Yukon River Exposures
A complex history of volcanism revealed
Overview
This map shows the flow footprint of the Pelly and Black Creek Eruptive Centers, as well as the locations of outcrops visited on this Virtual Tour. Modified from Jackson and Huscroft (2023).
In the area of Fort Selkirk, Yukon, impressive lava flows form cliffs along the banks of the Yukon River. The lava flows are a part of the Fort Selkirk volcanic field (FSVF) and mainly emanated from two different eruptive centers: 1) The Pelly eruptive center, active from around 1.5 to 1.3 million years ago; and 2) the Black Creek eruptive center, active around 441 thousand years ago ( 1 ) . Volcanic activity from these eruptive centers affected the Yukon River's drainage system at least five times, including interrupting or changing its course, and possibly blocking flow completely. A complex history can be constructed by looking at river exposures, several of which are explored below ( 1 ) .
This virtual field trip visits locations that are within the traditional territory of Selkirk First Nation. The content for this virtual field experience was collected in July 2023.
Virtual Field Trip
Join us in the field to explore sites along the banks of the Yukon River using our Kuula Virtual Field Trip! Click anywhere on the site tour to launch it, then make sure to click the full-screen button in the top right corner to get the full experience. Once in the tour, you can click and drag the image to look around the 360° view, zoom in and out to see more detail, and interact with the site by clicking the hotspots and information symbols.
Navigate below the virtual tour on this page to find additional information on volcanic features in this area.
Join us in the field as we explore outcrops along the Yukon River!
Using river exposures to unravel geologic history
The basalt flows that outcrop along the banks of the Yukon River tell a story of a changing landscape. Geologists have unraveled the geologic history of the area by examining volcanic structures such as pillow basalts and an exposed lava delta.
Pillow Lavas
Pillow lavas are primary igneous structures that form when lava erupts underwater. Pillows commonly form on the ocean floor in a rift (extensional) setting, but they can form anywhere that lava enters a body of water, including lakes and rivers.
The first pillow forms when lava erupts into water. As lava continues to flow it ruptures the initial pillow, forming a second pillow. This process continues and new pillows form, one after another until the lava supply decreases and formation of the pillow lava chain is halted.
When lava comes in contact with cold water the outer surface quenches, turning the lava into dark, fine-grained volcanic glass. This quenched layer is called a chilled margin and is a mixture of volcanic glass and microcrystals from the lava. The chilled margin acts as a thermal insulator, allowing the center of the pillow to remain hot and molten.
As new lava flows into the pillow, pressure builds up inside, causing the chilled margin to rupture. Lava will flow out of the rupture site and begin to form a new pillow. As this process continues, long chains of pillows are created, and through time, they can solidify on top of one another, forming layers of pillows. When the lava supply becomes low, pillow creation ceases and terminal pillows cool and crystallize.
This sample is a wedge of a pillow basalt that detached along a radial fracture plane and allows the inside of a pillow to be examined. The chilled margin is located on the top crust of the sample, with a ring of vesicles just below the chilled margin.
A common feature of pillow basalts is the glassy chilled margin. A chilled margin is the outside layer of the pillow, exhibiting a fine-grained texture and dark or black colouration that may be iridescent.
Pillows also typically have tension cracks that develop as a pillow cools. They form perpendicular to the cooling surface (the pillow top) in a radial pattern. Another common feature found in pillows is a concentric ring of vesicles. Gas bubbles from the lava will rise part-way to the pillow's surface and get trapped by previously solidified basalt. The solid rock stops the gas bubbles from escaping, and when the basalt cools and crystallizes, the gas bubbles remain as a ring of vesicles.
Mafic pillow lava is the most common type of lava found on Earth today. In fact, there is likely some forming on the sea floor right now! The presence of pillow basalt in an outcrop indicates that there was a subaqueous depositional setting at the time of formation. If the shape or size of a pillow has changed since formation, geologists infer that the rocks have been deformed.
Crystal Huscroft, Associate Teaching Professor at Thompson Rivers University, explains how pillow lavas form in the field.
Sheilany Bouchard, étudiant en Earth Sciences, explique comment les laves en coussin se forment sur le terrain.
Lava Deltas
At a site referred to by researchers as Pillow Point along the Yukon River, downstream from Fort Selkirk, geologists have identified rocks that form an ancient lava delta structure. Deltas are common features formed in fluvial (river) systems, where moving water flows into a standing water body (lake or ocean) and drops its sediment load, gradually building a fan-shaped sedimentary structure. A lava delta is the igneous equivalent, formed when lava flows into standing water and builds a similar fan-shaped structure. Follow the illustration sequence below to learn more about the formation of lava deltas and the evidence for this structure at Pillow Point.
River deltas form when a river flows into a standing body of water, and with the reduction in energy of the system, the sediment load drops out of suspension. Layers of sediment eventually build a fan-shaped landform that extends out into a lake or ocean.
Pictured here is the Yukon River delta, where the river empties into the Bering Sea on the Alaskan coast. Over time, the fan-shape was built by the sediment carried by the river into the Bering Sea.
As sediment continues to deposit into the body of water, layers of sediment build on top of each other and outwards into the basin. This creates a prograding (advancing basin-ward) sequence of deltaic sediments that are inclined toward the basin center (deeper water). Geologists refer to these inclined sediment packages as foreset beds.
Similarly, lava deltas are constructed when lava flows across the land surface and makes contact with a water body, rapidly cooling and forming pillow lava . Layers of pillow lava build up and in a basin-ward direction through time, advancing (prograding) into the basin.
The volcanic layering within the advancing delta front (foreset bedding) is not horizontal, but inclined or dipping toward the basin center, just like a river delta!
