believe that this type of folding is the product of the slumping of the soft gelatinous silica-rich sediments, contorting them before they are fully hardened into rock. This slumping may have occurred on the flank of an oceanic mountain range as earthquakes shook the tectonically active mid-ocean ridge. Another theory is that the chert beds were folded by compressive forces developed as the Franciscan Complex was created by the scraping and addition of oceanic rocks to the western margin of North America.

Diabase FAQ

Where does diabase form?

Diabase is an intrusive igneous rock with the same mineral composition as basalt. It cools under basaltic volcanoes, like those at mid-ocean ridges. Diabase cools moderately quickly when magma moves up into fractures and weak zones below a volcano. There, it forms dikes (tabular igneous rock bodies that cut across pre-existing rock layers or bodies) or sills (tabular igneous rock bodies that form parallel to pre-existing rock layers). The moderate cooling rate allows small visible crystals to form in the rock.

Why does the diabase have large and small crystals?

Igneous rock with some large crystals among the smaller crystals is called a porphyry. The different crystal sizes are the result of different rates of cooling as the magma body moved upward. The large crystals, called phenocrysts, in diabase are feldspar crystals that grew as the magma cooled slowly deep in a magma chamber. Later the magma with the large phenocrysts moved upward quickly, causing more rapid cooling of the rest of the magma and the formation of the small crystals that make up the rest of the rock.

Granite and Granodiorite FAQ

Where do granite and granodiorite form?

Granite and granodiorite are intrusive igneous rocks that slowly cool deep underground in magma chambers called plutons. This slow cooling process allows easily visible crystals to form. Both rocks are the product of the melting of continental rocks near subduction zones.

What is the difference between granite and granodiorite?

These rocks are both classified as granitic, because they both are rich in quartz. Granite contains mostly potassium feldspars and has a low percentage of dark iron and magnesium minerals. In contrast, granodiorite contains more plagioclase (calcium and sodium) feldspar than potassium feldspar and has more dark minerals. Thus it is a darker color than granite. Chemical and x-ray analysis of granite and granodiorite can be used to "fingerprint" these rocks, telling their exact composition and where they may have formed.

Where do you find granite and granodiorite?

Granitic rocks are found on continents around the world near active or past plate boundaries. They formed as magma cooled many kilometers below the Earth's surface. The granitic rocks were then uplifted to the surface as the volcanic mountains above them eroded away. In California, granitic rocks form the core of the Sierra Nevada, cooled from rock melted during the subduction process that also formed the rocks of the Franciscan Complex. Granite and granodiorite are also found west of the San Andreas fault near Monterey, Pacifica, and Point Reyes, where granite from the south end of the Sierra range has been transported northward by San Andreas fault movement. Some granite was also carried to the California coast from China by humans. Chinese granite sometimes filled the holds of sailing ships on their way to San Francisco during the Gold Rush.

Why do some grantitic rocks have both large and small crystals in them?

This type for rock is called a porphyry. The different crystal sizes are the result of different rates of cooling as the magma body moved upwards. The large crystals, called phenocrysts, are usually feldspar crystals. Feldspar is one of the first minerals to form large crystals as magma solidifies. They grew as the magma cooled very slowly deep in the magma chamber. Later, the magma with the phenocrysts moved quickly upward into cooler rock, causing more rapid cooling of the remaining molten rock to form the smaller crystals that make up the rest of the rock.

Developing an Essential Question

The essential question of our geology curricula, How do I recognize evidence of geologic change in my environment?, is intended to be a theme or thread, linking all curriculum components together. The question has the following characteristics:

Transportable: The essential question can be asked in any educational setting: National Park, school, neighborhood, Earth, and during any part of the Rocks on the Move curriculum: pre-site visit, on-site program, and post-site lessons.

Multi-sensory: The essential question can be answered using many senses. We can observe geologic changes, visually, or by feel (shaking ground), smell (volcanic activity), taste (salt air), hearing (hear the rumble of an earthquake or landslide).

Universal: All students have experienced geologic change of some sort or another by being residents of planet Earth, so all students have personal experiences to draw upon and share.

Process-oriented: The essential question addresses process (geologic changes and how they occur) rather than solely observation/description (rock identification and mapping).

Limestone FAQ

Where is limestone deposited?

Limestone is a sedimentary rock rich in the mineral calcite, which is made of calcium carbonate. Franciscan limestone is formed mostly from the tiny carbonate shells of single celled marine animals called foraminifera. This type of limestone forms in ocean settings where there is not enough continental mud to dilute the slow "rain" of carbonate shells, and where the ocean is not so deep that all calcium carbonate dissolves into the water before it can be buried in the sediment to form rock. In today's oceans, all calcium carbonate dissolves below the depth (called Carbonate Compensation Depth or CCD) of about 4 km. Franciscan limestone is thought to have formed on the tops and sides of underwater volcanoes in water less than 4 km deep.

What can the fossil shells tell us?

Scientists have studied foraminifera found in sedimentary rocks. After identifying these fossils, geologists develop an evolutionary sequence of the species. This sequence, called a biostratigraphy, helps geologists determine the age of the rocks. The limestone in the Marin Headlands is too metamorphosed and recrystallized to contain original fossil shells, but other local Franciscan limestone contains fossils that tell us it was deposited from about 125 million ago to about 90 million years ago.

Serpentinite FAQ

Were does serpentinite form?

Serpentinite is a metamorphic rock that forms at tectonic plate boundaries deep within the Earth. In the Franciscan Complex, it formed when ocean water carried down with subducting ocean crust was heated and moved through upper mantle and basal ocean crust rocks, hydrating their magnesium- and iron-rich minerals, like olive and pyroxene, to form magnesium-rich serpentine minerals.

What makes serpentinite look and feel the way it does?

Serpentinite rocks are almost exclusively made of serpentine minerals. The most common serpentine mineral in Franciscan rocks is antigorite. This mineral gives the serpentinite its characteristic light to dark green color. Serpentine minerals are made of tiny sheets of silica tetrahedrons that are loosely held together. The weak bonds between these sheets gives serpentine its greasy or scaly look, and slippery feel (like a snake skin). Serpentinite often contains many veins, some of which may be filled with the fibrous mineral chrysotile (a form of asbestos). Chrysotile is a serpentine mineral in which the silica sheets are rolled into tiny tubes to form hollow fibers. Loose asbestos fibers cause lung disease if you inhale them. Pay attention to serpentinite - if you see fibrous asbestos, don't handle the rock.

How does serpentinite get to the Earth's surface?

Scientists do not fully understand how serpentinite makes its way from deep in the Earth, where it often forms, to the surface of the Earth. In the process of formation, serpentinite rock actually becomes less dense (more buoyant) as water is added. Thus it essentially floats upwards, buoyed up by the denser rock around it. Serpentinite is also a very plastic and greasy rock that can be easily squeezed into and even lubricate the many faults at plate boundaries. It then may get dragged or "squirted" to the surface along the faults.

How can water molecules become part of a rock?

Hot water forced through tiny fractures and pores in the mantle rock dissolves some silica from the rock into it. This silica-rich water then chemically reacts with mantle rock minerals like olivine to produce serpentine minerals by the formula below:

3Mg2SiO4 (olivine) + SiO2 + 2H2O converts to 2Mg3Si2O5(OH)4 (serpentine)

In this chemical reaction, the water is converted to hydroxyl groups (OH) that are part of the serpentine minerals.

Environmental Factors

The Golden Gate National Recreation Area is truly a park on the edge, located on the far end of the North American tectonic plate and flanking the Pacific Ocean. Situated above a large subduction zone where planetary crust is driven underground, destroyed, and recycled into new geologic resources, Golden Gate contains a wide variety of geologic features. From the Franciscan complex, representing tectonic events that took place hundreds of millions of years ago. To the Colma formation which tells of a time hundreds of thousands of years ago when San Francisco was an island and the Central Valley of California was an inland sea. To dune sands only a few thousand years old that migrated from the Sierra Nevada Mountains through the Sacramento River all the way to the coast. Whether scraped from the ocean floor under extreme heat and pressure like serpentenite, or built by millions of microscopic sea creatures under time and pressure like radiolarian chert, the Recreation Area has anything a rock lover could want.

Climatic shifts and geologic processes continue to shape this environment as they have for millennia. More recently, human-caused factors such as air, noise, light, and water pollution, have had a much greater impact on natural resources world-wide. The Recreation Area strives to keep a healthy environment for wildlife and world-weary humans alike.