This is a photograph of Pillow Point looking upstream. The modern-day Yukon River water level is low, exposing basalt outcrops and the structure of the lava delta. The horizontal layers of basalt forming cliff bands near the top of the outcrop are subaerial lava flows. The chaotic-looking sloping unit on the lower part of the outcrop is subaqueous pillow basalt breccia. The layering in the pillow basalt unit is steeply dipping (inclined), as would be expected from foreset bedding in a delta structure.
This is an annotated photo of the outcrop at Pillow Point. Note the round basalt pillows near the base of the slope!
Dating the Lava Flows
This illustration represents the Earth’s magnetic field with a simple bar magnet as the core. The magnetic field has a "normal" polarity with the magnetic north pole approximately aligning with the geographic north pole.
The age of a rock sample is best constrained using radiometric dating. Minerals that contain unstable "parent" isotopes of certain elements (e.g. 238 U, 40 K) decay over time to form "daughter" isotopes (e.g. 206 Pb, 40 Ar). The concentrations of parent and daughter isotopes are measured in a laboratory and an age can be calculated from their relative amounts. However, rocks in the Fort Selkirk volcanic field are not ideal for radiometric dating because they are too young. To determine ages of the lava flows, the paleomagnetic signature of the basalt was also examined.
Paleomagnetism is the study of changes in the Earth’s magnetic field through time. Throughout Earth’s history the magnetic field has varied in both strength and position. The Earth’s magnetic field has also reversed (flipped) many times, meaning, the magnetic north pole has switched to the geographic south pole, and vice versa. Between reversals, the poles of our magnetic field are constantly moving. In 1948, magnetic north was located on Prince of Wales Island, Nunavut, but has since migrated northwest into the Arctic Ocean ( 2) .
This map shows the changing location of the Magnetic North Pole.
Certain rock types record the orientation of the Earth's magnetic field at the time of their formation. In igneous rocks, the magnetic fields generated by the spin of electrons in specific iron-bearing minerals, like magnetite (Fe 3 O 4 ), will align with the Earth’s external magnetic field as a mineral crystalizes. At temperatures above the Curie temperature (~570 °C for magnetite), this alignment will continue to change as the external field moves. However, below the Curie temperature this this magnetic signature gets locked in and can provide a record of earlier "paleomagnetic conditions."
Associate Teaching Professor Crystal Huscroft from Thompson Rivers University looking at four paleomagnetism sample core holes in a basalt outcrop that forms a part of the Pelly Eruptive Centre. These samples were collected several years prior as a part of a research project.
In order to 'read' the paleomagnetic record of rocks, like those at Fort Selkirk, samples must be carefully collected. Rocks are sampled using a drill that bores into the rock, extracting a long cylindrical sample called a core. The orientation of the core is measured in the field. In a laboratory, the magnetic orientation preserved in the sample is measured with a magnetometer. The magnetic orientation is compared with a paleomagnetic record, a known record of the Earth's magnetism through time. This will help the geologist constrain the age of the sample and can also provide location information (latitude/longitude) of the rock at the time of its formation.
Lava Flows on Sediment
Have you ever wondered what happens to the sediments and soil below lava flows? Just downstream from Fort Selkirk, at a site called Cave, recent river erosion has exposed the location in the stratigraphic sequence where basalt flows overlie laminated (finely layered), unconsolidated (loose) sediment. Check out the images below to learn more about this site!
Research suggests that lava flows from an early eruption from the Pelly eruptive center crossed the Yukon River and dammed it (1) . This created a lake on the upstream side of the flow in which sediment was deposited ( 1 ) . When the dam eventually breeched and the water drained away, a thick unit of lacustrine (lake) sediments remained.
Today, we can observe basaltic lava flows on top of the lacustrine, or slack water, sediment that indicate continued activity from the Pelly eruptive center sometime after 1.5 million years ago. At this location next to the Yukon River, this lava flowed onto unconsolidated (loose), wet sediment, protecting it from erosion and forming a unique deposit called peperite.
In the field, we can clearly see the boundary between the sediments and the overlying lava and can examine these deposits to learn more about this ancient environment.
The following sequence of images explores the sediments above and below this boundary.
This is a picture of the sediments underlying the lava flow. The finely laminated sediments (sand, silt and clay) were deposited in a low-energy subaqueous environment, where fine sediments can settle out of suspension. This sediment package records a long period of deposition, with each layer representing a slight change in sediment texture. Layers may be the result of changing energy environments within the drainage basin.
This image shows the unit immediately above the finely laminated sediments. As the hot lava flowed across the landscape and volcanic clasts mixed with non-volcanic sediments, likely in a fluvial or lacustrine environment, a unique rock type formed called peperite.
Peperite classified as a volcaniclastic rock, as it is comprised of both igneous and sedimentary components. It is a mixture of volcanic glass, crystal shards, sediments, and lithic fragments that are fused together upon contact.
Directly above the peperite is a unit of pillow basalt , which forms in a subaqueous environment. This indicates that lava was flowing into a river or lake at this time.
Above the pillow basalt unit is a thick unit of columnar basalt. The presence of columnar basalts indicates that the lava was solidifying in a subaerial environment.
This outcrop provides an example of how geologists use clues found in the rocks to reconstruct and link events in geologic history.
A stratigraphic record here shows changing depositional and volcanic environments, from lacustrine sedimentation to subaqueous and then subaerial volcanism.
Listen to Mary Samolczyk (Assistant Professor, Yukon University) and Crystal Huscroft (Associate Teaching Professor, Thompson Rivers University) talk about the history of the sediment preserved by a lava flow.
